Overview of Sedimentary Rocks
Weathering and Erosion
Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Sedimentary rocks can be composed of pieces of other rocks, which are broken down by the process of weathering.
Weathering occurs in two different ways: First, rocks can be physically broken into smaller pieces (e.g. two rocks bump into each other, and one breaks), which is called mechanical weathering. Alternatively, rocks can be decomposed and altered at the atomic level (e.g. dissolving salt in water), which is called chemical weathering. There are multiple ways each type of weathering can occur, and both the rate that rocks break down and how they break down varies dramatically depending on the area and environment.
The most prevalent type of mechanical weathering is the collision, breaking, and grinding of rock by the movement of rocks within a fluid, either water or air. The size of sediment that can be carried depends on the type of fluid and how fast it moves. For example, a rapidly flowing river can carry large particles and cause immense amounts of weathering, while a calm stream would hardly cause any weathering. The density of the fluid also controls the size of particle that can be transported; a denser fluid, like water, can carry larger particles than a less dense fluid, like air.
Another common method of mechanical weathering is called frost wedging, which occurs when water seeps into cracks in the rock and freezes. Water expands when frozen, which pushes the crack wider, eventually splitting off pieces of rock.
The addition and subtraction of heat or pressure can also cause rock to break. Rocks can shatter when cooled very quickly, and can split off in layers when immense pressure is released.
Finally, plants, animals, and humans can cause significant amounts of weathering. Weathered sediments then undergo erosion, meaning the sediments are transported away where the weathering happened. Eventually they will be deposited, and turned into a rock.
Rocks can also be chemically weathered, most commonly by one of three processes. The process of dissolution is when water breaks a mineral or rock into ions (atoms or molecules with an electric charge). Ions can then be transported with the water and redeposited if the concentration of ions increases. Evaporation is a common way that the ion concentration is increased.
Chemical weathering can also change the mineral content of a rock and weaken the original material. Minerals can undergo hydrolysis, where a hydrogen atom from a water molecule replaces the cation in a mineral; this will alters minerals like feldspar into a softer clay mineral. Additionally, a mineral can undergo oxidation, where oxygen atoms alter the valence state of a cation; this normally occurs on a metal and is commonly known as rusting.
Chemical and mechanical weathering can work together to increase the overall rate of weathering. Chemical weathering weakens rocks,