Travertine terraces are formed from limestone. Thermal water rises through the limestone, carrying high amounts of the dissolved limestone (calcium carbonate). At the surface, carbon dioxide is released and calcium carbonate is deposited, forming travertine, the chalky white mineral forming the rock of travertine terraces. The formations resemble a cave turned inside out. Colorful stripes are formed by thermophiles, or heat-loving organisms.
Travertine formations grow much more rapidly than the more common sinter formations in the park because of the “soft” nature of limestone. Due to the rapid deposition, these features constantly and quickly change.
As one early visitor described the Mammoth Hot Springs terraces, “No human architect ever designed such intricate fountains as these. The water trickles over the edges from one to another, blending them together with the effect of a frozen waterfall.”
Mammoth Hot Springs are a surface expression of the deep volcanic forces at work in Yellowstone. Although these springs lie outside the caldera boundary, scientists surmise that the heat from the hot springs comes from the same magmatic system that fuels other Yellowstone thermal areas. A large fault system runs between Norris Geyser Basin and Mammoth, which may allow thermal water to flow between the two. Also, multiple basalt eruptions have occurred in this area. Thus, basalt may be a heat source for the Mammoth area. 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.
Thermal activity here is extensive and has been present for several thousand years. Terrace Mountain, northwest of Golden Gate, has a thick cap of travertine. The Mammoth Hot Springs Terraces extend all the way from the hillside where we see them today, across the historic Parade Ground, and down to Boiling River. The Mammoth Hot Springs Hotel, as well as all of Fort Yellowstone, is built upon an old terrace formation known as Hotel Terrace. There was some concern when construction began in 1891 on the fort site that the hollow ground would not support the weight of the buildings. Currently, there are several large sink holes (fenced off) can be seen on the Parade Ground.
