10.2: Weathering and Erosion

Overview

Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Basically, sedimentary rocks are composed of the broken pieces of other rocks. The obvious place to start this chapter is a discussion of how rocks are broken down, which is a process called weathering. There are two basic ways that weathering occurs in nature. First, rocks can be physically broken into smaller pieces (imagine hitting a rock with a hammer), which is called mechanical weathering. Alternatively, rocks can be broken down and altered at the atomic level (imagine dissolving salt in a glass of water), which is called chemical weathering. There are multiple ways each type of weathering can occur and, therefore, both the rate that rock breaks down and how it breaks down vary 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 (like a rapidly flowing river) can carry large particles and cause immense amounts of weathering while a slow fluid (like 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, denser fluid (like water) can carry larger particles than less dense fluid (like air). Another common method of mechanical weathering is called frost wedging, which occurs when water seeps into a crack in the rock and freezes. Water has a unique property in that it 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 rocks to break, for instance spilling cold water on a hot light bulb will cause it to shatter. This breakage can also occur with rocks when they cool 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, which you are probably familiar with, is called dissolution. In this case, a mineral or rock is completely broken apart in water into individual atoms or molecules. These individual ions can then be transport with the water and then re-deposited as the concentration of ions increases, normally because of evaporation. Chemical weathering can also change the mineralogy and weaken the original material, which again is caused by water. A mineral can undergo hydrolysis, which occurs when a hydrogen atom from the water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into a softer clay mineral. A mineral can also undergo oxidation, which is when 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 making them more prone to breaking physically, while mechanical weathering increases the surface area of the sediment, which increases the surface area that is exposed to chemical weathering. Therefore, environments with multiple types of weathering can erode very quickly. As you go through the following sections (on rocks and environments) think about the types of weathering required to make the sediment that will then make up different types of sedimentary rocks as well as what types of weathering you would expect to occur in different environments.