Tufa

Tufa is a porous, soft limestone formed by the precipitation of calcium carbonate (CaCO3) from ambient-temperature, calcium-bicarbonate-rich waters in near-surface environments such as springs, streams, lakes, and spring outlets. The primary mineral is calcite, and the rock often has a microcrystalline, spongey texture. Tufa develops when dissolved calcium carbonate becomes supersaturated as carbon dioxide degasses from water or when biological activity raises the pH, promoting precipitation and accumulation around substrates like vegetation, rocks, or shells.

Tufa deposits exhibit layered sheets, shelves, and sometimes tower-like structures along shorelines, spring outlets, or waterfall margins. Environments range from subaerial spring vents to streamside and lake-edge settings; cave surroundings with seepage can host tufa as well, though stalactites and stalagmites are more accurately called speleothems. The term is sometimes contrasted with travertine, which is typically denser and forms in warmer, near-spring or hot-spring environments; however, the two terms are not always used consistently, and there is overlap in their usage.

Tufa holds value for geology and paleoclimatology, recording past water chemistry, temperature, and hydrological changes. Modern tufas can help illuminate regional hydrology and climate history, and the deposits may also preserve fossils and serve as landscape features in arid and semi-arid regions. Notable examples include tufa formations along lake and spring systems, such as those seen at Mono Lake, California.