An Entire Layer of Earth's History Could Have Been Ripped Away by Ice

An Entire Layer of Earth's History Could Have Been Ripped Away by Ice

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The Earth’s crust is a visual timeline that goes back billions of years. But all over the world, there’s a gap in the timeline—a huge chunk of crust that should be there yet isn’t. Now, scientists say that the crust may have been destroyed during Snowball Earth, a hypothetical period in which the globe was covered in ice.

The gap in Earth’s timeline is known as the Great Unconformity, and represents 250 million to 1.2 billion years of lost time. The crust timeline abruptly jumps from the Cambrian Period, which saw the most intense burst of evolution, and the Precambrian time, during which the Earth was formed—meaning that it skips over about one-fifth of Earth’s geological history.

So what happened? Well, that crust could have been ripped away by moving glaciers during Snowball Earth, argue scientists in a December 2018 article published in the Proceedings of the National Academy of Sciences. Glaciers’ intense weight causes them to scrape and erode land that they move across. After being scraped up by Snowball Earth glaciers, the missing crust could have fallen into the ocean as sediment, and then disappeared into one of Earth’s lower layers.

The scientists argue that a big geochemical shift happened around the time Snowball Earth’s worldwide glaciers formed, something that suggests Earth’s crust was being recycled. “Although this erosion didn’t apply evenly across the world, it amounts to an average sediment layer 1.9 to 3.1 miles deep being swept away,” reports National Geographic.

If correct, this theory explaining the Great Unconformity could help shed light on other aspects of Earth’s history, like why Earth lost most of its craters around 600 to 700 million years ago.

It could also help connect the dots between Earth’s formation and “Cambrian explosion” of life between 541 million and 530 million years ago. So far, we know there were large animals before the explosion—the oldest is the 558 million-year-old Dickinsonia, a sci-fi looking creature that could grow up to four and a half feet long. But there are still a lot of questions about what was going on before that, in the Earth’s gap years.

Frost heaving

Frost heaving (or a frost heave) is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated into the soil (the freezing front or freezing boundary). Ice growth requires a water supply that delivers water to the freezing front via capillary action in certain soils. The weight of overlying soil restrains vertical growth of the ice and can promote the formation of lens-shaped areas of ice within the soil. Yet the force of one or more growing ice lenses is sufficient to lift a layer of soil, as much as 1 foot (0.30 metres) or more. The soil through which water passes to feed the formation of ice lenses must be sufficiently porous to allow capillary action, yet not so porous as to break capillary continuity. Such soil is referred to as "frost susceptible". The growth of ice lenses continually consumes the rising water at the freezing front. [1] [2] Differential frost heaving can crack road surfaces—contributing to springtime pothole formation—and damage building foundations. [3] [4] Frost heaves may occur in mechanically refrigerated cold-storage buildings and ice rinks.

Needle ice is essentially frost heaving that occurs at the beginning of the freezing season, before the freezing front has penetrated very far into the soil and there is no soil overburden to lift as a frost heave. [5]


Evidence for ancient glaciation mounts Edit

Long before the idea of a global glaciation was established, a series of discoveries began to accumulate evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson who found ancient glacier-reworked material (tillite) in Islay, Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding that came to be known as "Reusch's Moraine" was reported by Hans Reusch in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection of continental drift. [5]

Global glaciation proposed Edit

Sir Douglas Mawson (1882–1958), an Australian geologist and Antarctic explorer, spent much of his career studying the Neoproterozoic stratigraphy of South Australia, where he identified thick and extensive glacial sediments and late in his career speculated about the possibility of global glaciation. [6]

Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and those of other continents where low-latitude glacial deposits are found, have remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes.

In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. [7] From this palaeomagnetic data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued for an ice age that was so extreme that it resulted in the deposition of marine glacial rocks in the tropics.

In the 1960s, Mikhail Budyko, a Soviet climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. [8]

While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had in fact never happened, because his model offered no way to escape from such a feedback loop. In 1971, Aron Faegre, an American physicist, showed that a similar energy-balance model predicted three stable global climates, one of which was snowball earth. [9]

This model introduced Edward Norton Lorenz's concept of intransitivity indicating that there could be a major jump from one climate to another, including to snowball earth.

The term "snowball Earth" was coined by Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning the biology of the Proterozoic eon. [10] The major contributions from this work were: (1) the recognition that the presence of banded iron formations is consistent with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a completely ice-covered Earth—specifically, the accumulation of CO2 from volcanic outgassing leading to an ultra-greenhouse effect.

Franklyn Van Houten's discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle". His studies of phosphorus deposits and banded iron formations in sedimentary rocks made him an early adherent of the "snowball Earth" hypothesis postulating that the planet's surface froze more than 650 million years ago. [11]

Interest in the notion of a snowball Earth increased dramatically after Paul F. Hoffman and his co-workers applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in Namibia and elaborated upon the hypothesis in the journal Science in 1998 by incorporating such observations as the occurrence of cap carbonates. [12]

In 2010, Francis A. Macdonald, assistant professor at Harvard in the Department of Earth and Planetary Sciences, and others, reported evidence that Rodinia was at equatorial latitude during the Cryogenian period with glacial ice at or below sea level, and that the associated Sturtian glaciation was global. [13]

The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. [14] According to modelling, an ice–albedo feedback would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° [15] to 30° [16] of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover.

Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove three things:

  1. that a bed contains sedimentary structures that could have been created only by glacial activity
  2. that the bed lay within the tropics when it was deposited.
  3. that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.

This last point is very difficult to prove. Before the Ediacaran, the biostratigraphic markers usually used to correlate rocks are absent therefore there is no way to prove that rocks in different places across the globe were deposited at precisely the same time. The best that can be done is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so. [17]

The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as 200 million years ago can be riddled with difficulties. [18]

Palaeomagnetism Edit

The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.

When sedimentary rocks form, magnetic minerals within them tend to align themselves with the Earth's magnetic field. Through the precise measurement of this palaeomagnetism, it is possible to estimate the latitude (but not the longitude) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator, [19] although the accuracy of this reconstruction is in question. [17] This palaeomagnetic location of apparently glacial sediments (such as dropstones) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation, or the existence of localized, possibly land-locked, glacial regimes. [20] Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator. [21]

Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of Earth's core, it is possible that during the Proterozoic, the magnetic field did not approximate a simple dipolar distribution, with north and south magnetic poles roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a 'West Pole' rather than the North Pole. Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record the alleged motion of the north pole would occur around the same time as the Gaskiers glaciation. [22]

Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building orogeny releases hot water as a by-product of metamorphic reactions this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations. [15] Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles. [23] [24]

There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes its depositional date is well-constrained, and the signal is demonstrably original. [25]

Low-latitude glacial deposits Edit

Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, it is worth remembering that many sedimentary features traditionally associated with glaciers can also be formed by other means. [26] Thus the glacial origin of many of the key occurrences for snowball Earth has been contested. [17] As of 2007, there was only one "very reliable"—still challenged [17] —datum point identifying tropical tillites, [19] which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the Sturtian is accumulating. [27] [28] Evidence of possible glacial origin of sediment includes:

    (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena. [29] (annual sediment layers in periglacial lakes), which can form at higher temperatures. [30] (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by mudflows or tectonic movements. [31] (poorly sorted conglomerates). Originally described as glacial till, most were in fact formed by debris flows. [17]

Open-water deposits Edit

It appears that some deposits formed during the snowball period could only have formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years solid oceans would not permit this scale of deposition. [32] It is considered [ by whom? ] possible that ice streams such as seen in Antarctica today could have caused these sequences. Further, sedimentary features that could only form in open water (for example: wave-formed ripples, far-traveled ice-rafted debris and indicators of photosynthetic activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent "oases" of meltwater on a completely frozen Earth, [33] computer modelling suggests that large areas of the ocean must have remained ice-free arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models. [34]

Carbon isotope ratios Edit

There are two stable isotopes of carbon in sea water: carbon-12 ( 12 C) and the rare carbon-13 ( 13 C), which makes up about 1.109 percent of carbon atoms.

Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter 12 C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in 13 C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower 13 C/ 12 C ratio within organic remains, and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in 13 C.

During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of 13 C to 12 C. [35] Close analysis of the timing of 13 C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic. [36]

Banded iron formations Edit

Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of the Earth's atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide.

The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits.

For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ferrous oxide) could accumulate before it met an oxidant that would precipitate it as ferric oxide. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice, [10] while opponents suggest that the rarity of the BIF deposits may indicate that they formed in inland seas.

Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's Black Sea a sufficient input of iron could provide the necessary conditions for BIF formation. [17] A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments. [20] BIFs are also strikingly absent during the Marinoan glaciation. [ citation needed ]

Cap carbonate rocks Edit

Around the top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary limestone or dolomite metres to tens of metres thick. [37] These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry. [38]

These cap carbonates have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples. [39] The formation of such sedimentary rocks could be caused by a large influx of positively charged ions, as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The δ 13 C isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle—such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis.

The precise mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant CO
2 from the atmosphere to form carbonic acid, which would fall as acid rain. This would weather exposed silicate and carbonate rock (including readily attacked glacial debris), releasing large amounts of calcium, which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an abiotic "cap carbonate" sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis.

However, there are some problems with the designation of a glacial origin to cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within—starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. [17] An alternative mechanism, which may have produced the Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48 ‰— δ 13 C values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments. [40]

Changing acidity Edit

Isotopes of the element boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation. [41] This may indicate a buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation.

