Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
WEATHERING AND MASS WASTING
PowerPoint Presentation Outline Notes
Weathering Is The First Step In The Wearing Away Of The Continents:
- Mechanical or physical weathering is the physical disintegration of rock.
- Chemical weathering is the chemical decomposition of minerals by atmospheric gases and water.
Weathering Builds Soils
Weathering can be observed in graveyards.
- Mechanical weathering causes rock to expand and break into smaller pieces
- How can rocks break apart?
- Frost wedging
- Tree roots wedging
- Temperature change
- Salt crystallization
Talus Slopes in the Sierra Nevada
- Frost wedging breaks up rock.
- Falls to form talus slopes with 35 degree slopes.
- Constant expansion and contraction of minerals in a rock may break up the rock.
- In a desert, ground temperatures may reach 190° F.
- A sudden chilling cloud burst may shatter the rock.
Unloading - Exfoliation Domes
Pressure release unloading forms exfoliation domes.
- Rocks broken into smaller pieces are more vulnerable to chemical weathering.
- Rocks decompose and form new substances as a result of exposure to the atmosphere:
- Carbon dioxide
- Minerals in rock combine with oxygen from the atmosphere to form new byproducts.
- Iron minerals oxidize to form RUST (ferric oxide).
- Carbon dioxide readily dissolves in water to form a weak carbonic acid.
- Water + Carbon dioxide = Carbonic Acid
- Limestone (Calcium carbonate + Carbonic Acid = Calcium bicarbonate, a soluble)
- Carbonation of Limestone and Marble
Limestone and marble (CaCo3) can be completely dissolved by carbonic acid.
- Hydrolysis is a chemical change that produces new byproducts.
- Water molecules react with the mineral feldspar to produce clay minerals, solubles, and sand.
- Clay is the most common byproduct of weathering.
Simple solution occurs with easily dissolved substances like salt and gypsum.
Climate and Chemical Weathering
- In which climate will this gravestone weather most quickly?
- A cold, dry climate
- A hot, wet climate
- A hot, dry climate
- Weathering is fastest in hot, wet climates.
Classify These Weathering Processes:
- Tree-Root Wedging
- Frost Wedging
- Temperature Change
- Salt Crystallization
What is Mass Wasting?
The Downslope Movement of Rock and Soil Under the Influence of Gravity
Mass Wasting Follows Weathering
- Weathering weakens rock and produces a layer of rock debris called regolith.
- Mass wasting provides a short distance transit system for regolith.
- Streams are the long distance transit system for regolith.
How does regolith make its way downslope?
Rate of Movement
Regolith moves at rates that vary from less than a centimeter a year to thundering avalanches of 125 miles per hour.
Soil Creep is the Slowest Form of Mass Wasting
- Soil creep is the most widespread form of mass wasting
- Major cause of property damage in hilly areas
- It can be observed by its effects: Tilted fence posts and retaining walls, tilted trees, and strata
What causes soil creep?
- Wetting and drying of soil
- Freezing and thawing
- Trampling and burrowing by animals
Mudflows are Rapid Flow Movements in Canyons
- Mudflows are most common in desert and semi-desert regions.
- Mudflows occur after torrential rains on bare or burned-over hillsides.
- Common in Southern California, mud and debris flow rapidly down canyons like wet concrete.
What is the most common form of landslide in the Berkeley-Oakland hills?
- Slumps occur when regolith on steep slopes is saturated.
- Slumps combine a collapse movement at the top and a flow movement at the base.
- Slumps pull away from slopes with slip movement along a curved plane.
- Produce a breakway scarp and slump blocks
- Lower part of slump flows forward as toe
- Rockslides involve a rapid sliding of large masses of fractured rock and regolith.
- Rockslides are not common but they move millions of tons of rocks in a short time.
- Rockslides are extremely hazardous.
- Rock avalanches are the fastest moving slides.
How do people’s activities speed up mass wasting?
People can speed up the rate of mass wasting by:
- Adding weight to the top of slopes
- Removing support at the base of a slope
- Saturating slopes
- Removing vegetation cover
Go To Top Of Page
Return to Weathering and Mass Wasting Learning Module