46.4

The Mammoth Hot Spring Terraces have been a popular feature in Yellowstone since the early stagecoach routes up the Yellowstone River Valley. The Terraces, first described by the 1871 Hayden Survey, were given the name of White Mountain Hot Spring, even though they were well known and named before then.

The step-like terraces form as heated water moves along the Morris-Mammoth Fault. The hot water carries dissolved calcium and bicarbonate to the surface of the terraces where pressure lessens. Carbon dioxide then escapes as gas and the carbonate combines with calcium to precipitate as travertine.

The Mammoth Terraces are constantly changing shape and color. Springs which were active one to five years ago may be dry and lifeless now, yet activity may later resume. Along with changes of thermal activity come changes in color. Fresh travertine is bright white in color and as it weathers it changes to gray. Bright colored cyanobacteria and algae mats which were dependent upon a stable temperature and a flow of water also change as the microorganisms die creating a stark, bleak landscape.