The Four Types of Thermal Features

GeoThrmlFtrSketch.jpg
Sketch of the four thermal features - all being charged by superheated groundwater.


GeyserCone.JPGGeyser: A geyser is a hot spring with the intriguing habit of tossing underground water into the air. Water falling as rain or snow seeps through porous layers of rock. Eventually that water comes into contact with extremely hot rocks that have been heated by magma underneath the park. This super hot water (up to 700 degrees F) then rises through a series of cracks and fissures underneath the surface of the Earth. In a sense, these fissures are the "plumbing system" of a thermal feature. A geyser is the equivalent of a giant pressure cooker; even though the temperature of water deep down may be well above boiling (199 degrees F in Yellowstone), the weight and pressure of the water above prevents that boiling from happening. Unlike the plumbing of the other thermal features, the fissures in a geyser have a constriction. Eventually, though, the pressure builds enough to push the water in the upper reaches up and out, causing an overflow. That overflow, in turn, relieves the pressure on the super-heated water below, causing it to flash into steam. That flash, that explosion through a narrow, constricted place in the rocks, is what sends water shooting into the air,

Hot Spring:
Hot springs let off enough heat by boiling or surface evaporationHotSpring.JPG to avoid the kind of steam explosions common to geysers. Also, they have unrestricted "plumbing". Some of Yellowstone's hot springs take the form of quiet pools. Others are flowing. The waters of many of this latter type, such as those at Mammoth Hot Springs, become charged with carbon dioxide while underground, creating a mild carbonic acid. That acid dissolves underground limestone rocks and carries the mixture to the surface of the Earth. Once on the surface, the carbon dioxide gas escapes. Without carbon dioxide, the water is less able to carry the dissolved limestone. The dissolved limestone precipitates out, creating travertine which forms beautiful terraces. In areas underlaid with volcanic rock, as opposed to more easily dissolved limestone, a modification of the plumbing system—perhaps through small earthquakes—can easily turn a hot spring into a geyser.

Fumarole
(also called steam vent): In simplest terms, a fumarole is a vent in the Earth's crust. The supply of water around fumaroles is not as plentiful as in hot springs and geysers. Modest amounts of groundwater come into contact with hot rocks underground and are turned to steam. This steam rushes up through a series of cracks and fissures and out the vent, sometimes with enough force to create a loud hiss or roar.

Mudpot:
In this feature, steam rises through groundwater that has dissolved surrounding rocks into clay; various minerals in the rocks make wide variations in the color of the mud. More often than not, such water is quite acidic, which help is the breaking down and dissolving process,

from: http://www.yellowstonenationalpark.com/geology.htm