Types of Sedimentary Rocks

Sedimentary rocks form at Earth’s surface and their formation involves a long list of external geologic processes. Sedimentary rocks are classified by the processes responsible for their formation. These categories include Clastic, Biological, and Chemical.

Clastic sedimentary rocks form when sediments (small pieces of minerals and rocks) are lithified. Sediments are created when preexisting rock is weathered over time. Sediments are transported by wind or water (erosion) and deposited in another location (deposition). Over time, these sediments are compacted and squeezed and eventually become naturally glued together (lithification). In the image above, sand particles were deposited and formed layers of sandstone seen in Bryce Canyon National Park and Arches National Park in Utah. Sandstone is an example of a clastic sedimentary rock.

Biological sedimentary rock forms as a result of organic material being compacted over millions of years and exposed to some heat. The natural water in this organic material is squeezed and cooked out over time. A common biological sedimentary rock is coal. Biological sedimentary rock can also form from the bones or shells of marine organisms. Chalk is a type of limestone and is formed by the microscopic plankton that fall to the ocean floor and accumulate over time. The plankton have shells made of calcium carbonate, which is compressed over time, forming chalk and other types of limestones.

Chemical sedimentary rocks form as a result of minerals precipitating out of water. As water moves through preexisting rock, it dissolves the minerals contained within that rock. The water carries away these dissolved minerals and over time as the water evaporates, the minerals are precipitated out of the water. This is commonly seen in cave systems created from minerals precipitating out of groundwater. These mineral formations can be seen as stalactites and stalagmites in cave systems.