The last major caldera forming eruption occurred 600,000 years ago. For hundreds of thousands of years following that, subsequent lava flows slowly filled in most of the caldera. Even now, in some places, nearly molten rock resides as little as 2-5 miles below the surface. Heat from the volcanic activity makes its presence known by heating ground water and creating the thermal features we now see. The four basic types of thermal features present in the Park are geysers , hot springs , fumaroles , and mudpots .
Geysers are hot springs that erupt periodically.
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. Top of Page Hot Springs are similar to geysers, but their underground channels are large enough to allow rapid circulation of water.