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I hear it hiss. I feel it rumble. I check my watch. Get ready. It’s coming in 5…4…3…2…1! A blast of hot water shoots from the ground. T e spray soars 40 meters in the air, then rains down in a frothy mist. It’s another eruption of Old Faithful. I can always count on Old Faithful. T is hot


water geyser erupts almost every 90 minutes. Each time, it puts on a great show. We’re standing on top of one of the world’s


largest volcanoes. In fact, it’s so large, it’s called a supervolcano. We can’t see it, though. It’s hidden under the ground in Yellowstone National Park in the United States (U.S.). T is park is a natural wonder. It’s fi lled with


incredible geologic features. Let me tell you about about this place as we take a tour.


Heat Beneath Your Feet Earth is a dynamic place. It’s always changing. Some changes can take millions of years. Other changes happen right before your eyes. As a scientist who studies this supervolcano,


I see signs of these changes every day. I watch geysers erupt, and I can see layering in rocks that took millions of years to form. T ese forces dramatically change the landscape. T e cause of the changes here is hidden in a


place deep beneath our feet. T at spot began to heat up more than 15 million years ago. Here’s how. Under Earth’s thin crust lies the


mantle. T is is a layer of hot rock. Sometimes, an area of the mantle get superheated and becomes a hot spot. T e hot spot melts rock into a liquid called magma. Far below where you and I are standing lies


a churning cauldron of boiling magma. Over time, heat and pressure build up in this huge magma chamber. When the pressure becomes too great, the


magma bursts through Earth’s crust. T at’s when a volcano erupts. T is process repeats again and again each time the pressure builds. With a supervolcano, the eruption is usually big. T at’s what happened here about 2 million years ago.


18 NATIONAL GEOGRAPHIC EXPLORER


Too Hot To Handle Aſt er heat and pressure built up over thousands of years, this supervolcano blew its top. T e force of the eruption was so great, the ground beneath Yellowstone collapsed. It created a crater, or caldera, nearly 75 km wide. Massive amounts of lava and ash spewed


upward. T e eruption buried most of the western half of the U.S. in ash. Wind carried sulfur, ash, and other chemicals from the eruption around the planet. Towering gray clouds of ash blotted out the sun, and global temperatures dropped for a time. T e whole world felt the impact of this eruption. Since this event, there have only been two


other major eruptions here. T e second 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 fi ll the Grand Canyon in the U.S. T ese eruptions changed the landscape. T ese major eruptions happened a long


time ago. Yet Yellowstone is still active. As the pressure below ground changes and builds, the fl oor of this ancient caldera rises and falls as if it were breathing. T e ground shakes thousands of times a year here with minor earthquakes. All around the park, there are signs of activity. With each action, the landscape changes.


A Supervolcano


magma chamber


Boiling rock, called magma, collects in a magma chamber. Heat and gases build up over time, creating pressure.


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