October 22, 2001

Six hundred million years ago, the earth was covered in ice an average of one mile thick, according to Paul Hoffman.

Hoffman, a leading proponent of the theory, says that, given a certain set of climatological conditions, the earth may freeze all the way to the equator in a matter of months or years. This has happened twice in the last two billion years, he says.

Hoffman, the Sturgis Hooper Professor of Geology at Harvard University, spoke on Oct. 10 as part of the Edwin Way Teale Lecture Series on Nature and the Environment.

"We know that the ice front has advanced and retreated repeatedly," he said. In the model he supports, called "the freeze-fry cycle", the ice does not always stop advancing.

One of the reasons the earth could freeze so quickly is an effect called ice-albedo feedback, said Hoffman. Glaciers, thick slabs of ice, are white and tend to reflect a large amount of the sunlight that hits them. If the polar ice caps grow enough, they cool the earth by reflecting sunlight, he said. This, in turn, causes more ice to form and intensifies the effect.

If, as Hoffman said, the earth were completely covered in ice, how could it thaw? "The salvation," he said, "is plate tectonics." When the plates supporting the continents and seas shift, earthquakes and volcanic eruptions occur.

Hoffman said volcanic eruptions would not be hindered by the earth's coat of ice, and they would pump carbon dioxide into the atmosphere. Once carbon dioxide began to heat up the climate, he said, the ice would melt off the continents within 1,000 years.

Hoffman said he supports the theory because it explains some puzzling rocks he had been observing. The rocks were formed near the equator and bore substances formed by dead organisms, he said. But underneath, the rocks showed the telltale signs of deposits left by

glaciers.

The theory went unnoticed when it was first proposed by other geologists, but Hoffman has developed and promoted it after seeing its explanatory power. There are many paradoxes in geology, said Hoffman, "and this hypothesis explains all

of them."

Tim Byrne, professor and head of geology and geophysics, says the theory ties together ideas from different disciplines. "It's like a mini-revolution. This is so global and so controversial," says Byrne.

Other scientists are not convinced. "Biologists in general are very skeptical," says Peter Gogarten, a professor of molecular and cellular biology, because it is difficult to see how the life thought to exist at the time of the disasters could have survived: "How can we reconcile these very drastic events with the continuity we see in biology?" Still, Gogarten says, "it's a fascinating

theory."

Hoffman closed his talk by telling aspiring geologists that despite the fact that the rocks geologists study have been around for a very long time, there is still plenty of scope for research. It would seem, he said, that "after 200 years of serious investigation, surely all the juicy stuff would have been found." Instead, he said, "the earth has got a lot of surprises in store for us."

Brent C. Evans