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Monday, February 28

How Toba's eruption changed life on Earth


Source


March Resources @ National Geographic Magazine

Once upon a time a volcano killed almost everybody. It's a radical and scary thought, but there's reason to think it may be true.

The disaster happened about 74,000 years ago on the island of Sumatra. The scar of the event is easily visible today: a lake named Toba, about 60 miles (100 kilometers) across at its longest dimension. It was only in 1929 that a Dutch geologist recognized the lake as a caldera.

A caldera is essentially a great hole in the Earth where the surface has collapsed after a massive volcanic eruption. The central part of Yellowstone National Park is a 35-by-45-mile (60-by-70-kilometer) example.

Eruptions that leave calderas this big don't happen often (Toba seems to be on a 400,000-year cycle, give or take a hundred thousand or so), but when they do, the effects can be global. Toba appears to have ejected some 670 cubic miles (2,790 cubic kilometers) of material, as much as 560 times the amount produced by Mount Pinatubo in 1991. The ash and gas from Toba reached 30 miles (50 kilometers) into the stratosphere and shrouded the entire planet.

A super-eruption has multiple effects on the biosphere. Sulfur dioxide combines with water vapor to form sulfuric acid particles that scatter, reflect, and absorb sunlight. The planet's surface cools, the stratosphere heats, photosynthesis is reduced.

The more immediate effects are equally devastating. Bill Rose, a volcanologist at Michigan Tech University, is particularly interested in the fine ash produced by volcanoes. The ash rains from the sky in particles so small that they can penetrate an animal's lungs. "It's like smoking," he says.

"The birds die first," says Rose. "They get the ash in their feathers and they're immobilized. Then the larger animals start to die."

A lot of the humans died too, says Stanley H. Ambrose of the University of Illinois at Urbana-Champaign. Indeed, studies of mitochondrial DNA in humans point to a possible bottleneck of genetic diversity at roughly the same time as Toba's eruption, although it's impossible to prove a link.

Ambrose does believe, however, that human behavior shows signs of change after Toba. Prior to the eruption, there's little evidence that humans engaged in long-distance networking. Afterward, humans in Kenya, some 4,000 miles (6,400 kilometers) from Toba, appear to have traveled up to 200 miles (300 kilometers) carrying obsidian objects. Ambrose's theory is that humans who learned to cooperate and give gifts would survive another crisis better than those who lived in isolated groups and did not practice altruism or reciprocity.

So you might say gifts saved the world, a heartwarming ending to this disaster story.

—Joel Achenbach


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