1. Chapter 5: “Weathering, Soil, and Mass Movements”

2. Mechanical Weathering
Occurs when physical forces break rock into small pieces without changing its mineral composition.
Each piece has the same characteristics as the original rock.
There are three physical processes in nature that are responsible for mechanical weathering.
Frost Wedging
Unloading
Biological Activity

3. Frost Wedging
Water seeps into the cracks of rocks and eventually it will freeze and expand.
This processes causes the cracks to enlarge.
After many freeze-thaw cycles, the rock breaks into pieces.
Most common in mountainous regions in the middle latitudes.
Sections of rock that are wedged loose may tumble into large piles called talus.
These typically form at the base of steep, rocky cliffs.

4. Unloading and Exfoliation of Igneous Rocks
Large masses of igneous rock can be exposed to uplift and erosion of overlying rocks.
Pressure on the igneous rock is reduced, called unloading, and this causes the outer rock layers to expand.
These outer layers separate from the rest of the rock mass which creates exfoliation.

5. Weathering and Biological Activity
Activities of plants, burrowing animals, and humans can cause mechanical weathering.
Plants: roots grow into cracks & are wedged apart as the plant grows.
Burrowing animals: move rocks to surface to become exposed to weathering.
Humans: blasting areas in search of minerals or creating new roads; deforestation.

6. Chemical Weathering
Chemical weathering is the transformation of rock into one or more new compounds.
The most important agent in this type of weathering is water.
Water absorbs gases from the atmosphere and the ground.
Chemical Weathering of Granite
Weathering of potassium feldspar produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution.
Quartz remains substantially unaltered.

7. Chemical Weathering Weathering of Silicate Minerals
When silicate minerals undergo chemical weathering, the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater.
Produces insoluble iron oxides and clay minerals.
Spheroidal Weathering
When water enters the joints in a rock, it weathers the corners and edges most rapidly.
This causes the corners and edges of the rock to be more rounded.
The rock takes on a spherical shape.

8. Spheroidal Weathering

9. Rate of Weathering
Two other factors that affect the rate of weathering are rock characteristics and climate.
1. Rock Characteristics
Mineral composition and solubility
Physical features such as joints
2. Climate
Temperature and moisture are the most crucial factors.
Chemical weathering is most effective in areas with high tempeartures and abundant moisture.
3. Differential Weathering
Caused by variations in composition
Creates unusual and spectacular rock formations and landforms