ESCI 121 Lecture 15: Mass Movements and Landscape Evolution of Landscapes

Landscape Development and Landforms

• Definition of Landscape: Refers to the character and shape of the land surface in a specific region.

• Individual Landforms: Landscapes are composed of specific features including:     * Valleys and ridges.     * Mountains.     * Rivers.     * Mesas.     * Sand dunes.     * Escarpments.     * Craters.     * Beaches.     * Headlands and bays.

• Relief: Defined as the elevation difference between two specific places. Areas are categorized based on relief:     * Low relief areas: Categorized by minimal elevation differences.     * High relief areas: Categorized by significant elevation differences.

• Forces Influencing Landscapes:     * Constructional Processes: Driven by internal energy (tectonics), which may cause the landmass to uplift or subside (sink).     * Destructional Processes: Driven by external and gravitational energy.     * Movement of Sediment: Leads to erosional processes (destructional) and depositional processes (constructional).

• Factors Governing Landform Development:     * Eroding or Transport Agents: Water, ice, wind, and gravity.     * Relief: Whether the terrain is steep or flat.     * Climate: Includes temperature, wind, and precipitation patterns.     * Substrate Composition: Whether the material is resistant to erosion or easily eroded.     * Organisational Activity: Includes activities of plants, animals, and humans.     * Time: Landforms are not static and evolve over geologic time.

• Anthropogenic Erosion: Humans are currently responsible for more erosion than natural processes through both direct effects (e.g., quarries) and indirect effects (e.g., farming).

Introduction to Mass Movements and Mass Wasting

• Mass Movements (MM) / Mass Wasting (MW): Terms used interchangeably to describe the downslope movement of rock and/or regolith under the direct influence of gravity.

• Regolith: The loose rock material, including sediment and soil, that sits on top of the solid bedrock.

• Primary Driving Force: Gravity is the main agent. While water is often present and important, it is considered a secondary influence.

• Location and Rate: These movements operate on any sloping surface and exhibit a wide range of rates, from extremely fast to extremely slow.

• Fundamental Drivers: Climate and Tectonics are the governing controls.     * Tectonics: Responsible for placing rock in high-elevation positions.     * Weathering/Surface Processes: Speed along the breakdown of material.     * Gravity: Ensures that rock in high places does not stay there long.

• Debris Transfer: Weathering constantly generates loose material. This sediment is transferred away from the site via two primary processes:     1. Mass movement.     2. Transport by water or ice.

• The Rock Cycle: Mass movements are a crucial component of the rock cycle.

The Physics of Slope Stability

• Gravity Calculations: Gravity is a force that acts everywhere on Earth, pulling objects toward the center.     * Force ($F$) is defined as mass ($m$) times acceleration ($a$).     * Weight is the pulling force of gravity, expressed as $W = m imes g$, where $g$ is the acceleration due to gravity ($9.8\,m/s^2$).

• Force Distribution on Slopes:     * Horizontal Surface: The entire gravitational force acts toward the surface; there is no horizontal movement.     * Sloping Surface: Steeper slopes create a larger downslope force, creating an imbalance.

• Stability Determinants: Slope stability is determined by the relationship between two opposing forces:     * Downslope Force (Stress): The component of gravity pulling the material down the hill.     * Resistance Force (Strength): This includes binding forces of grains in rock, as well as friction and cohesion between loose grains or blocks.

• Stability Outcomes:     * If Resistance Force > Downslope Force, the slope is stable.     * If Downslope Force > Resistance Force, the slope is unstable, leading to slope failure.

• Triggers for Unstability:     * Increasing the Slope: Natural processes (e.g., a river undercutting a hillside to create an overhang) or human activities can increase the angle, thereby increasing the downslope force.     * Increasing the Load: Adding mass to a slope increases the stress.

• Angle of Repose: The maximum equilibrium angle at which loose, dry material can be piled without sliding. It is determined by the friction of the individual grains.

The Dual Role of Water in Slope Stability

• Low Saturation (Beneficial): Slightly wet unconsolidated material often has a higher angle of repose because of cohesion. Surface tension between the water and grains tends to hold the grains together, increasing the material's strength.

• High Saturation (Detrimental): When material becomes saturated, the angle of repose becomes very small.     * Water eliminates grain-to-grain friction.     * The material behaves like a liquid or slurry (liquefaction).     * The added water increases the weight of the mass (increasing downslope stress) while simultaneously reducing shear strength (reducing friction and cohesion).

