How Sedimentary Rocks Form

Sedimentary rocks cover about three‐fourths of the surface of the continents. There are three kinds of sedimentary rocks: clastic, chemical, and organic. Clastic sedimentary rocks form from the consolidation of material such as gravel, sand, or clay (sediment) derived from the weathering and breakdown of rocks. Chemical sedimentary rocks result from biological or chemical processes, generally under water, that crystallize minerals that accumulate on the sea floor. Organic sedimentary rocks, such as coal, have as their major component accumulations of organic remains from plants or animals that make the rock distinctive.

As the sediments become buried under other sediment layers, pressures and temperatures increase. The sediment hardens into a sedimentary rock, or lithifies, after it has gone through the stages of compaction, dewatering, and cementation. During compaction, the grains of sediment are packed more tightly together. With increasing pressure some of the water between the sediment particles is squeezed out, dewatering the sediment. This process reduces the pore space, or open spaces between the grains. At this point, pressure and temperature conditions are such that certain minerals, usually calcite or quartz, fill some or all of the pore spaces and adhere to the sediment fragments, cementing them into a sedimentary rock.

A rock formation is an occurrence of rock with a set of characteristics that distinguishes it from the rocks above or below it. A formation can then be broken down into smaller rock layers called members. A sedimentary contact is the boundary surface between two different kinds of rocks and is usually a straight line that represents the original surface where one sediment type was deposited on another.