1. Mechanical Weathering
Chapter 4
Mechanical Weathering

2. What is Weathering?
Weathering: the process of rocks breaking apart naturally.
Two types: mechanical (physical) and chemical

3. Physical Changes in Rock
Mechanical weathering: the breaking up of rocks by physical forces
Rocks are split apart, but their composition stays the same.

4. What causes mechanical weathering?
Ice wedging
Pressure release
Plant root growth
Abrasion

5. Ice Wedging When water freezes, it expands.
When this happens in cracks of rocks, it breaks the rocks apart.
Happens where temperature goes above and below 32 degrees F.

6. Pressure Release Rocks deep within Earth are under great pressure.
When they are exposed to the surface, there is a release of pressure that causes the rock to crack.
Exfoliation: when layers or sheets of rock are broken off (onion-skin weathering).

7. Plant Root Growth
As roots of plants grow, they can wedge open cracks in rocks.
This splits the rock.

8. Abrasion Abrasion: the process of wearing down by friction.
Water does this as it moves over rocks in riverbeds and along shorelines.

9. Chapter 4
Chemical Weathering

10. What is chemical weathering?
Chemical weathering: breakdown of rocks by chemical reactions that change the composition of the rock.
This happens when minerals dissolve or have reactions.

11. Dissolving Water is the main cause of chemical weathering.
Some minerals, like halite, completely dissolve in water.
Other minerals will dissolve in water that is slightly acidic (mixed with carbon dioxide).

12. Dissolving Air pollution also makes the water acidic.
Acid rain can form, causing rocks to weather very quickly.

13. Rusting
Oxygen in the air combines with water to create iron oxides, or rust.
Turns rocks a reddish color.

14. Weathering occurs at different rates
Surface area: more surface area exposed = faster rate of weathering.
Rock composition: different rocks are stronger than others and don’t weather as quickly (granite vs. limestone)
Climate: water and heat cause chemical weathering, but freezing and thawing causes mechanical weathering.

15. Soil Composition and Properties
Chapter 4
Soil Composition and Properties

16. Soil Composition
Made of weathered rock, organic matter (from living things), water, and air.
Soils differ depending on what type of rock particles they contain.
The organic matter that makes up soil is called humus.

17. Soil Composition
Made of weathered rock, organic matter (from living things), water, and air.

18. Soil Composition The kind of soil that forms depends on:
The kinds of rock in the area
The climate
The landforms in the area
The plants in the area
The animals/organisms
Time

19. Properties of Soil
Texture is determined by the size of the rock particles in the soil.
Sand: largest; does not hold water well.
Silt: smaller, cannot be seen without a microscope
Clay: smallest; absorbs water easily.

20. Properties of Soil
Color is determined by the materials that make up the soil.
Iron gives soil a reddish color, while humus gives soil a black or brown color.
Color can indicate how well water moves through the soil (bright colors drain well).

21. Properties of Soil
Pore Space: the spaces between soil particles.

22. Properties of Soil
Chemistry: how well the minerals in the soil dissolve in water.
The water’s pH (acidic measure) depends on this.

23. Chapter 4
Soil Horizons

24. Soil Horizons
As soil develops, it falls in horizontal layers known as horizons.
Each horizon has different properties than the layer above or below it.
Named A, B, and C.

25. The A Horizon The upper layer of soil Called topsoil
Most organic matter (humus)
Often darker in color

26. The B Horizon Just below the A horizon Little organic matter
Reddish/brown
Clay and minerals

27. The C Horizon Deepest layer of soil
Largest and least-weathered particles
Light yellowish/brown

28. The Soil Profile
The soil horizons in a specific location make up the soil profile.
Different locations have different profiles.
Horizons can be thicker in different areas, or missing altogether.