Space dust Edit

Earth's surface is very depleted in the element iridium, which primarily resides in the Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years, [42] but this does not necessarily imply a global extent to the glaciation indeed, a similar anomaly could be explained by the impact of a large meteorite. [43]

Cyclic climate fluctuations Edit

Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. [44] This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth.

In addition, glacial sediments of the Port Askaig Tillite Formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments. [45] The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Portaskaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth.

The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's albedo, which would result in positive feedback for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where solar radiation is most direct.

Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a supervolcano, a reduction in the atmospheric concentration of greenhouse gases such as methane and/or carbon dioxide, changes in Solar energy output, or perturbations of Earth's orbit. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen equator as cold as modern Antarctica.

Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium. [ citation needed ]

Continental distribution Edit

A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth. [47] Firstly, tropical continents are more reflective than open ocean, and so absorb less of the Sun's heat: most absorption of Solar energy on Earth today occurs in tropical oceans. [48]

Further, tropical continents are subject to more rainfall, which leads to increased river discharge—and erosion. When exposed to air, silicate rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form: Rock-forming mineral + CO2 + H2O → cations + bicarbonate + SiO2. An example of such a reaction is the weathering of wollastonite:

The released calcium cations react with the dissolved bicarbonate in the ocean to form calcium carbonate as a chemically precipitated sedimentary rock. This transfers carbon dioxide, a greenhouse gas, from the air into the geosphere, and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.

As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis. [49] Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth. [50]

Additional factors that may have contributed to the onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with methane in the atmosphere, oxidizing it to carbon dioxide, a much weaker greenhouse gas, [51] and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic. [17]

Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this 'negative feedback' process limits the magnitude of cooling. During the Cryogenian period, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This let ice advance beyond the polar regions. Once ice advanced to within 30° of the equator, [52] a positive feedback could ensue such that the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered.

Polar continents, due to low rates of evaporation, are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from the carbon cycle. A gradual rise of the proportion of the isotope carbon-13 relative to carbon-12 in sediments pre-dating "global" glaciation indicates that CO
2 draw-down before snowball Earths was a slow and continuous process. [53]

The start of snowball Earths are always marked by a sharp downturn in the δ 13 C value of sediments, [54] a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans.

In January 2016, Gernon et al. proposed a "shallow-ridge hypothesis" involving the breakup of the supercontinent Rodinia, linking the eruption and rapid alteration of hyaloclastites along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events. [55]

During the frozen period Edit

Global temperature fell so low that the equator was as cold as modern-day Antarctica. [56] This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect.

Breaking out of global glaciation Edit

The carbon dioxide levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere. [57] Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of siliceous rocks. Over 4 to 30 million years, enough CO
2 and methane, mainly emitted by volcanoes but also produced by microbes converting organic carbon trapped under the ice into the gas, [58] would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed [59] this would be darker than the ice, and thus absorb more energy from the Sun—initiating a "positive feedback".

Destabilization of substantial deposits of methane hydrates locked up in low-latitude permafrost may also have acted as a trigger and/or strong positive feedback for deglaciation and warming. [60]

On the continents, the melting of glaciers would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as phosphorus, which combined with the abundance of CO
2 would trigger a cyanobacteria population explosion, which would cause a relatively rapid reoxygenation of the atmosphere, which may have contributed to the rise of the Ediacaran biota and the subsequent Cambrian explosion—a higher oxygen concentration allowing large multicellular lifeforms to develop. Although the positive feedback loop would melt the ice in geological short order, perhaps less than 1,000 years, replenishment of atmospheric oxygen and depletion of the CO
2 levels would take further millennia.

It is possible that carbon dioxide levels fell enough for Earth to freeze again this cycle may have repeated until the continents had drifted to more polar latitudes. [61]

More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state. [62]

During millions of years, cryoconite would have accumulated on and inside the ice. Psychrophilic microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers. Once the ice started to melt, these layers would become visible and color the icy surfaces dark, helping to accelerate the process. [63]

Ultraviolet light from the Sun would also produce hydrogen peroxide (H2O2) when it hits water molecules. Normally hydrogen peroxide is broken down by sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, leading to an increase in atmospheric oxygen. [64]

Slushball Earth hypothesis Edit

While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the equator, allowing for a continued hydrologic cycle.

This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water, or rapidly moving ice (which would require somewhere ice-free to move to). Recent research observed geochemical cyclicity in clastic rocks, showing that the "snowball" periods were punctuated by warm spells, similar to ice age cycles in recent Earth history. Attempts to construct computer models of a snowball Earth have also struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet.

A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation. [65] Synthesised evidence has produced models indicating a "slushball Earth", [66] where the stratigraphic record does not permit postulating complete global glaciations. [65] Kirschivink's original hypothesis [10] had recognised that warm tropical puddles would be expected to exist in a snowball earth.

The snowball Earth hypothesis does not explain the alternation of glacial and interglacial events, nor the oscillation of glacial sheet margins. [67]

The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "snowball Earth" deposits. Evidence for such melting comes from evidence of glacial dropstones, [32] geochemical evidence of climate cyclicity, [44] and interbedded glacial and shallow marine sediments. [45] A longer record from Oman, constrained to 13°N, covers the period from 712 to 545 million years ago—a time span containing the Sturtian and Marinoan glaciations—and shows both glacial and ice-free deposition. [68]

There have been difficulties in recreating a snowball Earth with global climate models. Simple GCMs with mixed-layer oceans can be made to freeze to the equator a more sophisticated model with a full dynamic ocean (though only a primitive sea ice model) failed to form sea ice to the equator. [69] In addition, the levels of CO
2 necessary to melt a global ice cover have been calculated to be 130,000 ppm, [57] which is considered by to be unreasonably large. [70]

Strontium isotopic data have been found to be at odds with proposed snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation. Therefore, methane release from permafrost during marine transgression was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation. [71]

"Zipper rift" hypothesis Edit

Nick Eyles suggests that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure. [17] The "Zipper rift" hypothesis proposes two pulses of continental "unzipping"—first, the breakup of the supercontinent Rodinia, forming the proto-Pacific Ocean then the splitting of the continent Baltica from Laurentia, forming the proto-Atlantic—coincided with the glaciated periods. The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography this high ground could then host glaciers.

Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and anoxic waters to form however, the limited extent of the Neoproterozoic banded iron deposits means that they may not have formed in frozen oceans, but instead in inland seas. Such seas can experience a wide range of chemistries high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form.

Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels.

High-obliquity hypothesis Edit

A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's axial tilt was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce. [72] A less extreme possibility would be that it was merely Earth's magnetic pole that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depend on the magnetic and rotational poles being relatively similar. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today severe changes to Earth's climate are not necessary.

Inertial interchange true polar wander Edit

The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange true polar wander (IITPW). [73] [74] This hypothesis, created to explain palaeomagnetic data, suggests that Earth's orientation relative to its axis of rotation shifted one or more times during the general time-frame attributed to snowball Earth. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude. [75] While the physics behind the proposition is sound, the removal of one flawed data point from the original study rendered the application of the concept in these circumstances unwarranted. [76]

Several alternative explanations [ clarification needed ] for the evidence have been proposed. [ citation needed ]

A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing non-oxidized iron-rich rocks to form.

Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as stromatolites and oncolites prove that, in shallow marine environments at least, life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed. [77] Proponents counter that it may have been possible for life to survive in these ways:

  • In reservoirs of anaerobic and low-oxygen life powered by chemicals in deep oceanic hydrothermal vents surviving in Earth's deep oceans and crust but photosynthesis would not have been possible there.
  • Under the ice layer, in chemolithotrophic (mineral-metabolizing) ecosystems theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of volcanism or geothermal activity. [78]
  • In pockets of liquid water within and under the ice caps, similar to Lake Vostok in Antarctica. In theory, this system may resemble microbial communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 100 m thick, and at the temperatures predicted by models equatorial sublimation would prevent equatorial ice thickness from exceeding 10 m. [79]
  • As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period.
  • In small regions of open water in deep ocean regions preserving small quantities of life with access to light and CO
    2 for photosynthesizers (not multicellular plants, which did not yet exist) to generate traces of oxygen that were enough to sustain some oxygen-dependent organisms. This would happen even if the sea froze over completely, if small parts of the ice were thin enough to admit light. These small open water regions may have occurred in deep ocean regions far from the supercontinentRodinia or its remnants as it broke apart and drifted on the tectonic plates. [citation needed]
  • In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice. [80] Such environments may have covered approximately 12 per cent of the global surface area. [81]
  • In small oases of liquid water, as would be found near geothermalhotspots resembling Iceland today. [82]
  • In nunatak areas in the tropics, where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt pools, which would freeze at sunset. [citation needed]
  • Oxygenated subglacial meltwater, along with iron-rich sediments dissolved in the glacial water, created a meltwater oxygen pump when it entered the ocean, where it provided eukaryotes with some oxygen, and both photosynthetic and chemosynthetic organisms with sufficient nutrients to support an ecosystem. The freshwater would also mix with the hypersaline seawater, which created areas less hostile to eukaryotic life than elsewhere in the ocean. [83]

However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a mass extinction. With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16 million years. [84] Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed [85] —in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth. [43] One rebuttal to this is the fact that in many of these places where an argument is made against a mass extinction caused by snowball earth, the Cryogenian fossil record is extraordinarily impoverished. [86]

A snowball Earth has profound implications in the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials.

Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted, it would cover the oceans with a layer of hot freshwater up to 2 kilometres thick. Only after the hot surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state. [87]

The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Cryogenian period.

Effect on early evolution Edit

The Neoproterozoic was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles in this sense, snowball Earth episodes may have "pumped" evolution. Alternatively, fluctuating nutrient levels and rising oxygen may have played a part. Another major glacial episode may have ended just a few million years before the Cambrian explosion.

One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much affect the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The Snowball I episode (of the Huronian glaciation 2.4 to 2.1 billion years) and Snowball II (of the Precambrian's Cryogenian between 580 and 850 million years and which itself had a number of distinct episodes) are respectively thought to be caused by the evolution of oxygenic photosynthesis and then the rise of more advanced multicellular animal life and life's colonization of the land. [88] [89]

Effects on ocean circulation Edit

Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation. [90]

Neoproterozoic Edit

There were three or four significant ice ages during the late Neoproterozoic. Of these, the Marinoan was the most significant, and the Sturtian glaciations were also truly widespread. [91] Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long [1] Gaskiers glaciation did not lead to global glaciation, [47] although it was probably as intense as the late Ordovician glaciation. The status of the Kaigas "glaciation" or "cooling event" is currently unclear some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age. [92] It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that the Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis. [4]

Palaeoproterozoic Edit

The snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested. [93] [94] The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (

2.25 billion years old) and were deposited at tropical latitudes. [95] It has been proposed that rise of free oxygen that occurred during the Great Oxygenation Event removed methane in the atmosphere through oxidation. As the Sun was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing.

In the absence of this methane greenhouse, temperatures plunged and a snowball event could have occurred. [94]

Karoo Ice Age Edit

Before the theory of continental drift, glacial deposits in Carboniferous strata in tropical continental areas such as India and South America led to speculation that the Karoo Ice Age glaciation reached into the tropics. However, a continental reconstruction shows that ice was in fact constrained to the polar parts of the supercontinent Gondwana.

Snowball-driven dump

And that’s exactly what the researchers found. The zircons in the database span nearly the entire history of the Earth, and by far the most noticeable wiggle lines up neatly with the Great Unconformity. When they ran the numbers to see how much erosion would be required to explain a wiggle of that size, they found that it would be something in the neighborhood of 3 kilometers (or 2 miles) of rock shaved off all the world’s continents and dumped on the ocean floor.

Erosion alone can't explain all the details of this episode, and you need something that affects the entire globe. Is there anything else that can wipe a few kilometers of rock off the Earth's face? The authors propose that three periods of epic cold snaps in the 180 million years leading up to the start of the Cambrian—sometimes referred to as “Snowball Earth” periods—could be the key.

The first two of these episodes, in particular, are thought to have seen huge ice sheets draped over every continent for millions of years. There are still big questions about how these events played out, but glaciers are often pretty potent agents of erosion. If temperatures drop low enough, glaciers will freeze to the ground like the tongue of an unfortunate child stuck on a flag pole. But it doesn’t take much for normal geothermal warmth to keep that base thawed, and sliding ice will grind up a lot of bedrock.

On top of that, the growth of ice sheets on land comes with a lowering of global sea level, exposing vast areas of former seafloor to erosion. That also lowers the base level that glaciers and streams flow to, giving them a little more downhill energy.

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Hopscotching through these human disasters to the present day, we pass perhaps the most familiar historical climate event of all: the Little Ice Age. Lasting roughly from 1500 to 1850, the chill made ice rinks of Dutch canals, and swelled up Swiss mountain glaciers. Tent cities sprung up on a frozen Thames, and George Washington endured his winter of cold and privation at Valley Forge in 1777 (which wasn’t even particularly harsh for the times). The Little Ice Age might have been a regional event, perhaps the product of an exceptional run of sunlight-dimming volcanism. In 1816, its annus horribilis, the so-called year without a summer—which brought snows to New England in August—global temperatures dropped perhaps a mere half a degree Celsius. While it is perennially plumbed by historians for insights into future climate change, it is not even remotely on the same scale of disruption as that which might lie in our future.

As Europe emerged from its chill, coal from 300-million-year-old jungles was being fed into English furnaces. Although the Earth was now in the same configuration that, in the previous few million years, had invited a return to deep, unthinkable ice ages, for some reason the next ice age never took. Instead the planet embarked on an almost unprecedented global chemistry experiment. Halfway through the 20th century, the climate began behaving very strangely.

So this is the climate of written history, a seemingly eventful stretch that has really been the random noise and variability of a climate essentially at peace. Indeed, if you were to find yourself in an industrial civilization somewhere else in the universe, you would almost certainly notice such similarly strange and improbably pleasant millennia behind you. This kind of climate stability seems to be a prerequisite for organized society. It is, in other words, as good as it gets.

As we jump back 20,000 years—to yesterday, geologically—the world ceases being recognizable. Whereas all of recorded history played out in a climate hovering well within a band of 1 degree Celsius, we now see what a difference 5 to 6 degrees can make—a scale of change similar to the one that humans may engineer in only the next century or so, though in this case, the world is 5 to 6 degrees colder, not warmer.

An Antarctica’s worth of ice now rests atop North America. Similar sheets smother northern Europe, and as a result, the sea level is now 400 feet lower. The midwestern United States is carpeted in stands of stunted spruce of the sort that would today look at home in northern Quebec. The Rockies are carved up, not by wildflower-dappled mountain valleys, but by overflowing rivers of ice and rock. California is a land of dire wolves. Where the Pacific Northwest edges up against the American Antarctica, it is a harsh and treeless place. Nevada and Utah fill up with cold rains.

During World War II, at Topaz, the desolate Japanese American internment camp in Utah, prisoners combed the flats of the Sevier Desert for unlikely seashells, fashioning miraculous little brooches from tiny mussel and snail shells to while away their exile. The desert seashells were roughly 20,000 years old, from the vanished depths of the giant Pleistocene-era Lake Bonneville—the product of a jet stream diverted south by the ice sheet. This was once a Utahan Lake Superior, more than 1,000 feet deep in places. It was joined by endless other verdant lakes scattered across today’s bleak Basin and Range region.

Elsewhere, the retreat of the seas made most of Indonesia a peninsula of mainland Asia. Vast savannas and swamps linked Australia and New Guinea, and of course Russia shared a tundra handshake with Alaska. There were reindeer in Spain, and glaciers in Morocco. And everywhere loess, loess, and more loess. This was the age of dust.

Ice is a rock that flows. Send it in massive sterilizing slabs across the continents, and it will quarry mountainsides, pulverize bedrock, and obliterate everything in its path. At the height of the last ice age, along the crumbling margins of the continental ice sheets, the rocky, dusty spoils of all this destruction spilled out onto the tundra. Dry winds carried this silt around the world in enormous dust storms, piling it up in seas of loess that buried the central U.S., China, and Eastern Europe under featureless drifts. In Austria, not far from the site of the voluptuous Venus of Willendorf figurine, carved some 30,000 years ago, are the remains of a campground of the same age—tents, hearths, burnt garbage pits, hoards of ivory jewelry—all abandoned in the face of these violent, smothering haboobs. Ice cores from both Antarctica and Greenland record a local environment that was 10 times dustier than today. All of this dust seeded the seas with iron, a vital nutrient for carbon-hogging plankton, which bloomed around Antarctica and pulled gigatons of CO2 out of the air and deep into the ocean, freezing the planet further.

This parched Pleistocene world would have appeared duller from space, hosting as it did a quarter less plant life. CO2 in the atmosphere registered only a paltry 180 ppm, less than half of what it is today. In fact, CO2 was so low, it might have been unable to drop any further. Photosynthesis starts to shut down at such trifling levels, a negative-feedback effect that might have left more CO2—unused by plants—in the air above, acting as a brake on the deep freeze.

This was the strange world of the Ice Age, one that, geologically speaking, is still remarkably recent. It’s so recent, in fact, that today, most of Canada and Scandinavia is still bouncing back up from the now-vanished ice sheets that had weighed those lands down.

In 2021, we find ourselves in an unusual situation: We live on a world with massive ice sheets, one of which covers one of the seven continents and is more than a mile deep. For most of the planet’s past, it has had virtually no ice whatsoever. The periods of extreme cold—like the ultra-ancient, phantasmagoric nightmares of Snowball Earth, when the oceans might have been smothered by ice sheets all the way to the tropics—are outliers. There were a few other surprising pulses of frost here and there, but they merely punctuate the balmy stretches of the fossil record. For almost all of the Earth’s history, the planet was a much warmer place than it is today, with much higher CO2 levels. This is not a climate-denying talking point it’s a physical fact, and acknowledging it does nothing to take away from the potential catastrophe of future warming. After all, we humans, along with everything else alive today, evolved to live in our familiar low-CO2 world—a process that took a long time.

How long, exactly? Fifty million years ago, as our tiny mammalian ancestors were still sweating through the jungly, high-CO2 greenhouse climate they had inherited from the dinosaurs, India was nearing the end of an extended journey. Long estranged from Africa and the august, bygone supercontinent of Gondwana, the subcontinent raced northeast across the proto–Indian Ocean and smashed into Asia in slow motion. The collision not only quieted CO2-spewing volcanoes along Asian subduction zones it also thrust the Himalayas and the Tibetan Plateau toward the stars, to be continually weathered and eroded away.

