Topic IV: Weathering & Erosion

The “Destructive Geologic Forces”

When You Have Finished This Unit, You should be able to

answer the following questions...• How is the Earth’s crust affected by its

environment ?

• What are the products of weathering ?

• How are the products of weathering transported ?

• How are eroded materials deposited ?

• How is the Earth’s surface shaped by weathering, erosion, and deposition ?

Definitions:……..

• Weathering - the slow continuous process by which nature breaks down rock material into smaller particles. The two major types of weathering are mechanical & chemical

• Erosion - the breakdown and transport of weathered material by the agents of erosion - running water, wave and current action, glaciers, wind, and gravity.

Mechanical Weathering

• The breakdown of rock material into smaller particles of the same material.

• Example - Granite bedrock breaks down into smaller particles of granite

Agents of Mechanical (aka “Physical”) weathering include...• Ice wedging

• Frost action

• Unloading

• Plant Action

• Animal Action

Ice Wedging• As water seeps into

the pre-existing cracks in rocks, it may freeze.

• As the water freezes, it expands.

• The expanding freezing water wedges the crack in the rock further open.

Frost Action• As rock material is

heated, it expands. • The exterior of the

rock may be heated while the interior is still cold and contracted from the cold night temps.

• This pulls the outer layer of rock away in a process called exfoliation.

“Unloading”

• When rock which has been buried by other sediment, rock, or ice is exposed, it may expand under the released pressure.

• This expansion causes the outer layers of rock to “exfoliate”

Animal Action

• As animals burrow into rock and soil, they break the material down into smaller particles of the same material.

• Examples include worms, ground hogs, chipmunks, etc.

Plant Action• The roots of growing

plants will grow into existing cracks of rock and enlarge the openings.

• Smaller plants (lichens, e.g.) dig their tiny roots into small cracks with the same result.

Chemical Weathering…...

• involves the breakdown of rock material into new material with a new chemical make-up. In other words, the rock has turned into new rock material. (decomposition)

• Agents of chemical weathering include oxidation, carbonation, hydration, and dissolution.

Oxidation

• Oxidation is the chemical union of oxygen with a molecule within the rock. “Rust” is a common form of oxidation and is seen on many iron rich rocks around the O’Neill campus.

Carbonation

• As water falls through the atmosphere, a mild acid forms which can dissolve away limestone to form caves.

Hydration • The chemical union of

water molecules with mineral molecules is known as “hydration”.

• Iron and magnesium minerals will alter to clay minerals due to hydration. Feldspar also hydrates to form clay minerals.

Dissolution...

• Certain minerals will actually dissolve when exposed to water or mild acids. The sample shown here is halite, a highly soluble mineral more commonly known as “rock salt”

In order for erosion to occur...

• Material must first be broken down by weathering so that it can be transported by the agents of erosion.

• Remember that chemical weathering can only occur on exposed surfaces.