Sizzling Sites Pushed by magma and heat, the land here has risen up to 7 cm a year recently. We can’t see or feel the change as we hike. T ere are plenty of other signs of that bubbling heat deep beneath our feet, though. We reach our first example of a geothermal
hot spot, or a place where the heat comes from Earth’s interior. We’re standing at the edge of a huge hot spring. T is pool looks like a jewel. Its center is a deep, clear blue. A rainbow of colors—light blue, green, yellow, orange, and red—rim the pool. It’s pretty, but don’t dive in. T e water here
can reach 87°C, close to the temperature at which water boils. It’s so hot that steam rises from the water. T e rainbow colors come from algae and bacteria that thrive in the hot water.
Rising Heat T e hot spring is just one of the thousands of hydrothermal, or hot water, features that dot this park. T e water here starts as cool rain or melting snow. It seeps into the ground through cracks in the rock. Eventually, the water sinks so deep that it reaches the hot rock just above the magma chamber. It heats up. Hot water weighs less than cold water, so it
rises toward the surface. When it reaches the surface, the superheated water cools and gets heavier. Pulled by gravity, it sinks again. Over and over, the water sinks and heats,
then rises and cools. It follows a circular path, flowing in a convection current. T is pattern of movement is one way in which Earth’s heat is transferred from one place to another. It occurs in hot springs. It happens in geysers, too.
FAST FACT: Half of Earth’s geysers—more than 300—are in Yellowstone.
14 NATIONAL GEOGRAPHIC EXTREME EXPLORER
Gushing Geysers In a geyser, the hot water doesn’t pool. It blasts. T at’s because it travels through smaller cracks in the rock. It gets trapped. When that happens, Earth’s heat raises the water’s temperature past boiling. It changes to steam. T en, whoosh! T e steam squeezes through the cracks and erupts. A fountain of steam and water shoots sky high. A geyser will continue erupting until it uses
all of its water. T en more water seeps into the cracks. It heats up, turns into steam, and then erupts again. For some geysers, it’s a predictable pattern that we can rely on as we study them. Any change in this pattern could be a signal of changes deep underground. For now, Old Faithful continues to erupt on schedule. So do many of the hundreds of other geysers here.
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