Hydrothermal features

Simplified diagram showing the underground structure of a typical geyser basin. Fumaroles form when boiling occurs below the ground surface, discharging water vapor (steam) or liquid water. The temperature of hot springs is either at or below the temperature of boiling water at the ground surface. In a geyser, liquid water and steam erupt episodically after water and steam bubbles accumulate in a side reservoir (“bubble trap”) where pressure builds prior to an eruption. These underground bubble traps are likely only a small part of much larger subsurface fracture and reservoir complexes. The source of the heat is usually an active or old dormant magmatic system and the source of water is precipitation recharged into the subsurface.