Tufa is a variety of limestone formed when carbonate minerals precipitate out of water in unheated rivers or lakes. Geothermally heated hot springs sometimes produce similar carbonate deposits, which are known as travertine. Tufa is sometimes referred to as (meteogene) travertine. It should not be confused with hot spring (thermogene) travertine. Tufa, which is calcareous, should also not be confused with tuff, a porous volcanic rock with a similar etymology that is sometimes also called "tufa".
Tufa columns at Mono Lake, California
Barrage Tufa at Cwm Nash, South Wales
Rubaksa tufa plug, after drying of the river, in Ethiopia
Huanglong, Sichuan, China
Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain
This limestone deposit in the karst of Dinaric Alps near Sinj, Croatia, was formed in the Eocene.
Ooids from a beach on Joulter's Cay, The Bahamas
Ooids in limestone of the Carmel Formation (Middle Jurassic) of southwestern Utah.