Sedimentary rocks are often found around bodies of water or aqueous environments. This is because water is an important element in the formation of sedimentary rocks. An example of how a sedimentary rock forms is through erosion of a rock due to the forces of water in a river and soon, sediments form and build up from other types of eroded rocks until it becomes cemented and forms a new rock. The newly formed rock is what can be called a sedimentary rock. On this note, it is important to know that sedimentary rocks are first and foremost classified according to the type of rock it originated from. In the following sections, the different classifications of sedimentary rocks will be discussed.
Detrital/Clastic or Chemical
According to Sills in his 2003 published book entitled “Earth Science the Easy Way,” sedimentary rocks can originate from either an eroded rock or a chemically weathered soluble rock. The first origin of sedimentary rocks is as discussed in the introduction in which a rock undergoes erosion and a new rock is formed from other eroded particles that form or cement together. In this process, the solid components of the rock are of “detrital” or “clastic” origin. On the other hand, sedimentary rocks can also form from a soluble material that underwent chemical weathering. In this process, a solid material initially dissolves via chemical weathering but when the water that keeps the particles dissolved evaporates, the particles lose its solubility and turns to solid particles again that form the sedimentary rock. This sedimentary rock is said to be of “chemical” origin.
Clastic-derived sedimentary rocks
Sills enumerated five identified rocks under this classification that include conglomerate, breccia, sandstone, siltstone, and shale. These sedimentary rocks are composed mostly by quartz, feldspar, and minerals from clay. Moreover, it can be expected that these rocks contain minerals and fragments from other rocks because it was derived from other rocks. Conglomerate can be described as having rounded fragments as opposed to breccia in which fragments are angular. Sandstone contain fine to coarse particles while siltstone has very fine grains. The last type is shale in which its particles are very compact and may break easily.
Chemically and organically-derived sedimentary rocks
Rock salt, rock gypsum, dolostone, limestone, and coal are the five types of sedimentary rocks of chemical origin as listed by Sills. Rock salt is composed of halite; rock gypsum is made up of gypsum; and dolostone contains dolomite. These chemical components of the rock crystallized to form the rock as a result of evaporation. In addition to this, limestone is composed of calcite, which is of biologic origin such as skeletons of dead marine creatures. Finally, coal consists of carbon from the remains of plants. It is important to note that the first three types of chemically-derived sedimentary rocks have a crystalline texture, while the last two have a bioclastic texture because they originated from biological life.
Sedimentary rocks can be formed from particles of eroded rocks or from precipitates of soluble particles. These two origins of sedimentary rocks are called detrital/clastic origin or chemical origin, respectively. Each classification has five distinct rock names in which rocks are classified based on their physical characteristics and particle composition.