Digging for Fire: Burning peat underlies Mali’s hot ground

In regions of northern Mali, the parched ground is punctuated with smoking, glowing holes that reach 750C at their rims. Locals have long attributed the appearance of these holes and the superheated ground around them to evil spirits or, in a more scientific vein, to volcanic activity. But Norwegian geologists have found that a less truculent force is at work. A seam of peat-like material is smoldering about 2 feet below the surface.

SPIRIT, LAVA, OR FIRE? A Tuareg tribesman stands by a smoking hole previously suspected to be caused by volcanism but now known to be caused by subsurface peat fires. Volcanic Basin Petroleum Research

The geologists, from the University of Oslo and the Volcanic Basin Petroleum Research group in Oslo, carried out their study at the invitation of Mali officials, who worried that a recent intensification of the phenomenon could presage a volcanic eruption.

“For the first 3 days of our expedition, we were mapping what we thought was hydrothermal-vent activity from volcanism,” says Dag Kristian Dysthe of the University of Oslo.

But the slow, uniform migration of the underground heat in what appeared to be a “heat front,” was not typical of volcanic activity, Dysthe says. In a 10-month period, the front scorched a 2-square-kilometer area of rich vegetation as it advanced toward a village at a rate of several centimeters per hour.

To uncover the heat source, the geologists conducted a simple experiment: “We dug a hole,” Dysthe says.

With members of the Tuareg tribe gathered around, the scientists dug a 3-foot-deep trench, revealing the fire below. They took samples of the burning material and found that it was like peat, but with an 8 percent organic content–one-sixth that of normal peat deposits. They report the findings in the July Geology.

“I was surprised the deposits caught fire at such a low carbon content,” comments Susan Page of the University of Leicester in England. Because of the low carbon content, she says, the carbon dioxide and other gaseous emissions from the Malian fires are much lower than those from peat- and coal-seam fires in places such as Indonesia and Pennsylvania (SN: 5/10/03, p. 298: Available to subscribers at The Fires Below).

The volcanic description of the region comes from French naturalist Theodore Monod who, in the 1960s, erroneously identified rocks there as coming from magma–even though the region is a craton, a geological zone where scientists wouldn’t expect to find volcanic activity. Dysthe says that the survival of Monod’s theory is “a good story in the sociology of science.” Scientists following up on the work of Monod, a respected authority on the deserts of West Africa, didn’t question his theory.

The debunking of the volcanic theory was heartening news for the local people. No one has ever stopped volcanic activity, but it might be possible to contain subterranean peat fires, which can burn for years and render vast tracts of land unusable, Dysthe notes.


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