Travertine terraces at Mammoth Hot Springs, Yellowstone National Park

Calcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation.

Travertine is a terrestrial sedimentary rock. It is a natural chemical precipitate of carbonate minerals; aragonite or calcite. This happens from solution in ground and surface waters, and/or geothermally heated hot-springs. Similar (but extremely porous) deposits formed from ambient-temperature water are known as tufa.

The Colosseum built by the Romans, was made of travertine. view more...