As it turns out, weathering rocks—that is, breaking them down with CO2-rich rainwater—is one of the planet’s most effective long-term mechanisms for removing carbon dioxide from the atmosphere, one that modern geoengineers are frantically trying to reproduce in a lab, for obvious reasons.

Adding to this colossal Himalayan CO2 sink, the more recent buckling, tectonic mess that lifted Indonesia and its neighbors from the sea over the past 20 million years or so also exhumed vast tracts of highly weatherable ocean crust, exposing it all to the withering assault of tropical rainstorms. Today this corroding rock accounts for roughly 10 percent of the planet’s carbon sink. Over tens of millions of years, then, the stately march of plate tectonics—the balance of volcanic CO2 and rock weathering—seems to have driven long-term climate change, in our case toward a colder, lower-CO2 world. As we’ll see, humans now threaten to undo this entire epic, geologic-scale climate evolution of the Cenozoic era—and in only a few decades.

When Earth’s blanket of CO2 was finally thin enough, the planet’s regular wobbles were at long last sufficient to trigger deep glaciations. The ice ages began. But the climate was not stable during this period. The ice advanced and retreated, and while the descent into the wild episodes of the Pleistocene epoch could be leisurely—the depths of planetary winter taking tens of thousands of years to arrive—the leap out of the cold tended to be sudden and violent. This is where positive feedback loops come in: When the last ice age ended, it ended fast.

Glacial ice near the Torfajökull volcano, in Iceland

Coral reefs marking the ancient sea level—but today lying deep off the coasts of Tahiti and Indonesia—reveal that about 14,500 years ago, the seas suddenly jumped 50 feet or so in only a few centuries, as meltwater from the late, great North American ice sheet raged down the Mississippi. When a 300-foot-deep lake of glacial meltwater spanning at least 80,000 square miles of central Canada catastrophically drained into the ocean, it shut down the churn of the North Atlantic and arrested the seaborne flow of heat northward. As a result, tundra advanced to retake much of Europe for 1,000 years. But when ocean circulation kicked back into gear, and the dense, salty seawater began to sink again, the system rebooted, and currents carried the equator’s heat toward the Arctic once more. Temperatures in Greenland suddenly leaped 10 degrees Celsius in perhaps a decade, fires spread, and revanchist forests reclaimed Europe for good.

In Idaho, ice dams that had held back giant lakes of glacial meltwater about six times the volume of Lake Erie collapsed as the world warmed, and each released 10 times the flow of all the rivers on Earth into eastern Washington. The floods carried 30-foot boulders on biblical waves, through what were suddenly the world’s wildest rapids. They left behind a labyrinth of bedrock-scoured canyons that still covers the entire southeastern corner of the state like a scar. When the Earth’s climate changes, this is what it can look like on the ground.

As the ice sheets of the Northern Hemisphere finally lost their grip, darker land around the melting margins became exposed to the sun for the first time in 100,000 years, accelerating the ice’s retreat. Permafrost melted, and methane bubbled up from thawing bogs. Colder, more CO2-soluble oceans warmed, and gave up the carbon they’d stolen in the Ice Age, warming the Earth even more. Relieved of their glacial burden, volcanoes in Iceland, Europe, and California awoke, adding even more CO2 to the atmosphere.

Soon the Sahara would green again, Jericho would be born, and humans would start writing things down. They would do so with the assumption that the world they saw was the way it had always been. “We were born only yesterday and know nothing,” one of them would write. “And our days on earth are but a shadow.”

As we leap back in time again, we emerge before the final Pleistocene glaciation. We’ve gone tremendously far back, 129,000 years, though in some ways we’ve only returned to our own world. This was the most recent interglacial period, the last of many breaks between the ice ages, and the last time the planet was roughly as warm as it is today. Once more, the seas have risen hundreds of feet, but something is awry.

As the Earth’s wobble and orbit conspired to melt more ice than the poles have shed so far today, the planet absorbed more sunlight. As a result, global temperatures were little more than 1 degree warmer than today’s Anthropocene chart-toppers—or maybe even the same. But sea level was 20 to 30 feet higher than it is now. (A full third of Florida was sunk beneath the waves.) This is “sobering,” as one paper put it.

The Dallol sulfur springs in the Danakil Depression, Ethiopia, one of the hottest places on Earth

Modelers have tried and mostly failed to square how a world about as warm as today’s could produce seas so strangely high. Provisional, if nightmarish, explanations like the runaway, catastrophic collapse of monstrous ice cliffs more than 300 feet tall in Antarctica, which may or may not be set into motion in our own time, Very soon, we may well have warmed the planet enough to trigger similarly dramatic sea-level rise, even if it takes centuries to play out. This is what the Exxon scientist James Black meant in 1977 when he warned higher-ups of the coming “super-interglacial” that would be brought about—as a matter of simple atmospheric physics—from burning fossil fuels. But our trajectory as a civilization is headed well beyond the warmth of the last interglacial, or any other interglacial period of the Pleistocene, for that matter. So it’s time to keep moving. We must take our first truly heroic leap into geologic time, millions of years into the past.

We’re more than 3 million years in the past now, and carbon dioxide in the atmosphere is at 400 parts per million, a level the planet will not again see until September 2016. This world is 3 to 4 degrees Celsius warmer than ours, and the sea level is up to 80 feet higher. Stunted beech trees and bogs line the foothills of the Transantarctic Mountains not far from the South Pole—the last members of a venerable line of once-majestic forests that had existed since long before the age of the dinosaurs.

What we’ve glossed over in our journey back to this ancient present: the entire evolutionary history of Homo sapiens, three Yellowstone super-eruptions, thousands of megafloods, the last of the giant terror birds, a mass extinction of whales, and the glacial creation and destruction of innumerable islands and moraines. As we make our way backwards in time to the Pliocene, the glaciations get briefer, and the ice sheets themselves become thinner and more temperamental. About 2.6 million years ago they all but disappear in North America, as CO2 levels continue their slow climb.

When we arrive in the middle of the Pliocene, just over 3 million years ago, CO2 levels are high enough that we’ve escaped the cycle of ice ages and warm interglacials altogether. Lucy the Australopithecus roams a heavily forested East Africa. We are now outside the evolutionary envelope of our modern world, sculpted as it was by the temperamental northern ice sheets and deep freezes of the Pleistocene. But as to atmospheric carbon dioxide, 3 million years is how far back we have to go to arrive at an analogue for 2021.

Despite the similarities between our world and that of the Pliocene, the differences are notable. In the Canadian High Arctic—where today tundra spreads to the horizon—evergreen forests come right to the edge of an ice-free Arctic Ocean. Though the world as a whole is only a few degrees warmer, the Arctic, as always, gets the brunt of the extra heat. This is called “polar amplification,” and it’s why maps of modern warming are crowned by a disturbing fog of maroon. Models struggle to reproduce the extreme level of warming in the Pliocene Arctic. It’s a full 10 to 15 degrees Celsius warmer in the long twilight of northern Canada, and the pine and birch woodlands of these Arctic shores are filled with gigantic forest-dwelling camels. Occasionally this boreal world erupts in wildfire, a phenomenon echoed by the blazes that today sweep ever farther north. Elsewhere, West Antarctica’s ice sheet may have disappeared entirely, and Greenland’s, if it exists at all, is shriveled and pathetic.

A common projection for our own warming world is that, while the wet places will get wetter, the dry places will get drier. But the Pliocene seems to defy this saw for reasons not yet fully understood. It’s a strangely wet world, especially the subtropics, where—in the Sahara, the Outback, the Atacama, the American Southwest, and Namibia—lakes, savannas, and woodlands replace deserts. This ancient wetness might come down to inadequacies in how we model clouds, which are under no obligation to behave in physical reality as they do in simplified lines of computer code. Hurricanes were almost certainly more consistently punishing 3 million years ago, just as our storms of the future will be. And a more sluggish circulation of the atmosphere might have lulled the trade winds, turning El Niño into “El Padre.” Perhaps this is what brought rains—and lakes—to the Mojave at this time.

Angeles National Forest, California

Our modern coastlines would have been so far underwater that you’d have to take great pains to avoid getting the bends if you tried scuba diving down to them. Today, traveling east through Virginia, or North or South Carolina, or Georgia, midway through your drive you’ll pass over a gentle 100-foot drop. This is the Orangeburg Scarp, a bluff—hundreds of miles long—that divides the broad, flat coastal plain of the American Southeast. It comprises the eroded and smoothed-out rumors of once-magnificent sea cliffs. Here, waves of the Pliocene high seas chewed away at the middle of the Carolinas—an East Coast Big Sur. This ancient shoreline is visible from space by the change in soil color that divides the states, and is visible on much closer inspection as well: To the east of this strange drop-off, giant megalodon-shark teeth and whale bones litter the Carolina Low Country. Though warped over the ages by the secret workings of the mantle far below, these subtle banks 90 miles inland nevertheless mark the highest shoreline of the Pliocene, when the seas were dozens of feet higher than they are today. But even within this warm Pliocene period, the sea level leaped and fell by as much as 60 feet every 20,000 years, to the rhythm of the Earth’s sway in space. This is because, under this higher-CO2 regime, the unstable ice sheet in Antarctica took on the volatile temperament that, 1 million years later, would come to characterize North America’s ice sheet, toying with the ancient coastline as if it were a marionette.

