3.0 Crust of the Earth: Erosion and Mass Wasting

Dr. Zhiquan Li, Department of Geology, Lakehead University

Weathering: The processes involved in the breakdown of rock.
Erosion: The set of processes whereby regolith is picked up and transported from one place to another.
Fluid: Substances which change their shape easily under their own weight.
- Important parameters of fluids:
1. Density
2. Viscosity

Fluid: Any substance that continuously deforms when acted upon by a shear force.
Shear Force: A force acting in a direction parallel to a surface or to a planar cross-section of a body.
Shear Stress: The response to a shear force.
Primary Fluids in Focus: Water and air (as sedimentologists).

Laminar Flow:
- Different streamlines (the path of individual molecules) are parallel to each other and do not intertwine.
- The streamlines may move closer together or further apart but do not cross.
- Occurs in shallow, slow-moving channels with smooth walls and bases, with high fluid viscosity favoring laminar flow.
Turbulent Flow:
- Streamlines twist, turn, and cross each other, exhibiting significant movement in directions other than the overall direction of flow.
- Develops when flow velocity increases, viscosity decreases, the bed becomes rougher, or the channel becomes deeper.

Reynolds Number (Re): Dimensionless number used to determine the nature of flow (laminar or turbulent).
Re = \frac{\text{water depth} \times \text{velocity}}{\text{kinematic viscosity}}
Key in fluid dynamics to predict flow behavior.

Water is the most significant transport mechanism for materials.
- Sediments deposited by water are termed alluvial deposits.
Load Types in Water Transport:
1. Dissolved Load: Fraction in solution as ions.
2. Suspended Load: Fine portion (clay or silt) kept in constant suspension by electrostatic and viscous interactions.
3. Bed Load: Portion that cannot be kept in constant suspension (e.g., sand-sized particles).

Wind blowing over land transports loose particles over large distances, resulting in eolian sediments/deposits (e.g., sand dunes, loess).
- Ventifacts:
- Windblown sand and dust grind away at surfaces in deserts, creating smooth faces or facets on pebbles, cobbles, and boulders.
- If the rock rolls or tilts relative to the wind, new facets form, joining at sharp edges.
Wind carries two types of sediment loads:
1. Suspended Load: Dust-sized particles suspended in the air.
2. Surface Load: Sand particles on the surface, which tend to accumulate in dunes.

Ice is considered a fluid due to its ability to slowly move and transport debris across land surfaces over extended periods.
Features of Ice Transport:
- Chatter Marks: Crescent-shaped gouges formed by fragments dragged across bedrock by glaciers.
- Striations: Linear scratches left by moving glaciers.
- Asymmetrical landforms created by abrasion and plucking by glaciers show former ice-flow directions.

Gravity facilitates sediment transport down slopes.
- Talus: Accumulation of debris at the base of rock walls, formed from gravity-driven movement.
- Angle of Repose: Varies based on several factors, including particle size, shape, and moisture content.
- Larger, flatter, angular pieces remain stable on steeper slopes.
- Increased moisture increases stability until saturation, which can cause a collapse into fluid-like behavior.

Mass wasting processes can be categorized based on the type of material moved and the nature of movement.
Types of materials: Unconsolidated (soil or snow) to solid masses of rock.
Mass Movements: Nonspecific term for rapid movements of rock or soil.

Falls: Rapid, free-falling actions where moving material is not always in contact with the ground (often rockfalls).
- Talus Accumulation: Coarse rubble collected at the foot of slopes prone to rockfalls.
Slides: Cohesive units of rock or soil slip along a defined surface or plane.
- Rockslides: Complex movements involving multiple rock types along the same plane.
Creep: Slow downslope movement of earth materials (often triggered by frost heaving). This gradual motion leads to visual signs of displacement (e.g., tilting fences, trees).
Flows: Chaotic, disorganized movement of material with mixing particles, categorized into specific types:
- Earthflows: Dry soil movement, can travel at speeds of kilometers per hour.
- Mudflows: Saturated with water and can move even faster than earthflows.
- Debris Flows: Water-saturated debris carrying various materials.
- Debris Avalanche: Fastest type of flow, high water content moving down steep slopes.
- Slumps: Erosion where the surface is undisturbed but the block of material below moves, sometimes ending in flow.

Landslides can result from natural events such as earthquakes and floods.
- As streams cut into valleys, they may create unstable slopes resulting in landslides that block valley streams, forming temporary reservoirs.

Human activities such as quarrying, road cuts, and construction can destabilize slopes.
- Removal of material at the base of sloping layers can lead to unsupported masses of rock, increasing slide risks.
- Activities can create conditions that promote slides due to steep slopes exceeding the angle of repose.

Strategies to reduce slide potential include:
- Reducing slope angle
- Adding support material at slope bases
- Reducing loads on slopes by removing weight from high areas.
Ground covers or vegetation can stabilize soil; however, when inadequate, other structures such as retaining walls can be employed.
Rock bolts may be used to stabilize slopes or mitigate rockslides, particularly effective for coherent rocks on low-angle slopes.
The efficacy of retaining walls improves with lower, thicker designs positioned at slide toes to stop movements effectively, reducing water content and pressure within soils is another critical method to prevent mass wasting.