1. Weathering and Erosion
Chapter 14
Weathering and Erosion

2. Pressure/Cementation
The Rock Cycle
Pressure/Cementation
Sedimentary
Rocks
Weathering/Erosion
Sediment
Weathering/Erosion
Melting
Heat and Pressure
Weathering/Erosion
Igneous
Rocks
Metamorphic
Rocks
Heat and Pressure
Cooling
Melting
Melting
Magma

3. 14.1 Weathering Processes
Weathering: change in physical form or chemical composition of rocks at the earth’s surface
Mechanical Weathering: physically breaks rocks into smaller pieces without chemical change
Chemical Weathering: breaks rocks down by changing chemical make-up

4. Rock structure and weathering
When granite cools underground, it is under pressure.
As rocks on top are removed, pressure lessens and the rock expands, causing cracks.
These cracks or joints are often sites of weathering.

5. Types of mechanical weathering
Exfoliation: When rocks break off into curved sheets (like layers of an onion), caused by cracks in the rock

6. Royal Arches, an example of exfoliation

7. Types of mechanical weathering
Ice wedging: when water gets into cracks and freezes, it expands, breaking the rocks apart

8. Types of mechanical weathering
Organic activity: plant roots and digging animals can break apart rocks
Abrasion: when rocks rub against each other, breaking them apart (caused by gravity, running water, and wind)

9. Chemical weathering
Causes: interactions between minerals and other chemicals such as water (hydrolysis), carbonic acid (carbonation), oxygen (oxidation), or acid (from acid rain or plants)
Result: minerals are changed into a new substance, often that gets dissolved in water and carried away

10. 14.2: Factors that affect the rate of weathering
Rock composition: some rocks are either harder (resist mechanical weathering) or more chemically stable (resist chemical weathering) than others
Slow to weather rocks: quartz
Fast to weather: calcite, limestone, sandstone and shale

11. Factors that affect the rate of weathering
Amount of exposure: surface area—the more of the rock that is exposed to weather, the faster it will weather!
Small pieces will weather faster due to high surface area, just like small sticks burn easier than large logs

12. Factors that affect the rate of weathering
Climate: areas with rainfall and alternating hot/cold seasons have the fastest weathering. Areas that are hot and dry have the slowest
Why: cold + water leads to frost wedging, warm + water leads to fastest chemical changes

13. Factors that affect the rate of weathering
Topography: slope and elevation of the land
Steep slopes speed up weathering because gravity pulls rocks away from each other
More frost wedging at high elevations because it is colder

14. 14.3: Soil and Weathering
Regolith: layer of weathering rock fragments, midway between rock and soil
Humus: decaying organic matter (dead plants/animals)
Soil: mixture of weathered minerals, water, humus and gases (air).

15. Soil Composition
Soil texture: size of mineral particles that make up the soil
Clay: particles smaller than .002 mm in diameter, result from weathered feldspar
Silt: particles between .002 and .06 mm
Sand: particles between .06 and 2.0 mm, result from weathered granite and other rocks high in quartz

16. Types of soils Loam: even mixture of all three particle sizes
Transported soil: soil that forms from material that has been carried from one place to another by wind, gravity, or water.
Residual Soil: soil that has formed in one place from the rock below it.

17. Soil Profile
Soil Profile: Cross section of a residual soil, divided into layers called “horizons.”
A horizon: top, called topsoil, consists of humus and small rock particles (clay, silt, sand)
B horizon: called sub soil, consists of clay and chemicals that have been carried down by water (leached)

18. Soil Profile
C Horizon: bottom layer consisting of partially weathered bedrock (regolith)
Over time, C horizon grows into B than an A horizon.
Biggest difference between A & B—A has humus
Biggest difference between B & C—B is more weathered

19. Soil and Climate
Different climates produce different types of soil because climate affects weathering.
Tropical soils:
thick due to rapid weathering
low in minerals because rainfall leaches minerals deep into soil away from plant roots
lots of humus due to rapid decomposition
Not very fertile soils

20. Soil and Climate Desert soils Artic soils
Soil is thin and rocky due to slow weathering
Artic soils
Temperate soils: soils in areas with moderate rainfall and variation between hot and cool temperatures, 2 main types

21. Types of temperate soils
Pedalfer
Found in eastern US, areas with higher rainfall
High in clay, quartz and iron (reddish)
Pedocal
Found in western US, drier areas
High in calcite, tends to neutralize acids in soil and makes the soil more fertile

22. Soil and topography
Slopes have thin soil and flat areas have thicker soils
Why: erosion by water and gravity carry soil down off slopes and into flat areas

23. 14.4: Erosion
Erosion: process that moves (transports) the products of weathering. Done by gravity, wind, glaciers, and water.
Soil erosion: soil erodes quickly because the particles are light, can be a serious problem since soil is used to grow food, bad because it removes top layer where we find important plant minerals

24. Soil conservation Necessary to prevent soil erosion
One method is to slow or reduce water movement through crop fields
Contour plowing: place furrows perpendicular to slope rather than in straight lines
Terracing: break slopes up into flat steps to slow water

25. Soil conservation Keep plants on soil so roots hold soil in place
No-till agriculture: don’t remove old plants after harvest
Strip cropping: alternate strips of crop that exposes soil (like corn) with crops that don’t (like alfalfa)
Crop rotation: alternate crops from season to season, include crops that hold soil.

26. Mass movements
Mass movements: movement of rock fragments down a slope due to gravity
Talus: loose rock fragments at base of slopes, carried by mass movements

27. Exfoliation
Talus

28. Talus

29. Rapid Mass Movements
Rockfall: fall of rock from a steep cliff, fastest form of rapid mass movement
Landslide: movement of loose rock and soil, often involve large pieces of loose bedrock
Mudflow: movement of large masses of mud, what we usually call landslides