Icy tales of ancient eruptions.

The ice age was a depressing event, geologically speaking. A carpet of ice more than a mile thick weighed down Earth's crust in much of North America and Europe and in parts of Asia. Geologists in the past speculated that the melting of such ice at the end of the glacial epoch could have stimulated volcanic eruptions by unweighting the crust. That theory has now gained support from researchers who read a record of past eruptions stored deep within the Greenland ice cap.

U.S. scientists spent five summers drilling in the center of Greenland to collect information about how climate has changed in the last 100,000 years or more (SN: 9/14/91, p.168). As part of the Greenland Ice Sheet Project II (GISP 2), Gregory Zielinski of the University of New Hampshire in Durham and his colleagues analyzed sulfate ions deposited in the ice in the years following large volcanic eruptions.

At a meeting of the GISP 2 participants last month, Zielinski reported that the period following the ice age had three times as many eruptions as the last two millennia. The early eruptions were also much larger than most modern ones. "This could be crustal adjusting to the unloading of ice, almost like uncorking a bottle," says Zielinski, who described the volcanic record for the last 9,000 years in the May 13 SCIENCE.

Looking even further back in time, the ice core team has discovered evidence of a monstrous eruption 68,000 to 75,000 years ago. While most volcanic sulfate layers in the GISP 2 core span only a year or two, this ancient one extends over many years, indicating that debris from this eruption filled the skies for an unusually long time. From the date of the ice layers, Zielinski believes that the prominent sulfate deposit records a well-known eruption of Toba on the Indonesian island of Sumatra. The Toba blast occurred close to the start of the last ice age, prompting some scientists to speculate that it may have helped push Earth's climate into a deep freeze.