Even if a geyser didn’t shoot great streams of water into the air, it would be one of the most interesting marvels of nature. A geyser is really a hot spring, and a hot spring itself is quite amazing. Here is a hole in the ground filled with hot water. Where does the water come from? Why is it hot? And what makes it shoot up into the air if it’s a geyser?

In all geysers, a hole called a tube leads from the surface to underground reservoirs which serve as storage basins for the water. Most of the water comes from rain and snow.

Deeper in the earth, the rock is very hot. This is probably uncooled lava, which is called magma. Gases from these hot rocks, mostly steam, rise through cracks in the rock and reach the underground reservoirs. They heat the water there to boiling and above-boiling temperatures.

This is how a hot spring is created. Now what makes it a geyser? The tube, or passageway from the water to the hot rocks below (where the heat comes from), does not go straight down in a geyser. It is twisted and irregular. This interferes with the natural rise of steam to the surface. If the steam and water can rise freely from below, we have a steadily-boiling hot spring.

The geyser erupts because the water in the irregular, or trapped, section of the underground water system is heated to the boiling point and suddenly turns to steam.

Steam requires more room than the water from which it was formed. So it pushes up the column of water above it. As this steam moves up, it lowers the pressure below, and more water turns into steam. Instead of there just being an overflow at the surface, there is a violent eruption as a result of the steam bursting upward, and we have the spectacle of a geyser!