Chapter 4. Sedimentary Rocks
Overview of Sedimentary Rocks
Adapted by Lyndsay R. Hauber & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 10 “Sedimentary Rocks” by Bradley Deline, CC BY-SA 4.0. View source. Last edited: 8 Jan 2020
4.2 WEATHERING AND EROSION
Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Sedimentary rocks are composed of pieces of other rocks, which are broken down by a process called weathering. There are two basic ways that weathering occurs in nature. First, rocks can be physically broken into smaller pieces (e.g. hitting a rock with a hammer), which is called mechanical weathering. Alternatively, rocks can be broken down 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, therefore, 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 a fluid, either water or air. The size of the carried sediment depends on the type of fluid and speed of the movement. A fast fluid (e.g. a rapidly flowing river) can carry large particles and cause immense amounts of weathering, while a slow fluid (e.g. a calm stream) would hardly cause any weathering. The density of the fluid also controls the size of particle that can be transported; for instance, 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 puts pressure on the rock and can potentially split boulders. The addition and subtraction of heat or pressure can also cause rock to break, which can cause rocks to shatter when cooled very quickly or immense pressure is released. Finally, plants, animals, and humans can cause significant amounts of weathering. These sediments then undergo erosion, which is the transport of sediment from where it is weathered to where it will be deposited and turned into a rock.
Rocks can also be chemically weathered, most commonly by one of three processes. The first is called dissolution, where a mineral or rock is completely broken apart in water into individual atoms or molecules. These ions can then be transported with the water and redeposited as the concentration of ions increases, generally due to evaporation. Chemical weathering can also change the mineralogy and weaken the original material, which is also caused by water. A mineral can undergo hydrolysis, where a hydrogen atom from the water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into a softer cla