1. What is weathering? What is erosion?
Rocks & Weathering
What is weathering?
What is erosion?

2. Compare 2 US mountain ranges:

3. Sierra Nevada Mountains, CA

4. Appalachian Mountains, NC

5. What is weathering?
The process that breaks down rock and other substances at earth’s surface
Caused by different forces

6. What is erosion?
The removal or movement of weathered materials from one place to another
Caused by different forces

7. Weathering and Erosion work together CONTINUOUSLY
to wear down and carry away the rocks
at earth’s surface.
weathering
erosion

8. What causes weathering?
Mechanical
= physical
Rocks and other materials on earth’s surface break down into smaller pieces; no change in the composition of the object itself
Chemical
Rocks and other materials on earth’s surface break down due to chemical changes in the object; the rock itself is changed; it has a new composition

9. Weathering acts slowly over time – it is a destructive force that changes the shape of earth’s surface.
Both types of weathering work together to break down rock on earth’s surface.

10. Mechanical Weathering
Mechanical (or physical) weathering affects all the rock on earth’s surface.
What are the FORCES that cause mechanical weathering??

11. 1. Release of Pressure
“stuff” on top of the rock applies pressure on top of the rock, holding it in place. When that stuff erodes away, that pressure is released. This loosens the outer layer causing it to crack and flake off

12. Freezing and Thawing (ice wedging)
Water seeps into cracks and freezes; when ice freezes it expands and pushes cracks open wider; remelts so water seeps further in – repeats over and over, and eventually cracks get larger and pieces break off. Cracks become wider and deeper.
**big factor in areas where temperature warms in the day (above freezing), and cools at night (below freezing)

14. 3. Animal Actions
Animals burrow in the ground, loosening and breaking apart rocks in the soil
Traveling, walking, pacing across rocks can also wear down rocks

15. 4. Plant Growth
Roots and plants grow in cracks, widening cracks even further as roots push down and out; can eventually break rocks apart

16. 5. Abrasion
Grinding away parts of rock – caused by wind carried sand, rock particles and ice; wears away expoed rock surfaces. Works like sandpaper, reshaping rock over time.
Arches National Park, Utah

17. 6. Thermal Expansion
repeated daily heating and cooling of rock; heat causes expansion; cooling causes contraction.
Similar to release of pressure, layers crack when cooling contracts the rock

18. Chemical Weathering
Chemical weathering also affects all the rock on earth’s surface.
What are the FORCES that cause chemical weathering??

19. 1. Action of Water
This is the MOST IMPORTANT cause. Water itself can dissolve components that make up the rock itself, which changes the rock’s overall composition

20. 2. Oxygen (oxidation)
Metals (iron, copper) in the rock react with oxygen in the air, which creates a new substance
Rocks turn reddish, brownish, greenish

21. 3. Carbon Dioxide
Carbon dioxide is found in the air, when it dissolves in rainwater, rivers, groundwater, it makes a weak acid called carbonic acid – which weathers soft rock, dissolving it. Most caves formed this way – carbonic acid dissolving limestone.

23. 4. Living Organisms
The roots of plants, lichens, fungi and other microorganisms produce weak acids which dissolve rock around the roots.

24. 5. Acid rain
Pollutants in the air such as sulfur, carbon, and nitrogen combine with water vapor and rain to make acid rain. The rain then causes weathering.

25. How fast does weathering occur?
This depends on different factors:
Type of rock
Is the rock soft or hard? Softer rocks are more vulnerable to both kinds of weathering. Permeable rocks (filled with little tiny holes so water can get through) are more vulnerable too.

26. Climate
Warm climates – the heat speeds up chemical weathering
Cool climates – speeds up mechanical weathering (ice wedging and heat expansion)
Wet climates – speeds up BOTH chemical (all chemical weathering requires water) and mechanical

27. 3. Surface Area
Weathering is a SURFACE reaction – occurs wherever the atmosphere (oxygen) or water can get on or in the rock.
The more surface area that is exposed, more weathering that will occur – so as a rock breaks down into smaller and smaller pieces, it weathers more and more easily because more surface area is exposed to air and water.

29. 4. Presence of Plants and Animals
as both contribute to weathering
Topography
steep terrains more open to weathering as rock is usually more exposed there; less chance of “stuff” building up to cover and protect rock from weathering