Geological Survey Bulletin 1444 Geology and Thermal History of Mammoth Hot Springs

Aquifer. Body of rock containing substantial water in its fractures and open spaces.

Artesian pressure. Pressure exerted by the weight of a vertical column of water extending up to the surface of the water table.

Caldera. Large depression, commonly many kilometers in diameter, formed by collapse of the ground surface after violent eruption of gas-charged volcanic rocks; near-surface ground collapses into the space vacated by the erupted rock.

Extrusive rocks. Rocks originating from volcanic activity.

Fissure ridges. Linear mounds of travertine deposited from hot-spring vents along a medial fracture zone that may be partly concealed under the travertine. Fissure ridges range in height from 1 to 6 m and in length from a few meters to nearly 300 m; width at the base of a ridge is equal to or greater than its height. The term fissure ridge was first used by Hayden (1883) to describe the travertine ridge at the east edge of Prospect Terrace.

Fumarole. A vent from which vapors are emitted.

Intrusive rocks. Rocks formed from magma that moved from its place of origin but solidified before reaching the surface.

Isotopic. Pertaining to isotopes, which are different species of the same chemical element that differ in atomic weight.

Magma. Molten rock contained in a magma chamber.

Magma chamber. A reservoir of molten rock that supplies the material composing volcanic rock on the Earth's surface.

Orogeny. Process of mountain building.

Terracettes. Semicircular travertine ledges formed by deposition of travertine around slowly rising pools; usually are from about 0.3 to as much as 2.5 m wide,

Travertine. Calcium carbonate (CaCO₃) precipitated from hot-spring waters.