Geyser
Geyser image courtesy U.S. Geological Survey; photo by S. R. Brantley, 1983 (fair use policy)
Geysers may be the world's favorite geological characters—loud, boisterous, full of personality, and almost totally safe to watch from close up (as long as you stay behind the rail).
Geysers are the climax of a long quiet period during which a deep network of water-filled fractures and openings grows hotter from the heat of a shallow body of magma. At some delicate point, part of the water bursts into steam, and the change in pressure causes other parts of the network to do the same. The energy is released in the only place it can go—up and out the mouth of the geyser.
The buildup of white stone around the base of the geyser is from dissolved minerals in the water. As the water cools, calcite and silica minerals come out of solution. Geyser waters are "blood samples" from the kind of underground places where metal ores and other economic deposits form.
This is Castle Geyser, in Yellowstone National Park.
Here's an article with more about geysers.
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