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Under Pressure — Geysers

Under Pressure — Geysers

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 For centuries, geysers have captured our imagination, in places like Yellowstone National Park, where 4 million visitors flock each year. This is partly because geysers are spectacular, and partly because they’re rare. There are only about 1,000 worldwide, and nearly half of them are in Yellowstone. Most others occur in just five countries. Why are there so few? They require very specialized geology. At the surface, they need caprock, to trap water. In the subsurface, they need fissures in the rock, so water can flow into and collect in reservoirs and cavities. Below that, they need intense heat—all geysers are in volcanic areas. The bottom of the geyser’s water column is closest to the heat, under higher pressure—which raises the boiling point. So the water keeps heating without converting to steam. The heat travels up the water column, eventually reaching the top. There, the pressure is lower, so the water can boil. As it turns to steam, it releases pressure on the water just below it. Which can now boil, releasing pressure on the water further down, and so on. This chain reaction produces huge volumes of steam, which erupt out the top of the geyser. In large geysers, steam can carry thousands of gallons of boiling water into the air, in fountains that can last a few seconds to a few hours and reach heights up to 300 ft. If you go to see one, prepare to be amazed. It’s one of nature’s greatest shows.
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