Weathering, Mass Wasting, and Soil Formation
Chapter 4: Weathering and Mass Wasting
Types of Weathering
Mechanical Weathering
The breaking of rocks into smaller pieces.
Chemical Weathering
The chemical transformation of rock into one or more new compounds.
Factors Affecting Weathering Rate
Important Factors:
- Climate: Temperature and moisture are the most crucial factors. Chemical weathering is most effective in warm, moist climates.
Weathering Products of Granite
- Potassium feldspar produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution.
- Quartz, crystallizing last in Bowen’s Reaction Series, remains largely unaltered (hence its presence in many beach sands).
Most Resistant Mineral to Chemical Weathering
Quartz
Soil
Definition
Soil is a combination of mineral matter, water, and air.
Soil Profile
Soil-forming processes operate from the surface downward, creating zones or layers called horizons.
Horizons in Temperate Regions
- O Horizon: Organic matter
- A Horizon: Organic and mineral matter
- E Horizon: Little organic matter; characterized by eluviation (physical removal of soil particles) and leaching (chemical dissolution and removal of soil materials).
- B Horizon: Zone of accumulation (eluviated and leached material deposited here).
- C Horizon: Partially altered parent material
Topsoil
Comprises the O and A horizons.
Solum
“True soil,” consisting of the O, A, E, and B horizons.
Factors Controlling Soil Formation Rate
- Time
- Climate
- Parent material
- Topography
- Plants and animals
Slope Failure (Landslides)
Causes: Gravity is the primary cause, with normal and shear forces as components.
Factors Influencing Slope Movement
- Angle: Steep slopes often have poorly developed soils. Optimum is a flat-to-undulating upland surface.
- Orientation: The direction the slope faces influences sunlight exposure and soil temperature (e.g., north-facing slopes receive less sunlight).
- Moisture: Destroys particle cohesion and adds weight to slope material.
- Oversteepening: Unconsolidated granular particles assume a stable slope called the angle of repose. When the slope angle exceeds the angle of repose, the slope becomes unstable.
- Vegetation Removal: Fires can make soils hydrophobic, increasing runoff.
- Ground Vibrations: Earthquakes can trigger slope failure.
Water and Vegetation’s Effect on Slope Stability
Water reduces slope stability by decreasing cohesion and adding weight. Vegetation increases stability by anchoring soil.
Creep
Slowest and most common form of slope failure. Causes tilting of fences and utility poles.
- Solifluction: Slow movement in permafrost areas. The saturated active soil layer flows over the frozen subsurface.
Types of Mass Movements
- Debris Flow (Mudflow): Rapid flow of debris with water, often confined to channels. Common in deserts after thunderstorms.
- Earthflow: Rapid movement on hillsides in humid regions, often triggered by water saturation and liquefaction (sometimes associated with earthquakes).
- Rockslide: Rapid movement of bedrock blocks down a slope.
- Slump: Rapid movement along a curved rupture surface on oversteepened slopes. Can transition into an earthflow at the toe.
- Rockfall: Rocks falling down a slope (e.g., Half Dome in Yosemite). Triggered by freeze/thaw cycles and ground shaking.