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Extra Friday,
July 18
Cooling
Mali's volcanism
In the Timbuktu
region south of the Sahara in Mali, patches of ground have been smoking for
more than a century. Plumes of water vapor and carbon dioxide escape from red-hot
holes and fissures in the ground. In April 2001, the inferno below the surface
began to radiate outwards, scorching tree roots and leaving a trail of burnt
soils and fallen trees. Over the following 10 months, the subsurface fires consumed
more than 500 acres of richly vegetated land.
A subsurface fire in Mali ejects gases out of a fumarole. Photo courtesy of
Henrik Svensen and Volcanic Basin Petroleum Research.
Theodore Monod, a French humanist and scholar, first characterized the smoking
holes, or fumaroles, in 1961. He concluded they were signs that the region had
become volcanically active. Magma bubbling up from depth fueled the fumaroles.
With this explanation in mind, local officials feared that the recent increase
in activity foretold a disastrous volcanic eruption. The Ministry of Mines,
Energy and Water in Mali asked a group of Norwegian geologists who specialize
in volcanic basins to visit the sites and assess the hazard. According to their
report in the August Geology, in just four days, the geologists collected
the data they needed to overturn the long held belief in active volcanism —
and substantially reduce the calculated risk for the region. The authors found
that the spontaneous combustion of buried peat layers, not magma, caused the
subsurface fires.
In his original survey of the Lac Faguibine region, about 50 miles west of
Timbuktu, Monod found dikes mixed in with the soils around the fumaroles. He
suggested the dikes were basaltic and hosted the mineral nepheline, which could
only have come from magma. In the early-1990s, French geologists built a geologic
model of the region that supported the volcanism hypothesis by indicating that
a huge and dense mass, likely magma, sat just 2 kilometers beneath the surface.
They argued that a major east-west trending lineament had recently reactivated,
and that the energy from the pull-apart tectonics drove the magma intrusion.
Later, in 2000, a group of geologists from Sonatrach, an Algerian oil company,
surveyed the region and past research and concluded that the fumaroles did indeed
result from volcanism — fueling local concerns of imminent disaster.
When the Norwegian team arrived in Mali, they expected to find signs of volcanism.
Digging a 2.5-meter-deep trench at Haribibi, one of the sites with fissures
and escaping smoke, they found a thin layer of peat burning just 60 centimeters
below the surface. The temperature in the peat layer exceeded 800 degrees Celsius,
but dropped down to cool background levels just 70 centimeters further down.
"Then we had two possibilities," says Henrik Svensen, a geologist
at the University of Oslo, Norw and lead author of the Geology paper:
"the burning peat layer was the only source of the heat anomalies, or the
peat layer could have been ignited by, let's say, magma at 1,000 degrees Celsius.
In the latter case, magma would have to be present 1 to 1.5 meters below the
surface." But they found no signs of magma.
The group concluded that microbial decomposition of organic matter in the peat
had likely generated enough heat to set the peat on fire. The impacted sites
sit in a dried-out lake basin; the old sediments provide the rich organic material
needed to stimulate high rates of microbial decomposition. "Burning peat
is a relatively common phenomenon," Svensen says. Subsurface peat fires
burn in parts of Botswana, and surface peat fires occur frequently in the United
States and South Africa, he adds.
During the rainy season, the Niger River occasionally rises enough to flood
the Lac Faguibine region. Those floods have historically quelled the subsurface
heat, which matches the pattern expected from peat fires, not magma intrusions,
Svensen says.
The volcanism hypothesis persisted for over 40 years largely because Monod
was a legendary figure and a well-respected scientist, Svensen says. The minerals
that Monod initially argued formed from magma intrusion could just have easily
formed from the heating and melting of minerals in the burning peat, he adds.
Glenn Stracher, a geologist at East Georgia College in Swainsboro, Ga., who
studies subsurface coal fires, agrees. "People have not thought in the
past about the amount of heat that can be generated by fire, whether peat or
coal, which can create enough heat to generate what are, in essence, igneous
rocks," Stracher says.
The several thousand people living in villages and nomad camps in the Lac Faguibine
region no longer face the prospect of a volcanic eruption, says co-author Dag
Dysthe, a physicist at the University of Oslo. But the peat fires still pose
health and environmental risks. By burning tree and grass roots, the fires increase
the rate of desertification at the fringes of the Sahara, Dysthe says. Additionally,
Svensen says, the fires are extremely difficult to put out, and the gases emanating
from the fumaroles and fissures may contain hydrochloric acid and sulfur dioxide,
both toxic.
Greg Peterson
