The Debunker: Are Volcanoes Full of Lava?

by Ken Jennings

We usually think of volcanic eruptions as sudden and dramatic events, but that's not always the case. The Hawaiian volcano of Kilauea, for example, has been erupting continuously since January 3, 1983, covering 48 square miles of the state's "big island" with new lava. In honor of the thirty-fifth anniversary of Earth's longest-erupting volcano, Jeopardy!'s Ken Jennings will be here all month providing explosive corrections to a lot of popular misinformation about volcanoes. The results might just rock your world.

The Debunker: Are Volcanoes Full of Lava?

Just because I'm "The Debunker" doesn't mean I'm an irritating pedant full-time. But this particular installment does hinge, in part, on a bit of semantic hair-splitting. Yes, volcanoes can vent lava, which is molten rock. But that doesn't mean that a volcano contains lava. That's because molten rock is only called "lava" after it erupts onto the surface. When it's still underground, it's called magma instead.

The Debunker

But there's a deeper misconception underlying the whole magma thing. Many of us have a view of volcanoes formed by our elementary-school conception of the Earth: that it’s a thin rocky crust covering the hot, molten interior that make up the mantle and the core. So we imagine that the hot liquid rock that occasionally gushes out of the Earth is coming from that molten core or mantle.

But the fact is that the mantle isn't really molten at all. I mean, it's true that the mantle flows, and the surface plates of the hard lithosphere move around on top of it. But that fluid motion takes place on a timescale of millions of years. Over shorter time frames, mantle rock acts like any surface rock: hard and brittle. The mantle is usually said now to be "plastic"—it flows more like very stiff Silly Putty than like water.

So where does the molten rock of a volcanic eruption come from? Not from deep in the mantle, and not from the outer core, which is the only layer of the Earth that's entirely liquid. (The inner core is hot enough to be liquid as well, but the rock there is under so much pressure that it stays solid anyway.) The magma in volcanoes actually comes from isolated pockets in the lower crust or upper mantle, where convection has made it hotter than usual, or the rock's pressure and composition has lowered its melting point, or both. So you can stop picturing the Earth as a massive Cadbury Cream Egg, a hard shell that will vent its delicious gooey filling when cracked.

Quick Quiz: Quick Quiz: What would a Polynesian person do with a lava-lava?

Ken Jennings is the author of eleven books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.