1. Notes
Weathering

2. Over time, natural forces break rocks into smaller and smaller pieces.
How big are rocks?
Over time, natural forces break rocks into smaller and smaller pieces.

3. How big are rocks?
Rocks come in all different sizes, from a fraction of a millimeter to large boulders.

4. How big are rocks?
The Wentworth Scale is the scale used most by geologists to describe rock particle size.

5. How big are rocks?
Silt and clay are both so small that it is hard to tell the difference.
If you rub them between your fingers, silt feels gritty and clay feels smooth.

6. Weathering: the process by which natural forces break down rocks.

7. There are two types of weathering: physical and chemical.

8. Physical Weathering
If you smash a walnut with a hammer, it will break into a lot of pieces, but its pieces will still be walnut.

9. Physical Weathering
Like the walnut, physical weathering is when rocks split apart or break down but their composition does not change.

10. Frost Wedging
When water freezes in the cracks and holes of rocks, it expands and splits the rocks apart.

11. When rock erodes, the rock underneath experiences less pressure.
Exfoliation
When rock erodes, the rock underneath experiences less pressure.
This causes the rock to expand, like memory foam.

12. Exfoliation
This type of weathering often leads to exfoliation, where sheets of rock gradually break or peel off.

13. Sometimes, this can split the rock apart completely!
Root Wedging
As the roots of plants grow in the cracks of rocks, they wedge open the cracks.
Sometimes, this can split the rock apart completely!

14. Water, ice, or rock can wear down rocks by abrasion.
Abrasion is the process of wearing down by friction, the rubbing of one object or surface against another.
Water, ice, or rock can wear down rocks by abrasion.

15. Physical Weathering
The shape or appearance of a particle or grain of rock can tell a story about how it eroded.

16. Physical Weathering
Particles or grains with sharp edges and points might have been created by glaciers.

17. Smooth edges might have been created by streams, wind, or waves.
Physical Weathering
Smooth edges might have been created by streams, wind, or waves.

18. The farther a rock travels, the more spherical it will be.
Physical Weathering
The farther a rock travels, the more spherical it will be.

19. Chemical Weathering
Chemical weathering is the breakdown of rocks through chemical reactions.

20. Chemical Weathering
Minerals in rocks change chemically when they react with water and/or air.

21. Water is the main cause of chemical weathering.
Dissolving
Water is the main cause of chemical weathering.
Some minerals completely dissolve in water, like Halite (salt).

22. Many minerals will dissolve in water that is acidic.
Dissolving
Many minerals will dissolve in water that is acidic.
Sometimes rain is slightly acidic, which dissolves the minerals in rocks and causes weathering.

23. Air pollution can make rainwater even more acidic.
Dissolving
Air pollution can make rainwater even more acidic.
Acid rain causes rocks to weather faster than they would naturally.

24. Many minerals in rocks contain iron.
Oxidation
Many minerals in rocks contain iron.
When the minerals dissolve, oxygen and water combine with the iron to form rust (iron oxide).

25. Iron oxides form a coating on the rock which makes it appear red.
Oxidation
Iron oxides form a coating on the rock which makes it appear red.

26. Chemical weathering changes a rock’s Texture Composition Exfoliation
Review
Chemical weathering changes a rock’s
Texture
Composition
Exfoliation
Pore space

27. Review
2. A fast flowing river wears away small particles from the rocks in its bed. This is an example of
Exfoliation
Abrasion
Desertification
Rotation