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Old Faithful

Geysers spout up where a column of underground water is heated to boiling by "magma," or molten rock, pushing the water upward.

Old Faithful erupting

Photo: CircumerroStock (flickr)

Old Faithful erupts periodically because of an unchanging natural process of water boiling underground

In Yellowstone National Park you can visit the geyser Old Faithful.

It’s a spectacular sight, as each four-minute-long eruption sends boiling water over a hundred and fifty feet high. On today’s Moment of Science, we’ll learn how a geyser works.

Geysers spout up where a column of underground water is heated to boiling by “magma,” or molten rock. Like a pan boiling over on your kitchen stove, this boiling pushes the water upward.

Old Faithful got its name because it used to erupt– faithfully–every sixty-five minutes. It’s been more irregular since a 1959 earthquake disrupted its natural, underground plumbing.

If Old Faithful were exactly like a pan on a stove, you’d expect the water to bubble out continually. What makes it alternate between quiet dormancy and exploding action?

This has to do with the way a deep column of water boils. Water’s boiling point is dependant on its pressure. At sea level, water boils at 212°F. Deep under Old Faithful, where the molten rock is, the pressure is much greater. Water won’t boil here until it’s over 300°.

What happens is this: The water column gradually heats, but high pressure keeps the deeper parts from boiling. Finally, the column gets hot enough for the top part to boil off. As the top turns to steam, this reduces the pressure on the deeper water–and lowers its boiling point!

Remember, this deep water is already quite hot. Reducing the pressure makes it boil immediately, further reducing pressure for even deeper water. It’s a rapid chain reaction that makes the whole column boil at once, erupting a spout high into the air. After each eruption, the column refills with cool water from an underground source, and the whole cycle begins again.

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