1. Chapter 5: Weathering, Soil, and Mass Movements
Section 1: Weathering

2. Key Questions - Objectives
What is the difference between Mechanical and Chemical weathering?
What rock characteristics change the rate of weathering?
What environmental factors change the rate of weathering?

3. Notes Outline Mechanical Weathering Frost Wedging Unloading
Biological Activity
Chemical Weathering
Water
Weathering of Different Materials
Rate of Weathering
Rock Characteristics
Climate
Differential Weathering

4. Mechanical Weathering
Mechanical weathering is the physical destruction or breakdown of rock into smaller pieces called sediments.
Mechanical weathering does not change the chemical composition of a rock.
Think-Pair-Share
There are human causes and natural causes for mechanical weathering.
Can you think of some human causes of physical/mechanical weathering?

5. Mechanical Weathering
Frost Wedging
Frost wedging occurs when water freezes in a crack in a rock, and by expanding, breaks the rock.
How quickly the rock breaks depends on how many freeze-thaw cycles occur in a given amount of time (year, month, day).

7. Mechanical Weathering
Unloading
Recall the vocabulary term Regional Metamorphism.
The great weight of the land can metamorphose rocks.
What happens when the weight is lifted?

8. Mechanical Weathering
Unloading
Unloading occurs when intrusive igneous rock reaches the surface and breaks loose from it’s host bedrock.
Load (or weight) of surrounding rock on top.
Intrusive Igneous Rock

9. Loading

10. Loaded

11. Unloading

12. Unloaded

13. Exfoliation is the term used to describe the specific type of unloading where rock separates in thin layers.

16. Biological Activity
There are many ways that animals and plants can mechanically weather rocks.
One of the most impressive ways is called Root Wedging.

17. Chemical Weathering
When chemical reactions change the molecular form of rock into something else, it is called chemical weathering.
What are other real world examples that show the difference between chemical and physical changes?

18. Chemical Weathering Water
The most prominent and powerful agent of chemical weathering is water.
Water absorbs dissolved gasses like CO2 and O2
Water provides constant contact to the rock surface for dissolved gasses to act on rocks
Water reacts with some compounds (like CO2 ) to make weak acids

19. The Weathering of Different Materials
Weathering of Granite
When granite is chemically weathered, the quartz is resistant to carbonic acid (made by CO2).
The feldspar in granite will be mostly converted into clays.
The quartz will fall away (sand).
Dissolving Rocks
Magnesium, Calcium, Potassium, and Sodium present in rocks are dissolved by water and carried away, leaving behind Iron, Aluminum, other non-dissolvable elements which react with oxygen.

20. Have you ever wondered why the edges of rocks are often smooth?
This is because corners are weathered more rapidly than the rest of the rock.

21. Rates of Weathering
Three things effect how quickly or slowly rocks will be weathered.
Climate
Rock Characteristics
Mechanical Weathering
Mechanical weathering increases the amount of surface area available for chemical reactions. This speeds up the weathering process
Chemical reactions are the source of Chemical Weathering.

22. Rock Characteristics
Mineral composition and shape will change how quickly rocks are weathered.
Rocks with cracks will weather more quickly due to mechanical weathering.
The mineral (chemical) composition of rocks changes the rate by which those rocks weather.
Granite weathers more slowly than marble.

23. Climate
More rainfall, and more temperature cycling will cause rocks to weather more quickly.
Dry areas weather very slowly.

24. END 5.1