So this is the Pliocene, the world of the distant present. While today’s projections of future warming tend to end in 2100, the Pliocene illuminates just what sort of long-term changes might inevitably be set in motion by the atmosphere we’ve already engineered. As the great ice sheets melt, the permafrost awakens, and darker forested land encroaches on the world’s tundra, positive feedbacks may eventually launch our planet into a different state altogether, one that might resemble this bygone world. Nevertheless, human civilization is unlikely to keep atmospheric CO2 at a Pliocene level—so more ancient and extreme analogues must be retrieved.

We’re now deeper in the past, and the planet appears truly exotic. The Amazon is running backwards, and gathers in great pools at the foot of the Andes. A seaway stretches from Western Europe to Kazakhstan and spills into the Indian Ocean. California’s Central Valley is open ocean.

What today is the northwestern U.S. is especially unrecognizable. Today the airy, columnated canyons of the Columbia River on the Oregon-Washington border swarm with tiny kiteboarders zipping through gorges of basalt. But 16 million years ago, this was a black, unbreathable place, flowing with rivers of incandescent rock. The Columbia River basalts—old lava flows that spread across Washington, Oregon, and Idaho, in some places more than two miles thick—were the creation of a class of extremely rare and world-changing volcanic eruptions known as large igneous provinces, or LIPs.

Some LIPs in Earth’s history span millions of square miles, erupt for millions of years, inject tens of thousands of gigatons of CO2 into the air, and are responsible for most of the worst mass extinctions in the history of the planet. They live up to their name—they are large. But these mid-Miocene eruptions were still rather small as far as LIPs go, and so the planet was spared mass death. Nevertheless, the billowing volcanoes raised atmospheric CO2 up to about 500 ppm, a level that today represents something close to the most ambitious and optimistic scenario possible for limiting our future carbon emissions.

In the Miocene, this volcanic CO2 warmed up the world to at least 4 degrees Celsius and perhaps as much as 8 degrees above modern temperatures. As a result, there were turtles and parrots in Siberia. Canada’s Devon Island, in the high Arctic, is today a desolate wasteland, the largest uninhabited island in the world—and one used by NASA to simulate life on Mars. In the Miocene, its flora resembled Lower Michigan’s.

The sweeping grasslands distinctive to our cooler, drier, low-CO2 world had yet to take over the planet, and so forests were everywhere—in the middle of Australia and Central Asia and Patagonia. All of this vegetation was one of the reasons it was so warm. Forests and shrubs made this planet darker than our own world—one still painted pallid hues in many places by bare land and ice—and allowed it to absorb more heat. This change in the planet’s color is just one of the many long-term feedback loops awaiting us after the ice melts. Long after our initial pulse of CO2, they will make our future world warmer and more alien still.

As for fauna, we’re now so distantly marooned in time from our own world that most of the creatures that inhabited this leafy planet range from the flatly unfamiliar to the uncannily so. There were big cats that weren’t cats, and rhino-size “hell pigs” that weren’t pigs. There were sloths that lived in the ocean and walruses that weren’t related to today’s walruses. Earth’s largest-ever meat-eating land mammals, African juggernauts like Megistotherium and Simbakubwa, not closely related to any living mammals today, tore early elephants apart with bladed mouths.

And with CO2 at 500 ppm, the sea level was about 150 feet higher than today. Approaching Antarctica in the middle Miocene by sea, the waters would be warmer than today, and virtually unvisited by ice. To get to the ice sheet, you’d have to hike far past lakes and forests of conifers that lined the coast. Trudging past the trees and finally over endless tundra, you would come at last to the edge of a much smaller ice sheet whose best days were still ahead of it. An axiom about this land-based Antarctic ice sheet in paleoclimatology is that it’s incredibly stubborn. That is, once you have an ice sheet atop the heart of Antarctica, feedback loops kick in to make it exceedingly hard to get rid of. Barring true climatic madness, a land-based Antarctic ice sheet is essentially there to stay.

Clouds in Death Valley, California

But in the middle Miocene this young Antarctic ice sheet seemed to have a temper. It might have been “surprisingly dynamic,” as one paper cheerfully puts it. As CO2 increased from just below today’s levels up to about 500 ppm, Miocene Antarctica shed what today would amount to 30 to 80 percent of the modern ice sheet. In the Miocene, Antarctica seemed exquisitely tuned to small changes in atmospheric CO2, in ways that we don’t fully understand and that we’re not incorporating into our models of the future. There will undoubtedly be surprises awaiting us in our high-CO2 future, just as there were for life that existed in the Miocene. In fact, the Antarctic ice sheet may be more vulnerable today to rapid retreat and disintegration than at any time in its entire 34-million-year history.

In the 16 million years since this mid-Miocene heat, the volcanic hot spot responsible for the Columbia River basalts has wandered under Yellowstone. Today it powers a much tamer kind of volcano. It could cover a few states in a few inches of ash and disrupt global agriculture for years, but it couldn’t launch the planet into a new climate for hundreds of thousands of years, or kill most life on the surface. Unfortunately, there is such a supervolcano active on Earth today: industrial civilization. With CO2 likely to soar past 500 ppm from future emissions, even the sweat-soaked, Siberian-parrot-populated world of the middle Miocene might not tell us everything we need to know about our future climate. It’s time to go back to a global greenhouse climate that ranks among the warmest climate regimes complex life has ever endured. In our final leap backwards, CO2 at last reaches levels that humans might reproduce in the next 100 years or so. What follows is something like a worst-case scenario for future carbon emissions. But these worst-case projections have continued to prove stubbornly accurate in the 21st century so far, and they remain a possible path for our future.

We’re now about to take our largest leap, by far, into the geologic past. We hurdle over 40 million years of history, past volcanic eruptions thousands of times bigger than that of Mount St. Helens, past an asteroid impact that punched out a gigantic crater where the Chesapeake Bay sits today. The Himalayas slump India unhitches from Asia and the further back we go, the higher the CO2 level rises and the warmer the Earth gets. The Antarctic ice sheet, in its death throes, vanishes altogether, and the polar continent instead gives way to monkey puzzle trees and marsupials. We have arrived, finally at the end of our journey, in the greenhouse world of the early age of mammals.

Today the last dry land one steps on in Canada before setting out across the ice-choked seas for the North Pole is Ellesmere Island, at the top of the world. But once upon a time there was a rainforest here. We know this because tree stumps still erode out of the barren hillsides, and they’re more than 50 million years old. They’re all that’s left of an ancient polar jungle now whipped by indifferent Arctic winds. But once upon a time, this island was a swampy cathedral of redwoods, whose canopy naves were filled with flying lemurs, giant salamanders, and hippolike beasts that pierced the waters. At this polar latitude, on some late-fall evening of the early Eocene, the sun tried and failed to lift itself from the horizon. A pink twilight reached deep into the jungle, but soon the sun would set entirely here for more than four months. In this unending Arctic dark, the stillness would be broken by the orphaned calls of tiny early primates, who hopped fearlessly over stilled alligators that would start moving again when the sun returned from beyond the horizon. In this unending night, tapirs hunted for mushrooms and munched on leaf litter that was left over from sunny days past and that in the far future would become coal.

We have no modern analogue for a swampy rainforest teeming with reptiles that nevertheless endures months of Arctic twilight and polar night. But for each degree Celsius the planet warms, the atmosphere holds about 6 percent more water vapor, and given that global temperatures at the beginning of the age of mammals were roughly 13 degrees warmer than today, it’s difficult to imagine how uncomfortable this planet would be for Ice Age creatures like ourselves. In fact, much of the planet would be rendered off-limits to us, far too hot and humid for human physiology.

Not only was this a sweltering age, but it was also one cruelly punctuated by some of the most profound and sudden CO2-driven global-warming events in geologic history—on top of this already feverish baseline. Deep under the North Atlantic, the Eocene epoch kicked off in style 56 million years ago with massive sheets of magma that spread sideways through the crust, igniting vast, diffuse deposits of fossil fuels at the bottom of the ocean. This ignition of the underworld injected something like the carbon equivalent of all currently known fossil-fuel reserves into the seas and atmosphere in less than 20,000 years, warming the planet by another 5 to 9 degrees Celsius. Evidence abounds of violent storms and megafloods during this ancient spasm of climate change—episodic waves of torrential rains unlike any on Earth today. In some places, such storms would have been routine, separated by merciless droughts and long, brutal, cloudless heat waves. Seas near the equator may have been almost as hot as a Jacuzzi—too hot for most complex life. As for the rest of the planet, all of this excess CO2 acidified the oceans, and the world’s coral reefs collapsed. Ocean chemistry took 200,000 years to recover.

The most jarring thing about the early age of mammals, though, isn’t merely the extreme heat. It’s the testimony of the plants. In higher-CO2 conditions, plants reduce the number of pores on their leaves, and fossil leaves from the jungles of the early Eocene have tellingly fewer pores than today’s. By some estimates, CO2 50 million years ago was about 600 ppm. Other proxies point to higher CO2, just over 1,000 ppm, but even that amount has long bedeviled our computer models of climate change. For years, in fact, models have told us that to reproduce this feverish world, we’d need to ramp up CO2 to more than 4,000 ppm.

