1. Section 4: Weathering and Erosion
Preview
Key Ideas
Bellringer
Physical Weathering
Chemical Weathering
Erosion

2. Key Ideas How does physical weathering affect rocks?
How are rocks affected by chemical weathering?
What is erosion, and what causes it to happen?

3. Bellringer
The Colorado River flows through the Grand Canyon. What do you think made the Grand Canyon?
What are some other environmental factors that change the land?
How do you think potholes form in a road?

4. Physical Weathering How does physical weathering affect rocks?
Physical, or mechanical, weathering breaks rocks into smaller pieces but does not alter their chemical composition.
Abrasion caused by particles suspended in wind or water is an example of physical weathering.

5. Visual Concept: Mechanical Weathering

6. Physical Weathering, continued
Ice can break rocks.
A common kind of mechanical weathering is called frost wedging.
Water seeps into cracks or joints in rock and then freezes.
When water freezes it expands, pushing rock apart.
Every time the ice thaws and refreezes, it wedges farther into the rock.

7. Physical Weathering, continued
Plants can also break rocks.
The roots of plants can also act as wedges as the roots grow into cracks in the rocks.
As the plant grows, the roots exert constant pressure on the rock, eventually causing pieces to break off.

8. Chemical Weathering How are rocks affected by chemical weathering?
Chemical weathering can cause rocks to weaken, decompose, or dissolve, and it can affect the chemical composition of rock.
Chemical weathering occurs when a rock is broken down due to chemical reactions with the environment.

9. Visual Concept: Chemical Weathering

10. Chemical Weathering, continued
Carbon dioxide can cause chemical weathering.
Carbon dioxide can react with water in the air to form carbonic acid.
Minerals dissolved by carbonic acid may be washed away, leaving underground pockets, or caves.
Water plays a key role in chemical weathering.
Some minerals react with water, which changes their physical properties.
Some minerals dissolve in water and are carried to new locations.

11. Chemical Weathering, continued
Acid precipitation can slowly dissolve minerals.
Sulfur dioxide and nitrogen oxides enter the air as a result of burning fossil fuels.
These chemicals can react with water in the air, forming sulfuric acid, nitric acid.
When this happens, the precipitation that results is acidic.
acid precipitation: precipitation, such as rain, sleet, or snow, that contains a high concentration of acids, often because of pollution in the atmosphere

12. Acid Precipitation

13. Erosion What is erosion, and what causes it to happen?
Erosion is the removal and transportation of weathered and nonweathered materials by gravity, running water, wind, waves, ice, and underground water.
erosion: a process in which the materials of the Earth’s surface are loosened, dissolved, or worn away and transported from one place to another by a natural agent, such as wind, water, ice, or gravity

14. Erosion, continued Water erosion shapes Earth’s surface.
Water is the most effective physical weathering agent.
Deposition occurs if a stream has too much sediment.
Rivers carry sediment to the ocean, and create canyons and riverbeds.
The faster the water flows, the larger the sediment it can carry.
deposition: the process in which material such as sediment is laid down, or deposited as a result of erosion

15. Erosion, continued Glaciers erode mountains.
Large masses of ice, known as glaciers, can exert tremendous forces on rocks.
Glaciers can carve U-shaped valleys in mountains.
Moving glaciers grind rocks below them into fine powder.
Glacial meltwater streams carry the sediment away from the glacier.

16. Erosion, continued Wind can also shape the landscape.
Fast moving wind can carry fine sediment.
Sediment carried by wind can smooth Earth’s surface and weather the landscape by abrasion.
Wind blows weathered particles away from their source.

17. Erosion, continued Gravity moves weathered materials downhill.
The movement of rock fragments down a slope in large quantities is known as mass movement.
Landslides and mudflows are common types of mass movement.
Humans can also cause erosion.
Humans often create conditions that speed up erosion.
Deforestation is an example of a human created condition that speeds up the erosion process.

18. Visual Concept: Erosion