Physical Weathering: Definition, Types & Examples

Physical Weathering: Definition, Types & Examples
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  • 0:03 Physical Weathering
  • 1:04 Types and Examples
  • 4:54 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

Learn what physical weathering is and the different ways in which it occurs. Examples of each type of physical weathering are covered, including exfoliation, abrasion, thermal expansion, and more.

Physical Weathering

Have you ever broken a fingernail, leaving a sharp, jagged edge? Most people use a nail file to smooth it out. A filed nail looks and feels different, but that doesn't change the fact that it is a nail or alter its makeup.

Something rather similar happens every day on the surface of the earth. No one is using a file to rub on the earth, but the rocks that make up the surface of the earth change in shape, texture, size, and firmness due to difference forces that act on them.

These changes are called physical weathering, or mechanical weathering. They occur without the rock moving. Things may drop on a rock, just like something drops on your hand, and the rock may break, just like your fingernail breaks. Something may rub on a rock, just like the file you used on your fingernail. The shape of the rock will change as long as whatever it is that rubs it is hard enough.

There are many different forces in nature that cause physical weathering of rocks. The forces that cause weathering fit into six different categories based on how they cause the rock to change.

Types and Examples

Exfoliation

The first type of weathering is exfoliation, also called unloading, which is when the outer layers of rock break away from the rest of the rock. Keep in mind that as some rocks form, they do so in layers. The layers become compact to become a solid rock.

Some types of rocks form beneath the surface of the earth. This causes them to form under increased pressure than that of rocks that form on the surface. If something happens to cause the rock to come to the surface (like an earthquake), then the pressure is released and the rock will expand. As it expands, it pushes the outermost layers of the rock outward until they break off.

An example of exfoliation is Half Dome in Yosemite National Park, which formed after glaciers caused surface rock to be removed. This released the pressure on rock under the surface and allowed it to expand, breaking off in sheets that slid off the side of the mountain leaving a half dome shape.

Abrasion

Abrasion is when moving material causes rock to break into smaller rock. Kind of like when you fall and brush your knee on the ground, breaking the skin open. Abrasions can occur to the point that rock becomes very fine, almost like powder. This most commonly occurs when rock rubs against other rock.

At the beach, waves move rocks over one another, causing them to break into smaller rocks. This continues until you get the very fine rocks that make up the sand that makes the beach. Yep, that soft sand that you love to walk on while at the beach formed from abrasion on rocks.

Thermal Expansion

We walk on beaches when the weather is warm, so let's talk about a type of weathering that happens when it's hot. Thermal expansion is when the outer layers of rock expand due to heat. Rocks don't tend to channel heat to their inner layers very quickly. As a result, only the outer layers that are hot begin to expand. When the temperature drops, the layers contract again. This happens repeatedly until the outer layers crack or break off.

An example of this is grus, which is granite rock that has crumbled due to thermal expansion. Grus most commonly occurs at the base of rocky structures where the rock is exposed to the widest range of temperature.

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