Just Ask: What Makes Volcanoes Erupt?

BY Jenny Marder  November 30, 2010 at 5:01 PM EST

It’s time for our weekly Just Ask feature, where the experts tackle your questions on science and technology.

John Eichelberger, a volcano expert with the U.S. Geological Survey, answers the question, “Why Does the Earth Have Volcanoes, and What Makes Them Erupt?”

Even though the earth is 4.6 billion years old, it is still generating heat, and that’s due to radioactive decay. And more heat is getting generated than can just calmly flow out — conduct out of the planet. So excess heat builds up, and this causes melting. So volcanoes are “earth sweat,” in a way.

So why does this stuff actually come up? Molten rock is less dense… that is, the amount it weighs per volume is less than the amount that solid rock weighs per the same volume, so the molten rock is going to always want to come up.

Now there’s something that makes this process even more spectacular, and that is that the molten rock contains gases dissolved in it, just like carbonated water contains a gas. You take the pressure off, and that gas comes out and makes bubbles. Well, it’s the same thing with molten rock magma. When it’s coming up, it’s feeling less and less pressure and that gas that’s dissolved comes out and makes bubbles. Well they take up a lot more space.

If you have a tube, say, going into the earth, and you start magma coming up that tube, say it’s coming up at walking speed, and it just has a few-weight-percent water in it, just a little bit… by the time it’s coming out the top, it will be going as fast as a jet plane. because all the expansion has to be in the vertical direction. So, what does that mean? It speeds up; it’s going faster and faster and faster and faster. So this stuff comes flying out of the volcano like a bullet aimed straight at the sky.

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