A geyser is a hot spring that erupts at intermittent intervals. Geysers are caused by ground water that seeps down into the earth until it runs into rock that is superheated by magma, molten rock. The magma heats the water which expands and begins to rise back up towards the surface through fractures and fissures in the earths surface. As the water rises it begins to cool and the cooler water starts to push its way back down while the warmer water is pushing up. This creates immense pressure which is eventually released through an eruption of water and steam. Since geysers require extreme heat they are usually found in areas that are commonly referred to as hot spots. Hot spots are volcanic regions where magma from the earths core rises up through cracks and fissures until it is relatively close to the earth's surface. Because these hot spots are not very common geysers are pretty rare. There are about a thousand known geysers around the world and about half of them can be found in Yellowstone National Park. That is where I took this pic of Cliff Geyser erupting.