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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 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: each eruption sends boiling water an average of 130 to 140 heat high. The eruptions range in length between about a minute and a half to five minutes. And they happen approximately every 35 to 120 minutes.

Old Faithful’s eruptions are still able to be predicted at a ninety percent confidence rate.

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 members of the Washburn expedition in 1870 noticed its incredible size and regularity. While there are are more regular geysers in Yellowstone than Old Faithful, eruption times can still be calculated at a ninety percent confidence rate.

A common misconception about Old Faithful is that it’s become more irregular since a 1959 earthquake. While the water supply to the geyser has decreased, thus increasing the amount of interval time between each eruption, it’s still possible for the eruption rates to be calculated and timed with enough accuracy for Yellowstone visitors to plan to watch Old Faithful in action during a visit to the park.

How Cone Geysers Work

What makes Old Faithful 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: cooler groundwater begins to transude beneath the Earth’s crust. Transude means to seep through a substance; think of it as similar to a drip coffee maker where water seeps through coffee grounds. When the cool groundwater approaches a heat source that’s beneath it, usually a hot magma chamber, it begins to heat up.

The water reaches a point where it becomes superheated. The water wants to boil, but can’t because of the pressure of cooler water on top of it, as well as the pressure of being below the ground. Then the water either becomes hot enough or the pressure or in some other way all that pressure is reduced and the water becomes steam.

And this is when the eruption happens! The pressure is reduced 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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