1. Earth’s Surface: Chapter 4 Section 1 Mechanical and Chemical Weathering
Learning Targets:
1) I can describe how mechanical weathering breaks down rocks
2) I can explain how chemical weathering breaks down rocks.
3) I can identify factors that affect the rate at which weathering occurs

2. Weathering Breaks Rocks into Smaller Pieces
Weathering is a process by which natural forces break down rocks.
One kind of weathering occurs when a rock is physically broken apart
Another kind of weathering occurs when a chemical reaction changes the makeup of the rock

3. Mechanical Weathering Produces Physical Changes in Rocks
Mechanical weathering is the breaking up of rocks by natural forces
In this natural process, physical forces split rocks apart but do not change their composition
Ex: Ice wedging, pressure release, Plant root growth, and Abrasion

4. Mechanical Weathering Produces Physical Changes in Rocks
1. Ice Wedging
When water freezes in the cracks and pores of rocks, the force of its expansion is strong enough to split rocks apart.
This process is strong enough to break up huge boulders.
Common where temperatures rise above and below the freezing point of water, 32 degrees F.
2. Pressure Release
Over time, Earth’s forces can push rock up to the surface.
The pressure inside the rock is still high, but the pressure on the surface of the rock is released.
This release in pressure causes the rock to expand and form cracks in it.
Exfoliation is a process in which layers or sheets of rock gradually break off

5. ICE WEDGING

6. PRESSURE RELEASE

7. Mechanical Weathering Produces Physical Changes in Rocks
3. Plant Root Growth
Trees, bushes, and other plants may take root in cracks in rocks.
As the roots of these plants grow, they wedge open the cracks.
The rock can be completely split apart
4. Abrasion
Abrasion is the process of wearing down by friction
The force of moving water alone can wear away particles of rock.
Water also causes rocks to tumble downstream.
Tumbling rocks wear down as they grind against the riverbed and
each other.
* Ocean waves beating against a rocky shore also wears down rocks by abrasion

8. PLANT ROOT GROWTH

9. ABRASION

10. Chemical Weathering Changes the Mineral Composition of Rocks
Minerals and rocks also undergo chemical changes when they react with water and air
Chemical weathering is the breakdown of rocks by chemical reactions that change the rocks’ makeup, or composition.
When minerals in rocks come in contact with air and water some dissolve and others react and are changed into different minerals

11. Water is the main cause of chemical weathering
DISSOLVING
Water is the main cause of chemical weathering
Many more minerals dissolve in water that is slightly acidic.
In the atmosphere, small amounts of carbon dioxide dissolve in rainwater creating a weak acid.
The rainwater moves through the soil picking up more carbon dioxide from decaying plants.
Air pollution can make water even more acidic.

12. Rusting The oxygen in the air is also involved in chemical weathering.
Many common minerals contain iron
When the oxygen in the air and the water combines with iron is produces rust (iron oxides)

13. RUSTING

14. Weathering Occurs at Different Rates
Most weathering occurs over long periods of time: hundreds, thousands or millions of years
Factors such as: surface area, rock composition, and climate influence the rate of weathering.
1. Surface Area: The more of a rock’s surface that is exposed to air and water, the faster the rock will break down.

15. Weathering Occurs At Different Rates
2. Rock Composition: Different kinds of rocks break down at different rates
Ex: Granite breaks down much more slowly than limestone.
3. Climate: Water is needed for chemical weather to occur.
Heat speeds up chemical weathering.
Therefore, chemical weathering occurs faster in hot, wet regions.