Evidence
The Snowball Earth hypothesis was originally devised to explain the apparent presence of glaciers at tropical latitudes. Modeling suggested that once glaciers spread to within 30° of the equator, an ice-albedo feedback would result in the ice rapidly advancing to the equator (further modelling shows that ice can in fact get as close as 25° or closer to the equator without initiating total glaciation). Therefore, the presence of glacial deposits seemingly within the tropics appeared to point to 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 two things:
- that a bed contains sedimentary structures that could have been created only by glacial activity;
- that the bed lay within the tropics when it was deposited.
During a period of global glaciation, it must also be demonstrated 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 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.
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 latitude of landmasses even as little as 200 million years ago can be riddled with difficulties.
Read more about this topic: Snowball Earth
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