Classification of Mass Movements

• Criteria 1: Type of Motion (Mechanism):     * Free Fall: Material falls through the air, bounces, or rolls downslope.     * Slide: Rock or regolith moves along a specific rupture surface (failure surface).     * Flow: Continuous movement of rock and/or regolith that behaves as a high-viscosity liquid.

• Criteria 2: Character of Material:     * Rock: Refers to solid rock material regardless of fragment size.     * Debris: Coarse-grained material; between $20\%$ and $80\%$ of fragments are larger than $2\,mm$.     * Earth: Fine-grained material; $80\%$ or more of fragments are smaller than $2\,mm$ (sand or smaller).

Specific Types of Mass Movements

• Rock/Debris Fall: Individual pieces of rock or debris fall off steep slopes. This often creates a talus pile at the base of the slope, typically positioned at the angle of repose. Falls may evolve into avalanches.

• Slides: Material moves along a failure surface. These failure surfaces can be planar (such as joints or bedding planes).

• Slumps: A type of slide characterized by rotational movement along a curved surface (rotational glide). Features include a "scarp" at the top and often result in back-tilted trees and pavement.

• Sediment Flows: Solid particles move in a way that they interact with each other. These can be dry or wet.     * Creep: The slowest form of flow. It involves extremely slow movement of rock or soil caused by expansion-contraction cycles (freeze-thaw or wet-dry). Visible signs include curved tree trunks, tilted fence posts, and broken retaining walls.     * Solifluction: A very slow downslope flow of water-saturated soil.     * Debris Flows: Viscous mixtures of rock fragments, mud, and water. Their high viscosity allows them to carry large boulders and trees. Velocities range from $1\,km/yr$ to $100\,km/hr$. They are often triggered by intense rainfall.     * Mudflows: More fluid versions of debris flows consisting of smaller particles and more water.     * Lahars: A specific type of mudflow or debris flow composed of volcanic materials, commonly triggered by eruptions or melting ice.     * Rock Avalanches: The fastest form of dry mass movement, often triggered by earthquakes or initial falls. Example: Sherman Glacier, Alaska ($1964$), which moved approximately $2.5\,km$.

Historical Case Studies and Statistics

• Frank, Alberta (1903): A rock avalanche that resulted in the loss of $76$ lives and the destruction of most of the town of Frank.

• Yungay, Peru (1970): An earthquake broke a huge mass of ice off a glacier. The ice melted into a mudflow that carried boulders, burying the town and killing $18,000$ people.

• Gros Ventre Slide (1925): A planar slide caused by the orientation of rock layers, undercutting by a stream, and saturation from rain/snowmelt. The scar remains prominent today, measuring approximately $70\,m$ deep.

• Mount St. Helens (1980): The largest mass movement ever witnessed by humans. A slide/debris avalanche was triggered by bulging magma, which in turn triggered lahars.

• New Hampshire (2003): The "Old Man of the Mountain" natural rock formation collapsed due to mechanical weathering and gravity.

• Appalachians: Debris flows are a key evolutionary process for landforms in this region, occurring somewhere in the area approximately once every $3\,years$.

• Hurricane Camille (1969): Caused significant debris flows in western Virginia.

• Venezuela (1999): In Los Corales and Caraballeda, over $4\,m$ of rain fell in two days, triggering mud and landslides that claimed between $10,000$ and $50,000$ lives.

• Taipei, Taiwan (2010): Site of a major landslide.

• California Mudslides (1982): Triggered by heavy rains, burying vehicles.

• Granby, Colorado (1985): A minor landslide caused the derailment of a passenger train.

• Hawaii Nuuanu Debris Avalanche: A massive subaqueous slide measuring $23,000\,km^2$, $235\,km$ in length, and $35\,km$ in width.

Prevention and Mitigation Strategies

• Revegetation: Planting vegetation has two benefits:     1. Removing water from the soil through evapotranspiration.     2. Roots bind and anchor the regolith.

• Redistributing Mass:     1. Regrading: Reshaping the slope to an angle below the angle of repose.     2. Terracing: Cutting steps into the slope to catch debris and reduce the mass loading.

• Drainage: Dewatering the slope through pipes or ditches to reduce weight and increase shear strength.

• Erosion Control: Slowing or eliminating undercutting by removing the agent of erosion or using riprap (large rocks) to protect the base of the slope.

• Engineered Structures:     * Retaining Walls: Barriers designed to pin the base of the slope and trap falling rock.     * Covers: Fencing or coatings (shotcrete) draped over outcrops.     * Rock Bolts: Steel bolts drilled into the rock to hold loose facing material to the solid bedrock.