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.