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

Sorcha McDonagh

In regions of northern Mali, the parched ground is punctuated with smoking, glowing holes that reach 750°C 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.

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 http://www.sciencenews.org/articles/20030510/bob9.asp).

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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References:

Svensen, H., D.K. Dysthe, et al. 2003. Subsurface combustion in Mali: Refutation of the active volcanism hypothesis in West Africa. Geology 31(July)581–584. Abstract available at http://dx.doi.org/10.1130/0091-7613(2003)031<0581:SCIMRO>2.0.CO;2.

Further Readings:

Harder, B. 2002. Wildfire below: Smoldering peat disgorges huge volumes of carbon. Science News 162(Nov. 9):291. Available at http://www.sciencenews.org/articles/20021109/fob1.asp.

Page, S., et al. 2002. The amount of carbon released from peat and forest fires in Indonesia during 1997. Nature 420(November 7):61–65. Abstract available at http://dx.doi.org/10.1038/nature01131.

Perkins, S. 2003. The fires below. Science News 163(May 10):298–300. Available to subscribers at http://www.sciencenews.org/articles/20030510/bob9.asp.

Sources:

Dag Kristian Dysthe
Physics of Geological Processes
University of Oslo
PO Box 1048
Blindern
N-0316 Oslo
Norway

Susan Page
University of Leicester
Geography Department
Bennett Building F57
University Road
Leicester LE1 7RH
United Kingdom

From Science News, Volume 164, No. 2, July 12, 2003, p. 22.