I hear it hiss. I feel it rumble. I check my watch. Get ready. 5…4…3…2…1! Hot water and steam suddenly blast from the ground. T e spray shoots 40 meters (131 feet) in the air. I can always count on Old Faithful.
It’s a geyser. T at’s a pool of hot water that erupts. Old Faithful erupts about every 90 minutes. T e reason why is deep under our
feet. We’re standing on top of one of the world’s largest volcanoes. It’s so big, we call it a supervolcano. We can’t see it, though. It’s hidden under the ground here in Yellowstone National Park. It’s in the United States (U.S.).
Heat Beneath Your Feet T is supervolcano has caused many changes to Earth’s crust, or surface. Some changes took millions of years. Others happen right before our eyes. Slow or fast, the changes here all
start in the same place. Under Earth’s thin crust lies the mantle. It’s a layer with hot, melted rock called magma. Magma here has pushed up into the
crust. It fills a big chamber far below our feet. Heat and gases from it build up. T ey press on the crust. If they get too great, the supervolcano can erupt. It happened here long ago.
18 NATIONAL GEOGRAPHIC EXPLORER
magma chamber
Boiling rock, called magma, collects in a magma chamber. Heat and gases build up over time, creating pressure.
Too Hot To Handle T e eruption was huge! T e ground beneath Yellowstone collapsed. T is created a wide crater, or caldera. Ash flew into the sky. It blocked
the sun. Temperatures dropped all around the world. Bits of ash floated to the ground. T ey buried most of the western half of the U.S. Since this blast, this volcano has
erupted twice more. T e last one happened 640,000 years ago. T at’s a long time ago. Yet this supervolcano is still active. I see the signs all around me. T e
floor of the caldera slowly rises and falls. Earthquakes shake the ground. Steam suddenly blasts. Let’s visit some of these places.
A Supervolcano E
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