The geologic record holds evidence of at least four periods when glaciers covered parts of the earth.
1) Paleoproterozoic: ~2.2 b.y.
The Gowganda Formation (north of Lake Huron) contains:
2) Neoproterozoic: ~850-600 m.y. "Snowball Earth"
Glacial tillites, striations, etc., are found on almost all continents. There were apparently about four separate glacial episodes. Many glacial sediments were apparently deposited at low latitude as shown by the association of carbonate sediments and glacial tills. Extensive limestone only forms at low latitudes today and calcite is soluble in cold water. Also, low inclination paleomagnetic data from glacial sediments indicates low latitude when they were formed. Was the earth locked in a global deep freeze?
Proposed mechanisms:
A] High Obliquity: Earth's axis of rotation was lying down, almost flat, in the plane of its orbit around the sun. This would decrease the average annual amount of solar radiation received at the equator and supposedly allow glaciation in the tropics.
However, the earth's climate would be characterized by extreme seasonality. Winter near the poles would be very long and cold, but summer would be very hot, so winter snow and ice should all melt. The tropics would experience cold periods at the solstices (northern and southern hemisphere winter/summer) but would have hot equinoxes (spring/fall) which also should melt any snow and ice. Another problem is how earth's axis could change to mostly upright (a collision?) without disrupting the moon's orbit around the earth.
B] Freeze/Fry Cycles
i) 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 this unusual placement of land masses, more light was reflected from the earth's surface and less heat absorbed than is usual. So the earth cooled.
Freeze Cycle
ii) 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. If the cooling were strong enough and the ice caps could extend down to around 30š latitude, models suggest that the ice reflection effect would cause runaway cooling and the ocean surface would freeze all the way to the equator. "Snowball Earth"!
iii) 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 (about 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
iv) 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. The ice would melt very quickly, perhaps in a century. With the high concentration of CO2 global temperatures would be very high.
v) High CO2, temperatures, and rainfall (b/c high temp -> 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.
3) Late Paleozoic Glaciation in Gondwana
i) Mountain Building and resulting weathering of newly exposed rocks during the Paleozoic, especially during Permo-Carboniferous times, would have resulted in a draw-down of CO2 from the atmosphere, thereby tending to cool global climate.
ii) Parts of Gondwana drifted over or near the south pole for most of the Paleozoic. During Permo-Carboniferous times the position of the south pole ran across Africa, Antarctica, and Australia yielding glacial deposits on those continents. Paleomagnetic data for Gondwana are in agreement with the paleoclimate indicators for the position of the south pole with respect to Gondwana during this time period.
4) Cenozoic Glaciation
The Cenozoic saw general cooling resulting from decreasing seafloor spreading rate (decreased outgassing) and increased mountain building (increased weathering). Glaciation in Antarctica began in the mid-Cenozoic as that continent became isolated over the south pole. The northern hemisphere "ice ages" began 2-4 million years ago when the closure of the Isthmus of Panama caused an intensification of the Gulf Stream.
(see notes on Cenozoic climate)