What is a geyser?

Old Faithful

  A geyser is a special kind of hot spring. Hot water does not flow from a geyser all the time as it does from a hot spring. Instead, a geyser is quiet for a time. Then it suddenly erupts and shoots water high into the air.
For a geyser, there must be hot rock not far below the surface of the ground. There must also be a narrow, crooked passage leading up from the hot rock.
  The eruption of a geyser comes about in this way: Water fills the crooked passage. The water at the bottom gets very hot. If the passage were big and straight, the cold water at the top would gradually sink down and push up the hot water. As it is, the hot water is bottled up. It gets so hot that it begins to boil and form steam. The steam pushes some of the cold water out of the top of the tube. As soon as a little comes out, there is less cold water to press down on the hot water. The hot water then changes to steam very fast and shoots the water above it high into the air.
  Geysers are found in only a few places. There are about 200 in Yellowstone Na­tional Park in Wyoming. There are also some in Iceland and New Zealand.
  The most famous geyser is Old Faithful in Yellowstone Park. It erupts every 65 minutes or so. Visitors seldom have to wait more than an hour to see Old Faithful perform.