Sinian World

Geoscientists in Scotland announced in 2004 they had evidence to disprove the controversial 'Snowball Earth' theory - the idea that the planet was almost completely encased in ice just over 600 million years ago.

The team, from the University of St Andrews, published its findings in the scientific journal Geology, after studying rocks in the west of Scotland, Ireland, Namibia, and California. Drs Dan Condon, Tony Prave, and Doug Benn said that they had found evidence of sedimentary material, which could only have been derived from floating ice on open oceanic waters.

This, they believed, indicated that Earth's oceans could not have been frozen during the snowball years.

Life 'explosion'

The controversial theory that for millions of years the planet was entirely smothered in ice, up to one kilometre thick, had been kicked around for more than fifty years.

It rested on the apparently anomalous evidence of past glaciation in places which should have been much too hot - very near the equator. Even during the most severe ice age, many scientists thought that the ice only reached as far down as Northern Europe and the middle of the USA.

'Snowball Earth' supporters were claiming that the supposed freeze would have caused severe environmental stresses upon early life. This would have resulted in repeated mass extinctions followed by an 'explosion' of more complex (multi-cellular) lifeforms on Earth once the thaw came. But Drs Condon, Prave, and Benn said their work cast severe doubt on the Snowball Earth idea.

Floating ice

The team examined a suite of rocks called Port Askaig Tillite, south-west of Oban in Scotland and directly north of Jura, which is supposed to record Snowball Earth glaciations. However, they found that the rocks similarly contained evidence which showed that Earth's oceans remained unfrozen during the snowball period.

Dr Prave thought that what was interesting about the Port Askaig Tillite is that it was the first rock unit ever described (in 1871) to be attributed to these ancient glaciations, thereby firmly grounding the roots of the Snowball Earth model on Scottish soil.

Dr Condon stated the team were claiming that in reality there was no totally frozen snowball Earth and that even during the coldest conditions large regions remained ice-free. If the planet's oceans were not totally covered in thick sea ice, there would have been areas of open sea which would have offered refuge for early marine life forms.

Different approach

This meant that there would have been places in which marine micro-organisms lived and survived during the glaciation. Dr Condon stated that others had approached the debate with a geochemical argument by examining isotopes of carbon while the St Andrews team had looked at the glacial rocks themselves.

The team examined rocks in Namibia upon which the Snowball Earth theory is based, as well as samples from County Donegal in Ireland, and Death Valley in California, which would have formed during the glaciations.

Dr Condon added that what was important and different about their findings was that they had found physical evidence for the glacial rocks being deposited in an environment in which there were areas of open sea, contradictory to the 'hard' snowball model.

Some images and original text copyright © BBC or affiliates. Reproduction is made on a 'fair dealing' basis for the purpose of disseminating relevant information to a specific audience. No breach of copyright is intended or inferred.