Bubbling Mud Earth’s heat doesn’t just show up on the planet’s surface as rainbow pools or exploding steam. Sometimes, it bubbles up through mud—and stinks. T at smell is how we know we’re getting close to our next geothermal feature. A smell of rotten eggs fills the air. T at tells
us that we’ve reached the mud pots. T ey’re hot springs with hardly any water. Here, steam and gases rise to the surface. Big bubbles bulge in the mud. T ey grow bigger and bigger and bigger, until pop! T ey break, mud spatters, and a stinky smell is released. By now, we know where the heated water
comes from. T e rotten egg smell comes from the same place. It’s hydrogen sulfide, a gas that comes out of magma. T e gas doesn’t just smell. It also can turn into an acid that breaks rock down into clay, forming a pool of mud. Mud pots might not be as exciting as other
Yellowstone features, but they add to the park’s chorus of sounds. While geysers rumble and roar, mud pots gurgle, burp, and pop.
Steam Vents Next, we head to Roaring Mountain. It feels like a diff erent world here. Steam pours out of the ground like white clouds. We hear a hissing sound all around us. T at’s because fumaroles surround us. T ese geothermal features are also called “dry geysers.” T e underground water in a fumarole never
reaches the surface. Earth’s heat turns it into steam first. T en the steam and gases hiss out of the ground through vents. T ese fumaroles are the hottest geothermal features here. We’ve measured temperatures up to 138°C at some. Few trees and plants can live amid the heat
here, so the land is mostly bare and empty. We scientists love it, though. It helps us understand more of what’s going on down below.
16 NATIONAL GEOGRAPHIC EXTREME EXPLORER
Forest of Stone Our last stop on the tour isn’t hot now. It once was, though. Welcome to one of the world’s largest petrified forests. Here, volcanic heat and minerals turned the trees into stone. A thick forest grew here about 50 million
years ago. Redwood and other tall trees reached for the sky. Dogwood trees flowered. Breadfruit and walnut trees grew fruit and nuts. T en nearby volcanoes erupted. Mud, ash,
and other volcanic debris buried the trees. T e covering was so thick that oxygen couldn’t get in. As a result, the trees couldn’t rot. Instead, minerals in volcanic rock seeped into the wood. T ey hardened into stone. T is petrified forest shows us the power of supervolcanoes. T ey can change materials and living organisms on Earth’s surface.
Ready to Rumble? T e big question is whether this supervolcano will erupt again. T e answer is yes, but we don’t know when. T ere are no signs it’ll happen any time soon. We know the volcano is alive and well, though. T e hissing fumaroles, bubbling mud pots, and gurgling geysers tell us that. All of this activity is normal. It’s nothing
to worry about. Other signs will tell us if the supervolcano is getting restless. An increasing number of small and medium earthquakes may rattle the park. Lava may seep out of cracks in the ground and flow across the land. T ese signs will tell us that the supervolcano’s magma is rising closer and closer to the crust. We aren’t seeing any unusual signs now. If
we ever do, we’ll have weeks, or even months, of warning. T at’s a good thing. We’ll need time to prepare. Just like millions of years ago, when this supervolcano erupts, it will change life on Earth. For now, though, its heat has created one of the most amazing places in the world to visit.
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