Seems like a story out of science fiction, but several prominent scientists have proposed just such a theory, and feel they have the evidence to support it. The theory was first postulated by Brian Harland at Cambridge University in 1964. In 1992, the term "Snowball Earth" was first coined by Joseph Kirschvink, a geologist from the California Institute of Technology, to describe the evidence of a ice enshrouded Earth. Kirschvink and others have since championed their theory in the face of a mounting group of scientists who feel the "snowball Earth" theory goes to far and think reality was just a bit different. Recently the Discovery Channel ran a special on the "snowball Earth" controversy and the current (April, 2003) issue of Natural History magazine has an article about the theory titled "The Longest Winter", written by Gabrielle Walker. So was the Earth covered entirely in ice at one time in its history? If so, what is the evidence? If not, what evidence does the theories detractors present?
"Snowball Earth" - The Theory
First, let us take a look at the idea behind the "Snowball Earth". In summary, the ideas presented by Joseph Kirschvink and later by Paul Hoffman and Daniel Schrag were built on an earlier idea postulated by Brian Harland. The basic theory of the "Snowball Earth" is that during the Late Proterozoic (900 - 570 million years ago (MA)) the continents aligned in the right manner, all located near the equator, allowing sea ice to form over both poles. This sea ice grows and grows until it reaches a point of no return. At this point the amount of ice is so great that more sunlight (and thus heat) is reflected back into space than is absorbed by the Earth. From this point the ice sheets continue to grow until they cover the entire globe. There are a few hot zones where volcanoes protrude through the ice. These volcanoes belch out various gasses, including carbon dioxide, and slowly build up the amount of CO2 in the atmosphere until, after tens of millions of years, a greenhouse effect occurs heating the planet and melting the ice. In the process, multi-cellular life, which has clung on near the hydrothermal vents and volcanic hot zones, explodes forth giving rise to the modern forms seen today in what is known as the Cambrian Explosion.
