So, when we find a limestone, we know that there was a large lake or ocean present at that location in the past. When we find evaporites, we can tell that we were probably in a hot and dry environment such as a desert playa. Find Your Rock:. Halite, Gypsum Selenite, Limestone, and Limestone with sea creatures. Chemical Sedimentary Rocks Chemical sedimentary rocks form by precipitation of minerals from water. In many such areas, carbonate-rich sediments also accumulate in quiet lagoons, where mud and mollusc-shell fragments predominate Figure 6.
Limestone also accumulates in deeper water, from the steady rain of the carbonate shells of tiny organisms that lived near the ocean surface. The lower limit for limestone accumulation is around 4, m.
Beneath that depth, calcite is soluble so limestone does not accumulate. Calcite can also form on land in a number of environments. Tufa forms at springs Figure 6. Similar material precipitates within limestone caves to form stalactites , stalagmites, and a wide range of other speleothems. Dolomite CaMg CO 3 2 is another carbonate mineral, but dolomite is also the name for a rock composed of the mineral dolomite although some geologists use the term dolostone to avoid confusion.
All of the dolomite found in ancient rocks has been formed through magnesium replacing some of the calcium in the calcite in carbonate muds and sands. This process is known as dolomitization , and it is thought to take place where magnesium-rich water percolates through the sediments in carbonate tidal flat environments.
In some cases, chert is deposited along with limestone in the moderately deep ocean, but the two tend to remain separate, so chert beds within limestone are quite common Figure 6. In other situations, and especially in very deep water, chert accumulates on its own, commonly in thin beds.
Interesting Facts About Quartzite. Differences Between Extrusive and Intrusive Rocks. Why Is the Rock Cycle Important? Most Common Types of Igneous Rocks. The Types of Seafloor Sediments. Three Types of Aqueous Reactions.
Marble Vs. Factors Affecting Weathering Processes. What Factors Determine the Rate of Weathering? If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.
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If no button appears, you cannot download or save the media. Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock. Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth.
Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image Sedimentary Rock An example of a sedimentary rock, which is, by definition, composed of many, smaller rocks. Photo courtesy of Alamy Stock Photo.
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