What Are Clastic Rocks?

Formation

• Clastic rocks are formed when weathering causes large rocks to break apart into smaller pieces, known as sediments. These sediments are then moved away from the original site by natural processes like wind, rain and the action of rivers. Once the transport process loses enough energy, the sediments are deposited, with larger pieces settling more quickly than smaller ones. Seawater or groundwater traveling through the new sediment deposit leaves behind chemical traces that form mineral crystals, known as cement. This cement binds the sediments together, allowing the formation of rocks, a process known as lithification.

Classification

• Clastic rocks are classified by the size of the particles, or grains, that comprise them. The size of the particles within a rock reflect the energy of the process that formed it, with larger particles reflecting higher energy processes. The scale for particle size has six categories based on the diameter of the grains within the rock. Going from largest to smallest, the categories are: boulders, cobbles, pebbles, sand, silt and clay.

Sorting

• Clastic rocks can be further classified by the uniformity of the grains present. If most of the particles within a rock are of similar size, it is said to be "well sorted," whereas a rock with a wide range of grain sizes is "poorly sorted." Well-sorted rocks have usually been exposed to processes that selectively concentrate grains of similar sizes and continually rework them over time, like the action of waves or the wind.

Examples of Clastic Rocks

• Breccia and conglomerate are clastic rocks that are composed of at least 30 percent gravel-sized fragments. Breccia has angular cornered fragments, while the fragments in conglomerate have rounded corners. Sandstone is made up of sand-sized particles. The most common types of sandstone are quartz and feldspar, also known as arkosic sandstone. Clastic rocks made up of silt, clay, or a mixture of the two, including siltstone and shale, have particles too small to see with the naked eye and so are classified by the properties of the rock as a whole.