Snowball Earth

Tuesday, July 01, 1997
If Dave Evans is right, Earth’s last ice age was a mere cold snap compared with a much earlier planetary deep freeze. Evans, a geologist at Caltech, has found evidence that 2.2 billion years ago, ice sheets covered almost the entire planet, reaching to perhaps 11 degrees of the equator, about the latitude of Costa Rica. In comparison, at the height of the last ice age, 25,000 years ago, glaciers advanced only about as far south as Paris.

Evans, along with researchers from Rand Afrikaans University in South Africa, found his evidence beneath a lava deposit in South Africa that had fortunately covered ancient glacial deposits. The lava layers, known to be 2.2 billion years old, each contained a small amount of magnetic mineral grains. When the lava had originally cooled, those grains were aligned with Earth’s magnetic field like tiny compasses.

What you see is that at the North Pole, the magnetic direction is going straight down; at the South Pole it is going straight up; and at the equator it is flat, because the field lines are straight overhead, says Evans. So depending on your latitude, the angle between horizontal and the magnetic direction changes. The South African glacial samples, the geologists found, had been laid down just 11 degrees, plus or minus 5, from the equator. Because of continental drift, South Africa now lies some 30 degrees south of the equator.

Evans doesn’t suggest that the glacial ice covered the entire Earth. The equatorial regions were most likely spared, he says, because otherwise life might not have survived. The jury is still out on what put Earth into such a deep freeze, but there is some evidence about how it thawed. The lava deposits covering the glacial layers hint that large-scale volcanic activity might have released enough heat-trapping carbon dioxide into the atmosphere to warm the planet. Another possibility, says Evans, is that a meteor or asteroid may have vaporized tons of carbon-containing rock. But so far we haven’t found any crater or impact deposit.
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