This ancient planet is far more extreme than anything being predicted for the end of the century by the United Nations or anyone else. After all, the world that hosted the rainforests of Ellesmere Island was 13 degrees Celsius warmer than our own, while the current global ambition, enshrined in the Paris Agreement, is to limit warming to less than 2, or even 1.5, degrees. Part of what explains this glaring disparity is that most climate projections end at the end of the century. Feedbacks that might get you to Eocene- or Miocene-level warmth play out over much longer timescales than a century. But the other, much scarier insight that Earth’s history is very starkly telling us is that we have been missing something crucial in the models we use to predict the future.

Mount Ruapehu and Mount Ngauruhoe volcanoes, in New Zealand

Some of the models are starting to catch up. In 2019, one of the most computationally demanding climate models ever run, by researchers at the California Institute of Technology, simulated global temperatures suddenly leaping 12 degrees Celsius by the next century if atmospheric CO2 reached 1,200 ppm—a very bad, but not impossible, emissions pathway. And later that year, scientists from the University of Michigan and the University of Arizona were similarly able to reproduce the warmth of the Eocene by using a more sophisticated model of how water behaves at the smallest scales.

The paleoclimatologist Jessica Tierney thinks the key may be the clouds. Today, the San Francisco fog reliably rolls in, stranding bridge towers high above the marine layer like birthday candles. These clouds are a mainstay of west coasts around the world, reflecting sunlight back to space from coastal California and Peru and Namibia. But under higher-CO2 conditions and higher temperatures, water droplets in incipient clouds could get bigger and rain down faster. In the Eocene, this might have caused these clouds to fall apart and disappear—inviting more solar energy to reach, and warm, the oceans. That might be why the Eocene was so outrageously hot.

This sauna of our early mammalian ancestors represents something close to the worst possible scenario for future warming (although some studies claim that humans, under truly nihilistic emissions scenarios, could make the planet even warmer). The good news is the inertia of the Earth’s climate system is such that we still have time to rapidly reverse course, heading off an encore of this world, or that of the Miocene, or even the Pliocene, in the coming decades. All it will require is instantaneously halting the super-eruption of CO2 disgorged into the atmosphere that began with the Industrial Revolution.

We know how to do this, and we cannot underplay the urgency. The fact is that none of these ancient periods is actually an apt analogue for the future if things go wrong. It took millions of years to produce the climates of the Miocene or the Eocene, and the rate of change right now is almost unprecedented in the history of animal life.

Humans are currently injecting CO2 into the air 10 times faster than even during the most extreme periods within the age of mammals. And you don’t need the planet to get as hot as it was in the early Eocene to catastrophically acidify the oceans. Acidification is all about the rate of CO2 emissions, and we are off the charts. Ocean acidification could reach the same level it did 56 million years ago by later this century, and then keep going.

When he coined the term mass extinction in a 1963 paper, “Crises in the History of Life,” the American paleontologist Norman Newell posited that this was what happened when the environment changed faster than evolution could accommodate. Life has speed limits. And in fact, life today is still trying to catch up with the thaw-out of the last ice age, about 12,000 years ago. Meanwhile, our familiar seasons are growing ever more strange: Flycatchers arrive weeks after their caterpillar prey hatches orchids bloom when there are no bees willing to pollinate them. The early melting of sea ice has driven polar bears ashore, shifting their diet from seals to goose eggs. And that’s after just 1 degree of warming.

Subtropical life may have been happy in a warmer Eocene Arctic, but there’s no reason to think such an intimately adapted ecosystem, evolved on a greenhouse planet over millions of years, could be reestablished in a few centuries or millennia. Drown the Florida Everglades, and its crocodilians wouldn’t have an easy time moving north into their old Miocene stomping grounds in New Jersey, much less migrating all the way to the unspoiled Arctic bayous if humans re-create the world of the Eocene. They will run into the levees and fortifications of drowning Florida exurbs. We are imposing a rate of change on the planet that has almost never happened before in geologic history, while largely preventing life on Earth from adjusting to that change.

Taking in the whole sweep of Earth’s history, now we see how unnatural, nightmarish, and profound our current experiment on the planet really is. A small population of our particular species of primate has, in only a few decades, unlocked a massive reservoir of old carbon slumbering in the Earth, gathering since the dawn of life, and set off on a global immolation of Earth’s history to power the modern world. As a result, up to half of the tropical coral reefs on Earth have died, 10 trillion tons of ice have melted, the ocean has grown 30 percent more acidic, and global temperatures have spiked. If we keep going down this path for a geologic nanosecond longer, who knows what will happen? The next few fleeting moments are ours, but they will echo for hundreds of thousands, even millions, of years. This is one of the most important times to be alive in the history of life.

This article appears in the March 2021 print edition with the headline “The Dark Secrets of the Earth’s Deep Past.”

End of the Sun

Any of the devastating scenarios above, while undoubtedly terrible for life, are just a fraction as bad as future Earth's ultimate fate. Gamma-ray burst or not, in about a billion years, most life on Earth will eventually die anyway due to a lack of oxygen. That’s according to a different study published in March in the journal Nature Geoscience .

The researchers suggest that our oxygen-rich atmosphere is not a permanent feature of the planet. Instead, in about a billion years, solar activity will cause atmospheric oxygen to plummet back down to the level it was at before the Great Oxidation Event. To determine this, the authors combined climate models and biogeochemistry models to simulate what will happen to the atmosphere as the Sun ages and puts out more energy.

They found that, eventually, Earth reaches a point where atmospheric carbon dioxide breaks down. At that point, oxygen-producing plants and organisms that rely on photosynthesis will die out. Our planet won’t have enough lifeforms to sustain the oxygen-rich atmosphere humans and other animals require.

The precise timing of when that starts and how long it takes — the deoxygenation process could take as few as 10,000 years — depends on a broad range of factors. But, in the end, the authors say this cataclysm is an unavoidable one for the planet.

Luckily, humanity still has another billion years to figure out other plans.

Fold Mountain

Fold mountains are created where two of Earth&rsquos tectonic plates are pushed together.

Earth Science, Geology, Geography, Physical Geography

Fold mountains are created where two or more of Earth&rsquos tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges.

Fold mountains are created through a process called orogeny. An orogenic event takes millions of years to create a fold mountain, but you can mimic it in seconds. Cover a table with a tablecloth, or place a rug flat on the floor. Now push the edge of the tablecloth or rug&mdashwrinkles will develop and fold on top of each other.

The vocabulary of fold mountains owes something to this simple tablecloth experiment. Some of the key structures in fold mountains are nappes. Nappes are common, dramatic folded rocks or rock formations. &ldquoNappe&rdquo is French for &ldquotablecloth&rdquo and it is believed the formations were named after the tabletop experiment.

The huge difference between the rock folds and cloth folds is that in the tabletop experiment, the table itself does not fold. In the creation of fold mountains, Earth&rsquos crust itself is warped into folded forms.

Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation.

At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation. For this reason, most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This can make them more susceptible to folding and warping. Most fold mountains are composed primarily of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present.

Young and Old, High and Low

Fold mountains are the most common type of mountain in the world. The rugged, soaring heights of the Himalayas, Andes, and Alps are all active fold mountains.

The Himalayas stretch through the borders of China, Bhutan, Nepal, India, and Pakistan. The crust beneath the Himalaya, the most towering mountain range on Earth, is still the process of being compressed. Here, the Indian plate is colliding northward with the Eurasian plate. The sedimentary rocks of the Himalayas include shale and limestone. Metamorphic rocks of the region include schist and gneiss. Dikes of igneous rock also intrude throughout the rock formations of the Himalayas.

The Andes are the world&rsquos longest mountain chain. They stretch along the entire west coast of South America, from Colombia in the north and through Ecuador, Peru, Bolivia, Chile, and Argentina to the south. Here, the dense oceanic crust of the Nazca plate is subducting beneath the less-dense continental crust of the South American plate. The Andes are mostly being folded and uplifted from the thicker, less-dense rocks of the South American plate. The sedimentary and metamorphic rocks of the Andes are dotted by active and dormant volcanoes.

The Alps roughly mark the top of the &ldquoboot&rdquo of the Italian Peninsula. The Alps stretch across Italy, Slovenia, Austria, Germany, Switzerland, Lichtenstein, Monaco, and France. Here, the tiny Adriatic microplate is colliding with the much larger Eurasian plate to the north. The J-shaped Adriatic microplate is a remnant of the African plate to the south, and today it carries the eastern Italian Peninsula as well as the entire Adriatic Sea. Alpine geology includes sedimentary and metamorphic rock, as well as igneous rocks that once were part of the ocean floor and were later uplifted in the process of folding.

Not all fold mountains are soaring peaks. The Appalachians, stretching along North America&rsquos east coast, are generally low-lying, gentle slopes. Millions of years ago, the Appalachians were taller than the Himalayas! Millions of years of erosion, however, have taken their toll. Today, some of the highest peaks of the Appalachians are less than a third of the height of Everest.

The crust that is now the Appalachians began folding over 300 million years ago, when the North American and African continental plates collided. Plate tectonics created this ancient mountain range, then called the Central Pangean Mountains . . . and plate tectonics tore it apart. As tectonic activity ripped apart the ancient supercontinent Pangea, the African, Eurasian, and North American plates drifted apart.

