Geology Chapter: Mass Wasting Overview

Mass Wasting: A Geological Overview

• Mass wasting (slope failure) is the large movement of rock, soil, and other geologic materials downhill due to gravity.
• Key measurements for documenting mass wasting events include rate (speed), motion (how material moves), and material (what moves).
• Rates vary from minutes/seconds (landslides, avalanches) to months/years (slumps), while motion includes falling, sliding, flowing. Material encompasses geological materials, vegetation, snow, and human structures.
• Mass wasting is infrequent due to the shear strength (resistance) of land, particularly in solid formations like mountains.

Causes of Mass Wasting

• Triggers include:
  • Increased water saturation: Heavy rainfall can oversaturate slopes, making soil unstable and causing erosion, though erosion can slowly undermine slope integrity leading to large-scale mass wasting.
  • Increased slope steepness: Steeper slopes have lower shear strength due to gravity.
  • Vegetation removal: Plant roots hold soil together. Loss of vegetation due to wildfires, disease, or unsustainable human practices (clearcutting) decreases soil stability.
  • Earthquakes: Earthquakes reduce shear strength by increasing movement within geologic materials, leading to landslides and other mass movements.

Types of Mass Wasting

• Avalanches (rock and snow): Rapid movement of geologic materials.
• Slumps: Slope failures with a distinctive curved shape.
• Rockfalls: Rocks falling from cliffs.
• Earth flows: Rapid movement of earth.
• Debris flows: Fast-moving flows of earth with debris (boulders, trees, infrastructure).

Impacts of Mass Wasting

• Direct impacts: Injuries, property damage, disruption to daily life, loss of wages (work), harm to livestock/wildlife, and damage to transportation.
• Indirect impacts: Reduced property values, decreased tourism, increased rebuild costs due to safety regulations (retaining walls).

Examples of Mass Wasting Events

• 2017 California landslide (Gorda): Heavy rainfall on a burn scar triggered coastal collapse.
• 2021 Nepal: Himalayan region experienced a 30-year high of mass wasting events due to heavy monsoon rainfall, impacting socioeconomic development.
• 2018 Japan: Numerous landslides affected highways and forests, highlighting the vulnerability of mountainous regions with high seasonal rainfall and earthquakes.
• 1994 California Northridge Earthquake: Caused over 11,000 landslides, demonstrating the significant secondary impacts of earthquakes.

Explore the principles of mass wasting, including its causes and mechanisms. Understand key concepts such as rate, motion, and material involved in these geological processes. Learn how factors like water saturation and slope steepness contribute to slope failures.