What is the Snowball Earth Hypothesis?

Michael Anissimov
Michael Anissimov

The Snowball Earth hypothesis refers to the idea that at some point during the Earth’s history, specifically during the Cryogenian period (850 to 630 million years ago), the entire surface was frozen over, including the oceans. The Snowball Earth hypothesis is controversial among paleontologists, but many believe it helps explain the presence of glacial deposits at tropical latitudes from this period, as well as other unusual aspects of the Cryogenian geological record. All scientists agree that there were huge glaciations in the Cryogenian period, the disagreement is on whether they became global in extent.

Volcanic activity has been linked to both global cooling and global warming.
Volcanic activity has been linked to both global cooling and global warming.

The Snowball Earth scenario, if it did indeed occur, is thought to have been initiated by the drifting of continents into an almost exclusively equatorial configuration. This would have caused the rapid weathering of continental rocks, which would have then absorbed large amounts of atmospheric carbon dioxide. As carbon dioxide is a greenhouse gas that plays a crucial role in keeping the Earth warm, these depletion would have caused runaway ice accumulation. As glaciers covered the Earth, they would have raised the Earth’s albedo (reflectivity), reflecting light energy back into space, further accelerating cooling. The eventual result would have been a planet entirely covered in ice, with equatorial temperatures similar to present-day Antarctica.

Some scientists believe that by about 850 million years ago, the entire planet Earth was frozen over.
Some scientists believe that by about 850 million years ago, the entire planet Earth was frozen over.

To dispel the deep freeze of the Snowball Earth would have required copious amounts of carbon dioxide. As silicate and carbonate rocks, the normal sources of carbon dioxide, would have been entirely covered up, the gas would have had to come from volcanic explosions. Over periods of time in the millions of years, volcanic explosions would belch enough CO2 into the atmosphere to initiate global warming. This warming would cause ice around the equator to melt and expose land, lowering the Earth’s albedo and causing positive feedback of warming.

Some scientists believe a “Slushball Earth” hypothesis is more realistic than Snowball Earth, where a region of thin-ice or ice-free ocean would exist around the equator. This is necessary for an active hydrological cycle, and indeed, some geological deposits from the time indicate the presence of one.

The Snowball Earth is sometimes cited as holding back the evolution of multicellular life, but the truth is, we don’t know whether this is true or not. Unicellular life certainly existed billions of years before the Cryogenian and survived through it, perhaps in deep-sea hydrothermal vents and other refugia. The survival of Earthly life through the Snowball Earth period is sometimes cited as an argument that life could exist in the ice-covered oceans of outer Solar System bodies such as the Jovian moon Europa.

Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Discussion Comments


@fiorite- Besides carbon isotope ratios, the planet earth has other buried secrets that support the snowball theory. The formation of banded iron striations in rock that is billions of years old, suggest that there was a lack of oxygen exchange between the oceans and the atmosphere. Iron precipitates out of the oceans, and the fact that the banded iron formations are not oxidized means that the water it precipitated out of was not heavily oxygenated. This suggests that the ocean was covered with something (like a sheet of ice) that did not allow oxygen exchange.

Other geologic clues include the presence of glacial deposits and formations at low latitudes. Things like dropstones, glacial till, and glacial striations in the tropics suggest heavy sheets of snow and ice once covered them. The verdict is still out on the snowball earth theory, but some of the evidence is supportive of the theory.


@anon69761- I will try to break it down for you in a reasonable manner. Carbon has three isotopes; C-12, C-13, and C-14. The numbers refer to number of neutrons present in each atom (carbon always has 6 protons). Light carbon isotopes are carbon atoms that contain six protons and six neutrons (C-12), making them lighter (in terms of atomic mass) than C-13 and C-14.

In reference to the snowball earth theory, the evidence would be in the ratio of C-12 to C-13 found in sedimentary rocks around the globe. Algae prefer light carbon to heavier carbon isotopes, so when they die and decay into the ocean sediment, the forming rock will contain more light carbon isotopes when photosynthetic life (algae) is present.

During a snowball earth scenario, photosynthetic life is not able to survive, so sediment from these times will have slightly lower levels of light carbon (C-12). When you look at the rock record from around the globe, there are four or five instances with elevated heavy carbon (C-13) levels, indicating that there was little photosynthetic life in the oceans (meaning they were covered with ice). I hope I made this explanation simple enough.


I learned about this hypothesis in Geology. However, it also mentions evidence for it are light carbon isotopes. What exactly are those- in an understandable language? I'm not too good with chemistry, etc., but love geology- just not sure what these "things" are! Thank you.

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