Lecture 7 Part 2
Weathering and Erosion Notes
Weathering Processes
Wetting and Drying
Wetting and drying cycles are significant, especially in landscapes with sedimentary deposits like old lake beds. These sediments are often fine-grained, sometimes including clays. The process involves:
Hydration: Some clays absorb more water than others, leading to differential expansion.
Breakdown: This differential expansion acts similar to hydrofracturing, causing the material to break down and form dramatic landforms such as hoodoos.
Clay Types and Expansion: Varying dramatically with water exposure, some expanding far more than others. For example:
Kaolinite: Doesn't swell much when wet.
Smectites (Sodium and molybdenum): Can swell up to 1000%.
Smectites (Calcium and molybdenum): Swell by 50-100%.
Example: Roads built on clay soils (e.g., in Queensland, Australia, specifically Toowoomba to Derby), are prone to cracking because the clay expands and contracts significantly with moisture changes, causing the road base to deform.
Air Pressure
Wetting can increase air pressure in the pores between grains in a rock, leading to disaggregation.
Landforms
Pinnacles/Hoodoo: Sedimentary deposits under wetting and drying cycles, often seen in areas with snowmelt (e.g., Bryce Canyon, US).
Cat Rock: Resistant cap rock atop pillars, protecting them from erosion.
Salt Weathering
Observed on coastlines (e.g., Southern Victoria Coast). Salt crystals grow within the rock, dislodging sand and mud grains, creating honeycomb-like patterns.
Organic Weathering
Roots physically break rocks apart. This can involve:
Root Wedging: Roots levering off pieces of rock.
Example: Tree roots growing through granite, causing exfoliation (lifting off sheets of rock).
Lichens: Breaking down rock surfaces.
Animals: Primarily secondary, by removing surface vegetation and exposing the underlying rock to weathering.
Physical vs. Chemical Weathering
Physical (Mechanical) Weathering: Breaking rocks into smaller pieces without changing their composition.
Chemical Weathering: Decomposing rocks by changing their chemical composition.
Processes: Solution, hydrolysis, carbonation, oxidation-reduction, and bio-exchange.
Importance: Crucial for soil formation, and typically works alongside mechanical weathering.
Surface Area and Weathering
Mechanical weathering increases the surface area exposed to chemical weathering.
Example: Dividing a 16 cm cube of rock four times increases its surface area from approximately to almost .
Climatic Influence on Chemical Weathering
Chemical weathering is heavily influenced by latitude and climate. The professor explained this using a graph of precipitation in millimeters per year, and temperature from 0 to 30 degrees Celcius, soil depth and latitudinal climate zones.
Polar Deserts: Dry and cold; weathering is slight.
Temperate Regions: Increased precipitation and temperatures; deeper weathering profile.
Subtropical Deserts: Dry due to high-pressure systems, high evaporation, little organic material; weathering is slight.
Tropics: High moisture and temperatures, abundant organic material (humic acids); deep weathering and produces the most amount of water
Abrasion Processes
After rocks are broken down by weathering, abrasion further reduces their size and alters their shape. This involves the smashing of rocks into each other.
Fluvial Systems
Rivers where rocks collide, contributing to abrasion.
Coastal Systems
Wave energy causes particles to rub together, resulting in abrasion.
Aeolian Systems
Wind-dominated systems (deserts) where wind causes abrasion. They're good at breaking down rock by collisions.
Glacial Systems
Glaciers have a huge amount of energy and grind rocks, reducing size during transport
Energy Levels
High-energy systems lead to higher rates of weathering.
Flooded Rivers and Sediment Transport
During floods, the energy of water transports rocks and sediment, with rocks banging into each other. The entrainment increases sediments within the stream.
Pothole Formation
Granites are good for these. In lower energy systems, grains of sand swirl within cracks, acting as an abrasive and gradually forming potholes.
Marine Environments
Constant wave action causes sediments to crush, grind, and abrade each other.
Glacial Erosion
Glaciers carve out entire valleys. They are very slow, but very strong.
Valley Shape: U-shaped (versus V-shaped for rivers).
Chatter Marks: Indentations in the rock caused by large rocks embedded in the base of the glacier being dragged across the surface.
Bergschrund: A crevasse at the head of a glacier, where ice pulls away from the rock, plucking rock off.
Mass Wasting - Sediment Movement
Once material is broken down, it moves downslope due to gravity. The speed varies based on slope angle and lubrication (typically water).
Creep: Very slow movement.
Glacier: Ice covered with debris (moraine).
As glaciers diminish, the debris replicates the original glacier shape but moves slowly.
Rates: Meter a day.
Flows: More liquid-like movement (e.g., earth flows, slumps).
Avalanches and Rockfalls: Very rapid (up to 200 km/h).
Slope Failures
Modes related to discontinuity, geometry, and friction strength.
Sliding: Failure along a plane.
Wedge Failure: A wedge of material moves downslope.
Toppling: Blocks topple over, common in limestone environments.
Rotational Slope: Occurs if a stream erodes the base of a slope.
Lateral Spreading: Blocks move and topple in an organized fashion.
Earthflow: Spoon-shaped, with a scarp at the top.
Debris Avalanche: Mixed material (sediment, slurry, sand, blocks) in a slurry.
Rock Wall Scree
Landscape Examples
Sedimentary Landscapes
Characterized by layered rocks, pillars, and evidence of both chemical and physical weathering.
Grand Canyon
Mix of igneous and sedimentary rocks, shaped by the Colorado River. The scale is difficult to comprehend, resulting from the combination of water and time.
Canyonlands National Park
Flat surface of sedimentary rock capped by volcanics. Canyons are formed by streams flowing through headward retreat.
Igneous Landscapes
Characterized by blocky structures.
Glacial Valleys
U-shaped valleys, often with seasonal melt streams. Processes include freeze-thaw and sediment transport by melt streams.
Antarctic Dry Valleys
Classic glacial valleys with linear dunes and frozen lakes.
Australian Deserts
Flat landscapes resulting from long-term erosion. Sand dunes are common.
Kosciuszko National Park
Australia's highest mountain, with glacial features. Active mountain building environment
Capped Landforms
Isolated hills, which result when there is igneous material that protects underlying rock from eroding.
In summary, the following points from the transcript are:
Weathering is fundamental to biogeochemical cycles and landscape evolution.
Understanding weathering processes is important for resource development and environmental management.
Principal weathering processes are mechanical/physical and chemical.
Rates of weathering can be influenced by human activity, such as emissions, air pollution, or vegetation removal.