On the second-from-the-last day of our week in Yellowstone, we took a day off from wildlife to see geologic features. At the end of the day, just after seeing the bear cub eating in the meadow, we stopped at Mammoth Hot Springs.
Here’s the hill of travertine at Mammoth Hot Springs. Unlike other Yellowstone hot springs that are rich in silica, Mammoth Hot Springs is a hot spring in a limestone area. Limestone, rich in calcium, is formed in oceans by coral. As hot water is pushed through limestone, the calcium dissolves and is carried to the surface, where the water evaporates and leaves the calcium behind. Limestone deposited from a hot spring is called travertine.
Most of the hill is white, but the terraces on the left are a more-interesting, rusty color. They’re called Minerva Terraces.
The photo below, taken at a wider angle, shows the wide variety of colors in the surrounding terraces. As with the other hot springs, the color comes from microbes or impurities.
Travertine is used as a building material; the Roman Colosseum is made of travertine. Limestone, rich in calcium, is a key ingredient in Portland cement; there is a cement plant in Cupertino, California, near our home.