The Appalachians are just one remnant of the Central Pangean Mountains. The Appalachians stretch from the province of Newfoundland, in southeastern Canada, through the southern state of Alabama in the United States. They are related to the gentle fold mountains of the Scottish Highlands (Eurasia) and the Little Atlas Mountains in Morocco (Africa)&mdashtheir orogenic sisters from the Central Pangean Mountains.

Types of Folds

Fold mountains are defined by complex, vital geologic forms known as folds. There are many, many different types of folds. Geologists primarily categorize folds by their shape&mdashdo they have sharp turns or gentle curves? Are the folds convex or concave?

A fold mountain usually displays more than one type of fold. Anticlines and synclines are the most common up-and-down folds that result from compression. An anticline has a &cap-shape, with the oldest rocks in the center of the fold. A syncline is a U-shape, with the youngest rocks in the center of the fold.

Domes and basins are often considered types of folds. A dome is a series of symmetrical anticlines, roughly shaped like half a sphere. Like an anticline, the oldest rocks in a dome are found in the center. A basin is a depression, or dip, in Earth&rsquos surface. Like a syncline, a basin has its youngest rocks in its center.

Fossil Site May Capture the Dinosaur-Killing Impact, but It’s Only the Beginning of the Story

It may be considered one of the worst days in the history of life on Earth. Sixty-six million years ago, an immense asteroid smacked into what is now the Yucatán Peninsula of Mexico, triggering global devastation and the world’s fifth mass extinction. The non-avian dinosaurs, pterosaurs and coil-shelled squid cousins called ammonites disappeared completely. Even groups that survived, like mammals and lizards, suffered dramatic die-offs in the aftermath. Who perished, and who survived, set the stage for the next 66 million years—including our own origin 300,000 years ago.

The Chicxulub impact was a catastrophic transition into a new world. The distinctive rock layer it left behind, spiked with an element called iridium often found in asteroids and meteorites, marks the end of the Cretaceous period and the beginning of the Paleogene, known by experts as the K/Pg boundary. This line in the stone is also the marker for the end of the Age of Dinosaurs and the beginning of the Age of Mammals, a shift that has been intensely debated and studied for decades. Now a fossil site in North Dakota is causing a new stir, said to document the last minutes and hours of the dinosaurian reign.

The fossil assemblage, nicknamed Tanis after the real-life ancient Egyptian city referenced in Raiders of the Lost Ark, was first described in an article the New Yorker. Excavated and studied by University of Kansas graduate student Robert DePalma and a team of international collaborators, the site contains glassy spherules of material believed to have come from the impact event, thousands of miles away. Also embedded in the rock and debris, the New Yorker reported, are delicately preserved fossil fish, marine organisms far from the nearest sea, ancient plants, prehistoric mammals, and, perhaps most significantly, dinosaur bones, eggs and even feathers.

Many paleontologists were quick to raise an eyebrow at the findings presented in the New Yorker, however, particularly because some of the claims in the article are not mentioned in a scientific paper about the site. That research, published by DePalma and colleagues, was released Monday in the Proceedings of the National Academy of Science. The only dinosaur fossil mentioned in the paper is a weathered hip fragment, but the study is nevertheless causing a stir as a window into the extreme effects caused by the asteroid impact.

Mass of articulated fish from the Tanis inundation surge deposit. (Robert DePalma / University of Kansas)

“Unfortunately, many interesting aspects of this study appear only in the New Yorker article and not in the scientific paper,” says Kirk Johnson, director of the Smithsonian’s National Museum of Natural History. “This is a sloppy way to conduct science and it leaves open many questions. At the present moment, interesting data are presented in the paper while other elements of the story that could be data are, for the moment, only rumors.”

As for the paper itself, the details are part of a broader picture of what transpired 66 million years ago in western North America, along the margins of a vanishing seaway that was draining off the continent at the time. According to DePalma and colleagues, seismic waves emanating from the asteroid impact reached the Tanis area within minutes. The disturbance sloshed local bodies of water in a phenomenon called a seiche—similar to water flowing back and forth in a bathtub—tossing fish and other organisms around in the wave. “As far as we can tell,” DePalma says in an email, “the majority of the articulated carcasses are from animals that were either killed when they were encapsulated by the muddy sediment, or very shortly prior as part of the same violent inundation surge event.”

In addition to articulated fish fossils with their scales still in place, the site contains shell fragments from seagoing mollusks called ammonites. DePalma and colleagues suspect that their presence is a sign that a previously unrecognized pocket of the Western Interior Seaway provided the water that ripped over the land and buried the Tanis site.

Sites demarcating the K/Pg boundary have been found all over the world, and vertebrate fossils at or within the boundary have also been discovered before. Part of what makes the Tanis site stand out, DePalma says, is that “this is the first known example of articulated carcasses, likely killed as a direct result of the impact, associated with the boundary.”

Despite the controversy over how claims of the site hit mass media before the peer-reviewed science paper was available, outside experts note that Tanis truly does seem to be an exceptional spot. “This isn’t the only site that preserves fossils at the K/Pg boundary, but it seems this might be the most sensational one ever discovered,” says Shaena Montanari, a paleontologist and AAAS science and technology policy fellow. The fossil preservation of the fish in particular stands out as unusual. “I thumbed through the pictures of the fossils included in the supplement and they look absolutely incredible,” Montanari says. Some of these fish have debris from the impact preserved in their gills, little pebbles of natural glass, perhaps sucked up from the water as the particles landed in ancient North Dakota shortly after the impact.

Tiny spherules thought to have been ejected from the Chicxulub impact and deposited at the Tanis site in North Dakota. (Robert DePalma / University of Kansas)

Much of what makes Tanis exciting, according to University of New Mexico postdoctoral fellow James Witts, is that it offers a range of geologic clues about what happened after the impact. “This study convincingly links evidence from impact ejecta, sedimentology and geochemistry with well-dated physical remains of animals and plants that appear to have been alive right at the time of the impact event.” It could be a snapshot of life not thousands or hundreds of years before, but during the cataclysm that shook the Earth.

How Tanis was created is also something of a novelty. Geologists have studied disturbances that the Chicxulub impact caused at other sites, but these spots represent what happened in the ancient ocean and not on land. If DePalma and colleagues are correct, then seiche waves washing over terrestrial environments is another effect of the impact that hasn’t been examined before, depositing the remains of sea creatures where they otherwise had no business.

A number of additional mysteries remain about the site as well. The marine fossils, for example, might not have come from a nearby remnant of sea but could have been fossils when the asteroid struck, ripped up by the seismic and seiche waves that buried Tanis. “It has to remain an open question as to whether the ammonites were reworked out of rocks that would have essentially been the bedrock at Tanis, or [if] they come from a population that lived in a reduced seaway to the east of Tanis that we have no record of because of later erosion,” Witts says.

Other geologic details of the site also merit further investigation. “It seems like the geochemical data are scant and in some cases being stretched a bit to make interpretations,” Montanari says, “although this is not a new thing for paleontology.” These data points can be used to measure when and how quickly the Tanis site formed, critical details when attempting to determine what the site actually records. Montanari says that additional data points and analysis would strengthen the case that Tanis represents a very short window of the last Cretaceous moments. “We need to be sure we are developing rigorous hypotheses and then testing them with the available evidence rather than trying to craft a scenario that fits exactly what is uncovered,” Montanari says.

Robert DePalma points to the K/Pg boundary impact fallout layer. (Robert DePalma / University of Kansas)

University of California, Berkeley paleontologist Pat Holroyd says that the estimations of when and how quickly the Tanis site formed are based on models without consideration of other possible interpretations. “I don't think there is any way to conclusively determine the exact amount of time represented in the site,” she says, “but it would have been useful to see how they estimated it.”

The details of what the site actually looks like, and how the layers were deposited, is not clear from what was published in the paper, Holroyd says. Such data is needed to compare Tanis to other K/Pg sites around the world. “Higher resolution images of the entire section would be of interest to many people as a resource for comparison to other types of deposits thought to be produced by seismic waves,” Holroyd says.

For now, Tanis is a localized phenomenon. It’s relevance to other sites in North America, and around the globe, awaits further study. “Seismic shaking from the impact could potentially have caused surges in other pockets far from the impact site, affecting that tapestry of microecologies as well,” DePalma says.

The site is also unique in that it appears to capture a small moment of geologic time. “It’s very tricky interpreting any rock outcrop as recording and preserving events operating on such a short timescale,” Witts says. The study does seem to show a rapid, violent event, but the details of the site will undoubtedly be further investigated and tested to see if the extraordinary claims hold up to scrutiny.

Witts hopes that the paper will help spur further discussion and analysis of other K/Pg sites around the globe. While geology is often thought of in terms of slow, gradual change, sometimes rapid transformation occurs. “I think Tanis reminds us geologists that sometimes it looks like the depositional stars align, and remarkable events could leave a signature preserved in the rock and fossil record,” he says.

Ultimately Tanis will be another part of a much broader story. The extinction at the end of the Cretaceous was a global event that played out over the course of days, weeks, months and years. Despite the fact that the site has been heralded as recording “the day the dinosaurs died,” there’s no way to know when the very last non-avian dinosaur went extinct. The last terrible lizard likely fell long after the events recorded at Tanis, likely in another part of the world.

DePalma says there is more to come from the Tanis site, and the mismatch between the claims made in the New Yorker article and the PNAS paper comes down to “triage” of what papers get priority. “We are already working on multiple follow-up papers and will be fully examining and reporting on everything found thus far,” he says.

