Geology And Geophysics

Evaporates Chemical Limestone and Tufa are three Types of Chemical Sedimentary Rocks

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Sedimentary rocks are created when particulates accumulate and are consolidated into rock. Chemical sedimentary rocks are a special type of sedimentary rock formed from chemical reactions. These rocks are made when materials dissolved in water precipitate and later harden with the other matter particles. This process occurs in oceans, lakes, and places where hot springs encounter the earth’s surface. There are three types of chemical sedimentary rock, distinguished by their method of formation: evaporates, chemical limestone, and tufa. 


When water evaporates from a lake or part of the ocean, evaporate sedimentary rocks are formed. When water evaporates it is changing from a liquid to a gas. The molecules and minerals dissolved in the water are not always capable of the same phase change, like calcium, sodium, and chlorine. The water remaining becomes increasingly rich with whatever is left behind. For chemical sedimentary rocks to form, they must be in the ideal climate where evaporation is faster than rainfall. Playa lakes in deserts that form and then evaporate as well as places where deserts border oceans are prime locations. Evaporates, like halite (NaCl, table salt), are light in color and readily recognized.

Chemical limestone

The majority of limestone is created in a biochemical process with the exception of travertine, or chemical limestone. Travertine tends to form outcroppings and other shapes like stalactites and stalagmites in caves. There is also a notable travertine terrace in Yellowstone National Park. These chemical limestone formations occur when water is rich in calcium and carbonate ions. The water either evaporates or the carbon dioxide escapes the carbonate ions. When either of those two processes take place, it leaves the calcium behind to build up. In the case of stalactites and stalagmites, the slow dripping of the water makes the calcium and carbonate bond together, making calcite. The water dripping down makes stalactites and as it drips to floor it builds up to make the stalagmites. Sometimes the two will merge together forming limestone columns. 


Natural springs flowing into lakes creates tufa. Calcium, carbonate, and other ions form naturally around the vent of the spring and as the spring seeps into the lake, precipitation forms. Tufa tends to create porous, cauliflower-shaped formations. As the waters of California’s Mono Lake lowered, towering tufa structures of calcium-carbonate were exposed and a reserve was established surrounding them in order to preserve them. The Trona Pinnacles in the Mojave Desert are also tufa formations, reminiscent of a time when the region was less dry.

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