Dear Graylon,
Your question takes us on a journey deep into the Earth. Figuratively speaking, of course. It’s really hot under Earth’s surface. It’s so hot it can melt rock. This melted rock is known as magma. And anything that erupts magma is a volcano.
Under volcanoes are giant pools filled with piping hot mush. That’s what I learned from my friend John Wolff, a geologist at Washington State University.
“It’s almost like thick oatmeal,” Wolff explained. Thick oatmeal that glows hot orange and sometimes stinks like a burnt match or rotten eggs. The mush is a mixture of rock that is not quite a liquid or a solid. It also has crystals.
In his rock collection, Wolff has a piece of dark grey basalt rock with white crystals. Basalt is one of the oldest, most common kinds of rock we find under volcanoes.
Scientists think crystals in the semi-melted rock help the mush keep its shape for such a long time. If half of the mush is crystals, it will stay mushy. When the mush has less than half crystals, it will start melting into more of a liquid.
“The crystal mush can’t flow,” Wolff said “But when it heats up and melts, all of a sudden it starts to move.”
Hotter magma down below heats up the mush. As it melts, it gets really hot. Since it’s the hottest thing around, it starts rising through a big tube in the volcano.
Until then, the mush just sits and waits. It can sit for tens of thousands of years. Under some volcanoes, the mush columns go all the way down to the top of Earth’s mantle. That’s about 20 miles deep.
Beneath Yellowstone National Park there is enough of this mush to fill about ten Grand Canyons. Yellowstone National Park is actually a super volcano. It stretches through parts of Wyoming, Idaho and Montana.
When the volcano blew thousands of years ago, it collapsed into itself. It formed a deep crater called a caldera.
Wolff actually tracks patterns of very hot spots under parts of southern Idaho. Earth’s rocky shell can move around and shift over time. It’s what scientists call plate tectonics. Parts of southern Idaho used to exist right over where Yellowstone National Park is today.
Wolff’s work also helps predict when volcanic eruptions will happen. Scientists don’t think Yellowstone will blow anytime soon. But when it does, the super volcano will create a super eruption.
We might not always think about it, but there are about half a dozen volcanoes erupting almost all the time. That’s a lot of magma spewing out from our planet’s volcanic lakes, oceans, mountains, and calderas.
Wolff says your question about what lies beneath volcanoes is a really good one.
“In one way or another, your question is the one most of us who work on volcanic rocks are trying to study,” he explains.
Graylon, you might just be a future scientist.
Sincerely,
Dr. Universe