The discussion about what Tanis means is only just beginning. “I’m sure paleontologists will be eager to see this material and do additional studies on Tanis,” Montanari says. “I can’t wait to see the rest of what’s to come.”

About Riley Black

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.

A few stupid mistakes causes New York City's 1977 blackout

In 1977, New York City lost all power for 25 hours. The consequences were devastating. American Experience (via PRI) says around 800,000 people were stranded in the subways and elevators, while others set to looting and pillaging on a medieval scale. There were about a thousand fires set, more than 1,500 businesses were looted, and by the time the lights came back on, there were damages up to about a billion dollars. Some have credited the blackout as the catalyst that sparked the hip-hop movement, which is a nice bonus.

And it all happened because someone didn't know what buttons to push.

Schneider Electric looked at just what happened on that hot summer night in '77, and it started with a few lightning strikes. That's not uncommon, they say, and most substations are prepared for it. This one — run by Con Ed — wasn't. After lightning tripped the breakers, Con Ed tried to restart the station's generators. The problem? No one was there. When employees got there and started running system-wide procedures to get everything back up and running, they ran the wrong procedures. Instead of dumping the necessary 1,500 megawatts of load, they ran one that got rid of only a few hundred. The station shut down, and the Big Apple went dark.

The Pre-flood Atmosphere

There is evidence that the atmosphere enveloping the early earth was very different than it is today. It seems that at one time the entire earth enjoyed a warm tropical environment and there was enhanced oxygen in the atmosphere. Organisms would have grown larger than their modern counterparts and could also have lived longer. For example, massive fossilized trees in the Florissant Fossil Beds National Monument of Colorado appear to be much older than their tree rings would indicate they actually were (Oard, Michael, “The Florissant Redwood Trees Deposited from a Flood Log Mat,” Journal of Creation, 2019, p. 91.)

Many creationists have attributed this special primeval atmosphere to a water vapor canopy that was created by God on the second day, the “waters above the firmament” (Genesis 1:7). This theory holds that a “vast blanket of invisible water vapor, translucent to the light of the stars but productive of a marvelous greenhouse effect which maintained mild temperatures from pole to pole, thus preventing air-mass circulation and the resultant rainfall (Genesis 2:5). It would certainly have had the further effect of efficiently filtering harmful radiation from space, markedly reducing the rate of somatic mutations in living cells, and, as a consequence, drastically decreasing the rate of aging and death.” (Morris, Henry, Scientific Creationism, 1984, p. 211.) Citing evidence of denser atmosphere in the past, Morris postulated that this vapor layer could have dramatically increased the atmospheric pressure on the surface of the early earth, again contributing to a healthier environment (like a natural hyperbaric chamber). Later the canopy would have collapsed in the form of rain (the “windows of heaven” in Genesis 7:11), contributing to the Flood water, and resulting in the dramatic drop-off in longevity after the deluge.
Genesis 9 tells how Noah planted a vineyard after the flood and became drunk from the fruit of it. This is an aberration in the life of this godly man. Some have suggested that Noah did not know his grape juice would ferment so quickly or so extensively in the post-flood atmosphere. Or perhaps the reduced atmospheric pressure made it harder for him to “hold his drink.” While this is only speculation, the removal of the vapor canopy could help explain this curious situation.

Some creationists emphasize other factors that may have caused the worldwide temperate conditions that existed before the Flood. They stress the evidence of far greater concentrations of carbon dioxide levels in the past and point out that the earth’s magnetic field was far stronger than today. This could have acted as the shield for cosmic radiation and produced the healthier environment. (Humphreys, Russel D., Starlight and Time, 1995, p. 63.) Creationist John Baumgardner suggested that the atmosphere surrounding the original earth was far thicker than it is today and that the exploding of the fountains of the great deep during the initial stages of the Genesis Flood stripped some of that original atmosphere away. Certain Bible scholars cite the language of the Psalm 148:4 as evidence against a vapor canopy. If the canopy had collapsed during the flood, they reason, why does the Psalmist still reference the waters above the firmament? But this poetic allusion could hark back to the original creation, or it could make reference to waters God expanded out into deep space as part of creation, or it could refer to some of the original water vapor (left over from the canopy) still in the outer reaches of our atmosphere. Computer modeling of a vapor canopy have shown that it is extremely effective as a thermal blanket, causing global warming. In fact, a canopy of significant size would result in extreme temperatures on Earth. This is a matter of ongoing creationist climate research.

It is interesting that scientists who would not subscribe to the water vapor canopy theory described above, have published articles that lend credence to portions of that theory. “Using evidence collected in South America and New Zealand, an international team of researchers has determined that climate changes – both warming and cooling patterns – during the late Pleistocene occurred rapidly and were global in scale. As giant iceberg armadas flooded the North Atlantic, alpine glaciers were simultaneously advancing across the Chilean Andes and Southern Alps of New Zealand. Thomas Lowell, associate professor of geology at the University of Cincinnati, and his colleagues published their findings in the September 15, 1995, issues of Science. …So, what did cause the climate changes? Lowell admits that he and his colleagues have no quick and easy answers. Possibly water vapors played a role. ‘A lot of water vapor in the atmosphere leads to a warmer climate,’ he states. ‘If there’s less vapor, temperatures become colder. Amounts of water vapor can change quickly, and the geological record indicates that climate changes could be very fast.'” (Anonymous, “Were Climate Changes Global During Ice Ages,” Geotimes,vol. 41, 1996, p.7, as cited in Morris, 1997, p. 305.) Additionally some scientists have been quite surprised to find water vapor in the freezing atmospheres of Jupiter, Uranus, Neptune and Saturn. (Dayton Daily News, April 8, 1998, p. 12A)

The water vapor canopy hypothesis would neatly explain yet another observed anomaly…too much water in Earth’s upper atmosphere. NASA satellites have confirmed far more hydroxyl in the hydrosphere than current models predict. The parent molecule of hydroxyl (OH) is water (H2O). Because ultraviolet radiation from the sun breaks down water in Earth’s upper atmosphere into hydroxyl and hydrogen, a large amount of water must have previously existed. Some have proposed a constant influx of mini-comets as a source for the mysterious water, but that theory has been strongly criticized as unworkable. (Matthews, Robert, New Scientist, July, 1997, pp. 26-27.)

It seems that another interesting feature of the early earth atmosphere was enhanced oxygen. Microscopic air bubbles trapped in fossilized tree resin were analyzed by Robert Berner of Yale and Gary Landis of the U.S. Geological Survey. The “gas bubbles enclosed in fossil amber may represent ancient air trapped at the time the original resin was exuded from its host tree” thus providing a glimpse into the ancient past (Berner, Landis, “Analysis of Gases in Fossil Amber, American Journal of Science 318:5, 2018, pp. 590-601.) The procedure involved clamping an amber sample “into a vacuum chamber of a quadrupole mass spectrometer, a device that identifies the chemical composition of a substance. As the machine slowly crushed the sample, the microscopic bubbles were released, exhaling up to 100 billion molecules. These breaths disclosed some surprising evidence: the ancient air contained 50 percent more oxygen than the air today.” Landis believes that the subsequent reduction in oxygen could have led to the dinosaur’s demise. (Discover, February, 1988, p. 12.)

Other studies of air bubbles in amber have found increased pressure as well as the greater oxygen levels. “One implication is that the atmospheric pressure of the Earth would have been much greater during the Cretaceous era, when the bubbles formed in the resin. A dense atmosphere could also explain how the ungainly pterosaur, with its stubby body and wing span of up to 11 meters, could have stayed airborne, he said. The spread of angiosperms, flowering plants, during the Cretaceous era could have caused the high oxygen levels reported by Berner and Landis, scientists said last week.” (Anderson, Ian, “Dinosaurs Breathed Air Rich in Oxygen,” New Scientist, vol. 116, 1987, p. 25.) A Yale study published in the March 3, 2000 issue of Science independently confirmed the high levels of oxygen present in the earth’s distant past. Some have even suggested that without such an atmosphere the relatively small lung capacity in certain dinosaurs could not have supplied their massive tissue with the needed oxygen.

In October 2006 Science Daily publicized a study led by Arizona State University staff entitled “Giant Insects Might Reign If Only There Was More Oxygen In The Air.” The article claims, “The delicate lady bug in your garden could be frighteningly large if only there was a greater concentration of oxygen in the air, a new study concludes. The study adds support to the theory that some insects were much larger during the late Paleozoic period because they had a much richer oxygen supply, said the study’s lead author Alexander Kaiser. The Paleozoic period…was a time of huge and abundant plant life and rather large insects — dragonflies had two-and-a-half-foot wing spans, for example. The air’s oxygen content was 35% during this period, compared to the 21% we breathe now, Kaiser said.” This research concurs with the biblical model of the early earth. In 2010 researchers at Arizona State University presented the results of experiments raising insects in various levels of atmospheric oxygen. Ten out of twelve varieties of insects studied decreased in size with lower oxygen. Some, like dragonflies, grew faster and became bigger in an enriched oxygen atmosphere (Science Daily, October 30, 2010.).

Some object strongly to using the scriptures to gain scientific insight into the natural world. While the Bible is not a science text, there are several clear lines of evidence that the Bible is God’s Word . If God’s word is truly inspired, it speaks accurately to all areas of knowledge: historical, political/economic, sociological, and scientific.

Watch the video: Layers of the Earth video for Kids. Inside Our Earth. Structure and Components (June 2022).

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