I hear it hiss. I feel it rumble. I check my watch. It’s coming in 5…4…3…2…1! A blast
of hot water shoots 40 m in the air, then rains down in a frothy mist. I can always count on Old Faithful. T is
geyser erupts every 90 minutes on average. It always puts on a great show. T at’s not the only reason I watch it, though. Each blast reminds me of what’s going on deep beneath my feet. I am standing on top of one of the world’s
biggest volcanoes. It’s so large that we call it a supervolcano. I can’t see it, though. It’s hidden under the ground here in Yellowstone National Park in the United States (U.S.). T is might be hard to believe, but it’s true.
T e signs are all around me. Even as the giant volcano sleeps, its heat energy aff ects the land above. Geysers spray. Mud boils. Forests turn into rock. Mini earthquakes rattle the land. Each spurt, bubble, and shake lets me keep
an eye on the supervolcano. T ey can warn me if its energy is building, and if it’s going to erupt any time soon. To see how, let’s tour some of my favorite spots above this supervolcano.
Heat Beneath Our Feet As we walk, think about what’s under our feet. If we dug about 8 km into Earth’s crust, we’d hit a chamber. It’s filled with magma, or hot and mostly melted rock. T is magma chamber is huge. What lies beneath it is even bigger. A giant plume of hot rock rises through the
mantle, the layer of Earth just below the crust. T e plume starts 644 km underground. Fed by the heat in Earth’s core, it reaches 1,925°C. This is the hot rock that feeds the supervolcano’s magma chamber. Over time, heat and pressure build up in the magma chamber. If the pressure becomes too great, the supervolcano will erupt. It’s happened here before.
12 NATIONAL GEOGRAPHIC EXTREME EXPLORER
Too Hot To Handle About two million years ago, massive amounts of magma and gases bubbled up. T ey pushed up the crust. A dome formed. T eir pressure cracked the crust. T en, KABOOM! T is supervolcano finally blew its top. T e
force of the eruption was so great, the ground here collapsed. T is created a giant crater, or caldera, nearly 75 km wide. Lava and ash spewed from the volcano. T e
eruption buried most of the western half of the U.S. in ash. Winds carried a cloud of ash, hot sulfur, and other chemicals from the eruption around the planet. Plumes of ash rose into the sky and blocked the sun. All around the world, temperatures dropped, plunging the planet into a volcanic winter. Since then, there have been two other major
eruptions here. One happened 1.3 million years ago. T e most recent was 640,000 years ago. Together, these eruptions produced enough ash and lava to fill the Grand Canyon. T is supervolcano is still active today. Heat
and pressure below ground constantly change and build. T is makes the floor of this ancient caldera rise and fall as if it were breathing.
Waking a Sleeping G See what happens when a supervolca
magma chamber
1. Gases and heat build in the magma chamber. This pressure causes small earthquakes. It pushes up the land.
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