Glacial tillites, striations, and dropstones in deep ocean sediments are found on almost all continents. There were apparently two major periods of glaciation called the Sturtian (~730-700 m.y.) and Marinoan (~665-635 m.y.). Many glacial sediments were apparently deposited at low latitude as shown by the low inclination paleomagnetic data from many of the glacial sediments. Dating of these global glacial deposits is improving. It is not yet completely clear if there were two major, long pulses of snowball earth conditions or if there were several advances and retreats of the ice during this period.
some proposed causes:
According to astronomical observations and theoretical calculations the sun produced only about 73% as much energy when the solar system was just formed about 4.5 b.y. ago. In the late Proterozoic the sun was not yet as bright as it is today. It was perhaps 8% weaker.
Bare land, lacking vegetation which had not yet evolved in the Proterozoic, reflects much more sunlight back into space and absorbs much less heat in comparison to the oceans. Most continental masses were lying at low and mid latitudes where the greatest amount of solar radiation strikes the Earth. Therefore, during this period with the unusual placement of land masses, more light was reflected from the earth's surface and less heat absorbed than is usual.
A supercontinent, called Rodinia had formed around 1 b.y. ago. The vast interior was dry because it was locked away from moisture from the oceans by rain shaddows from mountain ranges and simply by distance. As Rodnia broke up in the late Proterozoic the dry interior gained access to humid winds from the ocean, increasing the rate of weathering, especially in the equatorial region. Increased weathering would draw more CO2 from the atmosphere.
a proposed mechanism: Freeze/Fry Cycles
Freeze Cycle
Ice caps formed as sea ice at the poles and spread to lower latitudes. Ice reflects most of the incoming sunlight, so as the ice caps spread, more and more sunlight was reflected and less and less was absorbed by the earth. The cooling and ice formation resulted in more cooling due to the ice-albedo effect: a positive feedback. If the cooling were strong enough and the ice caps could extend down to around 30° latitude, models suggest that the ice-albedo effect would cause runaway cooling and the ocean surface would freeze all the way to the equator. "Snowball Earth"!
Heat from the earth's interior would keep the oceans from freezing solid. Gradually carbon dioxide would build in the atmosphere via outgassing at midocean ridges, hotspot volcanoes, and volcanic arcs. Although the CO2 would increasingly hold in infrared radiation emitted from the surface, most of the sun's radiation would be reflected by the ice as visible wavelengths. Therefore, a very large amount of CO2 would be required in the atmosphere (perhaps 350 times current CO2 levels) to melt the ice. At modern outgassing rates, it would take millions of years to acquire sufficient CO2.
Fry Cycle
Once sufficient greenhouse warming had occurred, ice would begin to melt in the tropics. This would decrease surface reflectance and increase the amount of heat absorbed by the oceans thereby increasing the rate of melting and further decreasing reflectance increasing heat absorption (positive feedback, ice-albedo effect). The ice would melt very quickly, within about 2000 years. With the high concentration of CO2 global temperatures would be very high. So the snowball earth episode would end with an ultra-greenhouse.
cap carbonates: High CO2, temperatures, and rainfall (b/c high temp yields high humidity) and high acidity in rain and surface waters would yield very rapid weathering of the continents. Weathering products including calcium ions (from rocks) and bicarbonate ions (from carbonic acid) would be transported to the ocean at high rates where the calcium and bicarbonate ions would combine to form calcium carbonate and be stored as carbonate rocks, thus accounting for carbonates overlying glacial tillites in the rock record.
Rapid weathering and storage of carbon in carbonate rocks would remove CO2 from the atmosphere, eventually returning the carbon balance to its previous level. If conditions for the chill down had not been removed then the freeze/fry cycle could begin again.
primary sources:
Hoffman and Schrag, 1999.
Hoffman, 2007: snowballearth.org