Topic IV: Weathering & Erosion

1. Topic IV: Weathering & Erosion The “Destructive Geologic Forces”

2. 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 ?

3. 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.

4. Mechanical Weathering • The breakdown of rock material into smaller particles of the samematerial. • Example - Granite bedrock breaks down into smaller particles of granite

5. Agents of Mechanical (aka “Physical”) weathering include... • Ice wedging • Frost action • Unloading • Plant Action • Animal Action

6. 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.

7. 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.

8. “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”

9. 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.

10. 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.

11. 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.

12. 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.

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

14. 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.

15. 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”

16. 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.