The four basic types of thermal features present in the Park are geysers, hot springs, fumaroles, and mudpots. Many of these are concentrated in Yellowstone’s major geyser basins: Upper, Midway, Lower, Norris, West Thumb, Shoshone and Heart Lake.

Geysers are hot springs that erupt periodically. The eruptions is the result of super-heated water below-ground becoming trapped in channels leading to the surface. The hottest temperatures are at the bottom of these channels (nearer the hot rock that heats the water) but the deep water cannot vaporize because of the weight of the water above. Instead, steam is sent upwards in bubbles, collecting in the channel’s tight spots until they essentially become clogged, leading to a point where the confined bubbles actually lift the water above, causing the geyser to overflow. This causes the pressure to decrease until suddenly violent boiling occurs throughout much the length of the column, producing a tremendous volume of steam which forces the water out of the vent in a superheated mass. This is an eruption. As the eruption continues, the heat and pressure gradually decrease, and the eruption stops when the water reservoir is depleted or the steam runs out. The two types of geysers are fountain geysers (which shoot water out in various directions through a pool) and cone geysers (which shoot water out in a fairly narrow jet, usually from a cone-like formation).