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Recipes for Calderas

Scientists look closely at the biggest volcanic explosions

I've written about how scientists have moved beyond kid stuff when making models of volcanoes—no more of the baking-soda-and-vinegar mixtures for them. Today computer simulations, field studies, and lab experiments all serve to advance the science of volcanology. A recent paper takes a look at the very largest eruptions, those that give birth to calderas.

Crater Lake caldera, Oregon. U.S. Geological Survey photo.
Calderas are huge holes in the ground that form when a volcano empties out, then collapses in upon itself. The eruptions that create them are beyond enormous. Crater Lake, Oregon, shown here, is a small caldera, about 9 kilometers across. When it formed, the corresponding eruption sent ash halfway across the continent. Ash piled ankle-deep nearly 1000 km away, and red-hot landslides swept as far as 60 km away in all directions. The Campi Flegrei, in the heart of Italy near Naples, is a caldera 13 kilometers across. The eruptions that accompanied its birth involved some 80 cubic kilometers of lava.

Map of Yellowstone caldera after U.S. Geological Survey.
One far larger than these is in Yellowstone National Park. This last caldera, almost 30 by 50 km in size, has vomited forth some 2500 cubic kilometers of volcanic debris in total. Scientists have never witnessed eruptions of that magnitude, but they estimate that the magma may have erupted at rates of more than a cubic kilometer every 30 seconds. A paper in the October 2000 Geology suggests that eruptions like that may give rise to a newly described kind of explosion called a sustained blast.

Volcanoes erupt because when magma rises near the Earth's surface, it's under less pressure, and the gases dissolved in the magma come out of solution as bubbles. The higher and faster the magma rises, the faster the bubbles form and expand. The kind of eruption that happens depends on the balance between the bubbles and the magma. A slow-rising magma lets the bubbles out gently, and an effusive eruption happens, with lava simply pouring out of the vent or, if it's a thick, sticky lava, it builds a dome. When magma rises faster, a runaway effect can happen as the bubbles expand faster than the magma can let them out. The result is an explosive eruption. Confined in the throat of the volcano, the expanding magma shoots straight upward, hurling fine ash as high as the stratosphere.

When magma rises still faster, according to the Geology paper, it may reach the surface before the bubbles can even form. Instead the gas comes out all at once in a shattering burst, and the lava explodes in all directions, unconfined by the volcano. As long as the magma comes up, this blast is sustained.

We have examples of volcanic blasts in the historic record. At Mount St. Helens in 1980, for example, the north side of the mountain collapsed and the pressurized magma beneath released a sideways, or lateral blast. The Russian volcano Bezymianny did the same in 1956. But Krakatau set the modern record in 1883 when its caldera-forming eruption was heard thousands of kilometers away.

Even that gigantic eruption, though, was a one-time blast. The sustained blast would keep going and going, perhaps for days. And there's no telling how that would affect the world's atmosphere or civilization itself. It will take more work beyond the first crude model in the Geology paper to make a better scientific guess.

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