4-1 Using Active Reading Techniques

Is the Earth Cooking up Another Super Volcano?

By Christopher Joyce

Every few million years or so, the Earth burps up a gargantuan volcano. These aren't like volcanoes in our lifetimes; these "super volcanoes" can erupt continuously for thousands of years. While they might be rare, you'd best look out when one hits. The ash and volcanic gases from these volcanoes can wipe out most living things over large parts of the planet.

Michael Thorne, a seismologist at the University of Utah, has some clues about what causes these big eruptions. Thorne uses seismic waves to get a picture of what's going on about 1,800 miles beneath the Earth's surface, where the planet's core meets the outer mantle. Think of the Earth as an avocado, and the pit is the core. The stuff you make guacamole with is the outer mantle.

Thorne has been watching two enormous piles of rock that sit on the boundary between the core and the mantle. One pile is underneath the Pacific Ocean; the other under Africa. Scientists have known about them for 20 years, but Thorne saw something different. "I think this is the first study that might point to evidence that these piles are moving around," Thorne says.

Moving perhaps, but slowly, and the piles are maybe 3,000 miles across. Thorne thinks, in fact, that the pile under the Pacific is actually two piles crushing up against each other. And where they meet, there's a blob.

"We call it a blob of partially molten material," he says. "I mean it's big ... this one that we found is an order of magnitude, maybe 10 times larger, than any of the ones we've observed before." The blob is the size of Florida, and there are other, smaller blobs around the edges of the piles, too.

So these great rock piles are being squished together and squeezing this huge molten blob at the middle of it like some kind of balloon, and it is going on right underneath us. Or at least, under Samoa. So should we care about these blobs?

"A possibility is that these blobs might represent sort of a deep-seated root, to where plumes arise all the way to the surface, giving rise to hot-spot volcanism," Thorne says. One example is the Yellowstone super volcano, which has blown its top three times in the past 2 million years.

Thorne published all this in the journal, Earth and Planetary Science Letters. He's rather calm about it, and says it is a slow process from blob to blowout — maybe 100 million years or so. Thorne says he has no plans to move just yet.

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