SNOWBALL EARTH

New Scientist editor Walker looks at the hottest issue in geology: whether or not the Earth of some 700 million years ago was covered in ice.

The prime mover in this debate is Paul Hoffman, a Harvard geologist here depicted as a driven man unafraid of making enemies. Trained in the Canadian Arctic, Hoffman discovered evidence that pre-Cambrian rocks in Africa had been formed by glaciers. When he learned of pre-Cambrian glacial deposits found in other areas believed to have been near the equator at the time, Hoffman adopted a theory originally proposed by Joe Kirschvink of Caltech: Earth was covered with ice from pole to pole for millions of years. The theory quickly polarized the geological community, divided as much by Hoffman's prickly personality as by disagreements about the evidence. The breakup of the ice, brought about by the accumulation of greenhouse gasses spewed forth by volcanoes, was accompanied by a sudden explosion of life. The primitive organisms that evolved in the long ages before worldwide glaciation had attained nothing more complex than colonies of bacteria; once the seas were again open to the sun, multi-cellular life almost immediately began to flourish. The inference strongly urged by Hoffman and his supporters is that the breakup of the snowball was in large part responsible for the evolution of modern complex life. Walker summarizes the evidence from chemical and magnetic analyses of the rocks, describes the various geological sites that provide evidence for the snowball theory, and gives warts-and-all portraits of the scientists involved. She offers viewpoints and opinions on both sides of the still unsettled issue, although her summary perhaps prematurely implies that Hoffman's viewpoint has prevailed.

A fascinating and well-written account of scientists at work in an often neglected discipline.