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What makes a geyser erupt?

A geyser needs a water source, usually from miles under the Earth's surface. The geyser also needs a heat source, supplied by the Earth's magma, about 3 miles (4.8 kilometers) under the surface of the Earth.  The magma heats the water, but hot water isn't enough.  Another key factor is pressure.

The water at the top of the geyser puts pressure on the water at the bottom of the geyser shaft. Water under pressure reaches very high temperatures and creates a lot of energy. When the water reaches the boiling point, it begins to come out of the geyser's mouth. The pressure on the water that's left inside the geyser decreases and the remaining water turns to steam as the temperature also decreases.  The steam expands, and the geyser erupts in an explosion of water and steam.  When the geyser runs out of water and cools down, the eruption stops.