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

Thermal water is heated by partly molten magma in a fault zone under the Norris-Mammoth corridor. During transport along the fault, the water is enriched in calcium and bicarbonate, the two ingredients necessary for the formation of travertine.

The trend of White Elephant Back Terrace and the tension fractures to the southwest of this fissure ridge suggest that the thermal water moves up through the old terrace deposits along preexisting vertical linear planes of weakness. As the water reaches the surface, pressure is released, carbon dioxide escapes as a gas, and bicarbonate in the water is partitioned into more carbon dioxide and carbonate; the carbonate then combines with calcium to precipitate calcium carbonate, forming travertine. Most travertine has precipitated rapidly from solution and is lightweight and porous; however, a few fissure ridges contain denser, vertically banded travertine layers that line the fissure. These vertical bands were apparently deposited over a long period of time. Dense travertine also forms beneath the surface of the terraces by deposition in the pore spaces of older deposits.

Some hot springs are pools with little or no deposition of travertine, whereas others form magnificent arrays of terracettes, cone-shaped deposits, or linear mounds called fissure ridges. Minerva Spring consists of an intricate collection of terracette deposits that are greatly enhanced by variegated algal growth. Terracette deposits of' New High land Springs occur as spectacular overhanging ledges. A few hot-spring cones are active on the upper terraces, but the best example of this type of deposit is the long-inactive Liberty Cap. Fissure ridges occur in numerous sizes and shapes. In some places water from new hot-spring activity becomes ponded behind fissure-ridge barriers or dams, with the result that travertine deposits eventually form large flat terraces.

Sporadic records of hot-spring activity at Mammoth suggest that most springs have been intermittently active at least since 1871. Another characteristic of the hot springs is the colorful algae and bacteria that thrive in the thermal springs and their discharge areas. When discharge of a hot spring ceases, the microorganisms die and the travertine deposits become white and then dingy gray. With the passage of time, the surface travertine deposits develop a loose soil capable of supporting the growth of small plants and eventually large trees, such as occur on Pinyon Terrace.