Weathering: Definition, Types, Causes & Rates

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  • 0:01 What Is Weathering?
  • 0:40 Physical Weathering
  • 1:51 Chemical Weathering
  • 2:42 Biological Weathering
  • 3:12 Changing Rates of Weathering
  • 4:00 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Ever wondered how erosion could create something as dramatic as the Grand Canyon? The simple answer is weathering. This lesson describes the three major types of weathering, their causes, and how weathering rates can change.

What Is Weathering?

Chances are you're pretty familiar with the idea of erosion, when water and air slowly strip away layers of soil and rock. However, how could air and water cause such dramatic landscapes, like canyons and fjords, from seemingly solid rock? The answer is relatively simple. Weathering wears down these otherwise solid structures, making it much easier for other processes to change a landform.

In this lesson, we're going to look at the three types of weathering and their causes. Finally, we will examine how rates of weathering can change and how their impact on landforms is of great interest to humanity. But first, the three types of weathering - physical, chemical, and biological.

Physical Weathering

Physical weathering occurs when there are physical changes to the landform's rocks or soil. While we may think of rock as pretty solid, it is actually full of tiny cracks and crevices. Water fills up these cracks and crevices and expands and contracts with changes in temperature. When water freezes, it forms ice crystals that are actually pretty hard. On a molecular level, these ice crystals weaken the bonds of the rocks.

Also, erosion can cause a slippery slope when it comes to weathering. Let's say that there is a solid rock surface, where the force of the rock pressing down is equal at any point under the landform. Then erosion comes along. As anyone who has seen the effects of erosion at places like the Grand Canyon can attest, erosion is not an even process. It leaves ridges and valleys on the landform itself. Since there is less mass of material pressing down, these ridges and valleys are pushed up by geological forces far underground. In turn, these ridges and valleys provide new opportunities for water to freeze and thaw, as well as for other forms of weathering.

Chemical Weathering

Chemical weathering occurs when a chemical reaction causes a change in the landform's geological profile. Take a hill that has considerable amounts of iron ore. If other factors expose that iron ore, a chemical reaction can occur between the iron and air, creating rust. Iron may be a very solid material, but rust can be brushed away with a firm brush. As a result, rust is easily pushed away by water and air, leaving space where there was once solid iron ore rock.

Many types of chemical weathering are increased in strength when rainwater is more acidic. This has become increasingly common due to elevated levels of pollution. The more acidic water leaches into landforms and reacts with the minerals that make up a given area. The reaction is especially dramatic when acidic rain affects limestone, a common layer for many landforms.

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