mass wasting

CHAPTER 10: MASS WASTING

1. INTRODUCTION

  • Gravity's Role: Everything on Earth's surface is pulled by the force of gravity, impacting all Earth materials including rock and regolith.

  • Mass Wasting: The movement of surficial Earth materials downslope due to gravity; it can have significant practical and theoretical implications, including loss of life.

  • Characteristics of Mass Wasting:

    • Mass of Material: Varies from tiny grains to large volumes.

    • Speed of Movement: Ranges from extremely slow (<1 m/year) to rapid (hundreds of meters/second).

    • Nature of Movement: Can be intact masses or highly mixed homogenized materials.

BACKGROUND: GRAVITY

  • Distinction: Gravitation refers to the attraction between bodies, while gravity is the force acting on objects at Earth's surface.

  • Newton's Law: The gravitational force between two masses is proportional to their mass product and inversely proportional to the square of the distance between them.

  • Weight vs. Mass:

    • Weight: The gravitational force exerted on a body.

    • Mass: Amount of matter in a body, independent of gravitational influence.

BACKGROUND: DOWNSLOPE COMPONENT OF GRAVITY

  • Force of Gravity: Directed towards Earth’s center, defining horizontal planes.

  • Component Analysis: The gravity force can be resolved into components, where the downslope component decreases on gentler slopes.

2. THE CONTROLS ON DOWNSLOPE MOVEMENT

2.1 Envisioning the Problem

  • Experiments:

    • Sand Pile: Building a steep sand pile leads to collapse at a certain angle, termed the angle of repose (30-35° for dry granules).

    • Tilting Plywood: Soil slides off a sheet at an angle, demonstrating failure under increased slope.

2.2 Analyzing the Problem

  • Force Analysis: When downslope gravity force exceeds friction, material fails and moves. Friction acts as shear strength to resist this motion.

  • Factors: Several factors determine the failure angle, including volume and depth of the material.

BACKGROUND: FRICTION

  • Essential Friction: Friction is crucial for motion and prevents complete collapse into depressions.

  • Definition: The force generated when materials slide past each other.

  • Friction and Shearing: Different kinds of friction apply across planes in various materials, affected by surface irregularities.

  • Coefficient of Friction: Describes the relationship between normal and frictional forces, typically less than one.

2.3 The Real World

  • Conditions for Failure: Factors include loading from new material, increased slope steepness, undercutting, and reduced shear strength due to increased pore pressure.

  • Pore Pressure: Increased pore pressure can lift materials, reducing particle-to-particle forces, facilitating sliding.

3. KINDS OF MASS WASTING

  • Factors in Complexity: Various factors include slope, material composition, fracturing, weathering degree, and water saturation.

  • Types of Mass Movement (in alphabetical order): creep, debris avalanche, debris flow, debris slide, earthflow, gelifluction, lahar, landslide, mudflow, rock fall, rock slide, slump, solifluction, sturzstrom.

  • Landslides: General term for detectable rock or regolith movement, excluding slow processes like creep.

3.1 Creep

  • Definition: Slow downslope movement of regolith, imperceptible without prolonged observation.

  • Processes of Creep:

    • Wetting and drying, heating and cooling, freezing and thawing, burrowing actions (e.g., earthworms).

  • Importance: It's a continuous process resulting in a net downslope movement over time; often modest effects rather than catastrophic.

  • Solifluction: A type of creep characterized by saturated regolith flowing over impermeable layers, critical in regions with permafrost.

5. LANDSLIDES

  • Categories of Landslides:

    • Rock Falls: Rapid falls of rock material.

    • Slumps: Movement along a discrete shear surface resulting in characteristic headscarps and earthflows.

    • Rapid Landslides: Large movements from weathered rock, often triggered by rain or undercutting.

  • Undersea Landslides: Largest landslides, often measured in cubic kilometers, increasingly recognized due to advances in imaging technology.

6. DEBRIS FLOWS

  • Characteristics: A mixture of water and sediment flowing downslope; speeds vary widely and are distinct from streamflow movements.

  • Composition: Debris flows can have high sediment volumes and poor sorting, with mixed particle sizes.

  • Mechanisms of Suspension: Includes buoyancy effects, cohesive forces, and dispersive effects during flows.

  • Hazards: Can devastate regions, especially where volcanic materials are involved, leading to lahars.