Hot rocks

Natural History, Oct, 2003 by Stephan Reebs

For centuries the nomadic Tuaregs of the Sahara, warned off by legends of diabolical fumes and flames, have avoided camping in the dry lake beds around Timbuktu, Mall. Some geologists noted similarities between the lakes' steaming cracks and the fumaroles of volcano craters--and wondered if magma might be brewing there. Problem was, West Africa has no active volcanoes and is tectonically stable. Henrik Svensen, a geologist at the University of Oslo in Norway, and several colleagues went to investigate.

The team took a direct approach: they dug an eight-foot-deep trench into the leading edge of a smoking, migrating heat front. What they found wasn't fiery lava but a layer of smoldering peat--the result of microbial decomposition of the organic residue left in the sediment when a lake has dried out. The decomposition generates so much heat that the buried peat self-ignites, roasting the ground above it to temperatures as high as 1,400 degrees Fahrenheit. ("Subsurface combustion in Mali: Refutation of the active volcanism hypothesis in West Africa," Geology 31:581-84, July 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior 'n the Aquarium and in the Wild (Cornell University Press).