First is a geyser.   A geyser is a hot spring that throws underground water into the air.  Rain and snow melt seep through porous layers of rock and eventually comes into contact with hot rocks, usually heated by molten magma.  The hot water rises through cracks and fissures in the ground.  Even though the temperature down deep may be well over boiling, the weight and pressure of water above prevents the boiling from happening.  Eventually, the pressure builds enough to push some of the upper water up and out.  That overflow relives the pressure on the super-heated water below, causing it to flash into steam.  The flash of steam in the narrow constricted cracks is what shoots water high into the air.

    The second thermal feature is a hot spring.  Hot springs let off enough heat from boiling or surface evaporation to avoid the steam explosions that make geysers.  Some are quiet pools while others flow freely.  While underground, the water often becomes charged with carbon dioxide,creating a mild carbonic acid.  This acid dissolves the limestone rocks while flowing upwards.  Once on the surface, the carbon dioxide gas escapes,  leaving the water less able to carry dissolved limestone.  The limestone then precipitates out creating beautiful travertine terraces.

   Fumaroles, or steam vents, are the third type of thermal.  Simply put, fumaroles are "smoking" vents in the Earth's crust.  The level of groundwater around fumaroles is much less than around hot springs or geysers. Water comes into contact with the hot underground rocks and is turned to steam.  The steam rises up through the cracks and fissures in the ground and rushes out the vent, often with a loud hiss or roar.

    The last thermal feature is mudpots.  Mudpots are created when steam rises through groundwater that has dissolved surrounding rocks into clay.  Minerals in the rocks color the mud a wide variety of shades.  Often, the water is quite acidic, which helps break down and dissolve the rocks.