Mass Wasting Dr Stephen Crabtree October 30 2019

Mass Wasting Dr. Stephen Crabtree October 30, 2019

Mechanical Weathering • Physical forces acting upon rocks, breaking them into pieces • Works from large to small scale • Cracks give water and wind weaknesses to exploit • Examples: – Shrinking/Cooling Cracks – Sheeting – Frost Wedging and Expansion – Thermal Expansion – Biological Activity – Abrasion

Method/Mode of Transport Gravity Water Ice Wind

Mass Wasting • Erosion by gravity • No transport medium required – But often made easier by water • Technically just the downslope movement of rock, regolith, and soil due to gravity

Mass Wasting • Typically occurs along pre-existing joints, fractures, or bedding planes

Mass Wasting – Parallel to slope surface, promotes movement: W x cos q θ 0. 5 sθ – Perpendicular to slope surface, promotes inertia: W x sin q 45° 0. 707 co • Weight in two components 1. 0 sin • Gravity acts vertically on particles, affected by particle masses 0. 0 0° 30° 60° 90° • Shallow slopes promote perp. motion, stability • Steep slopes promote motion parallel to slopes

Mass Wasting • Shear Strength: – Properties of matter that resist the effects of gravity – Three components: • Overall internal friction: Can grains slide past each other? • Effective normal stress: Stress perpendicular to slope, holding “grains” against each other, closing pore spaces – Strongly affected by presence/absence of pore water • Cohesion: Attractive force between particles – Strongly affected by moisture and clay contents

Mass Wasting • A body of material on a slope will remain in equilibrium so long as shear strength exceeds shear stress • Must consider stability threshold, as well as the need of a trigger for movement

Mass Wasting • Relative resistance of materials influences the slope that will develop – Steep slopes maintained for long periods only if rock and soil is tightly bound together – Gentle slopes stable for long periods

Classification Depends on several factors: • Type of movement • Type of material • Rate of motion • etc. . .

Movement Types Broad categories: • Heaving • Sliding • Flowing Classification tied to moisture and movement rate

Mass Wasting – Hot & Wet: Chemical Dryer Hotter – Cold & Dry: Mechanical Wetter Colder • Similarity of Chemical vs. Mechanical weathering strongly tied to rainfall and temperature

Material Types • Rock • Debris • Mud/Earth

Motion Rates Spectrum from very fast to extremely slow Ex: Soil Creep, Slumping Ex: Avalanches, Rockslides

Types of Mass Movement Heaves/Soil Creep • Disrupting forces act perpendicular to the ground surface • Expansive motion is vertical, not explicitly lateral Often a slow downslope precursor to more-rapid mass movements

Types of Mass Movement Heaves/Soil Creep • No continuous external stress – Spasmodic or periodic lowering of cohesion and frictional resistance • Often seasonal and shallow, particularly with “soil creep”

Types of Mass Movement Heaves/Soil Creep • Primarily driven by swelling, freezing, and thawing – Very minor impact of burrowing animals and subsurface vegetation • Distance and rate of motion decrease at depth

Types of Mass Movement Solifluction • Specialized wet-soil creep process in polar climates Continuous Creep • Driven by gravity alone • May affect consolidated rock • May occur in deep subsurface

Types of Mass Movement Slides and Falls • Cohesive blocks of material move on a well-defined surface • No internal shearing • Often seen as “slumping” along scarps or joints

Types of Mass Movement Rockfalls • Single mass of soil and rock travelling as a freely-falling body • Little or no interaction with other solids • Most common on steep slopes, from a Present a significant hazard well-jointed parent in alpine or seismic regions

Types of Mass Movement Rockfalls • Undercutting of the rock or soil face may accelerate the process • Overhangs may shear off unexpectedly New Hampshire’s Old Man of the Mountain: featured on quarter in 2000, collapsed 2003

Types of Mass Movement Topples • Similar to falls • Forward block movement caused by slow rotation around a fixed hinge • Important factor in cliff retreat

Types of Mass Movement Slides • Slope failures initiated by slippage along a welldefined “planar” surface • Sliding mass is generally undeformed – May partially disintegrate, transitioning to flowing • Slide plane not necessarily parallel to surface

Types of Mass Movement Translational Slides • Slides on shallow planar surfaces • Most-common slide type • Strong influence of structure and lithology – Foliation and inclined beds – Relict joints in rock or soil

Types of Mass Movement Rotational Slides • Slides on shallow, concave surfaces • Essentially slumping along a bowl shape • More common with poorly -lithified material or soil than with solid rock

Types of Mass Movement Rockslides • Usually associated with major structural features – Stratigraphy, joints, fabrics – Once fractures open, shear stress is concentrated on unfractured zones in rock • Shear strength entirely based on internal friction

Types of Mass Movement Flows • Moved entirely by differential shearing within the transported mass • No clear plane at the base

Types of Mass Movement Flows • Movement within the displaced mass resembles that of a viscous fluid – Velocity is greatest at the surface, reduced at depth • Often the final event in movement begun as a type of slide Abundant water is often a necessary component

Types of Mass Movement Debris Flows • Complex group intermediate between landslides and water flooding • Include a variety of sizes from boulders to clay, with water

Types of Mass Movement Debris Flows • Significant geologic hazards due to velocity and impact • Sediment contents ~70% to 90% mass – Remainder is water • Includes lahars

Types of Mass Movement Debris Flows • Originate, become mobilized by: – Abundant moisture – Abundant fine grained sediment – Relatively steep slopes

Types of Mass Movement Mudflows • A slurry composed of mostly finegrained material – Sand – Silt – Clay – Water

Types of Mass Movement Debris Avalanches • Extremely rapid • Typically generated on steep bedrock slopes with thin colluvial/soil cover

Types of Mass Movement Earthflows • Very common mass movement events • Range of speeds – If slow, started as slump induced by a rising water table – May have episodic, faster surges

Mass Wasting Hazard Risk Low High

Mass Wasting Triggers • External events often cause landslides – Intense rainfall

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation – Undercutting

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation – Undercutting – Oversteepening of slopes

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation – Undercutting – Oversteepening of slopes – Earthquakes

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation – Undercutting – Oversteepening of slopes – Earthquakes – Removal of vegetation

Mass Wasting Triggers • External events often cause landslides – Intense rainfall – Water-Saturation – Undercutting – Oversteepening of slopes – Earthquakes – Removal of vegetation • Some landslides may have multiple triggers

Alluvial Fans • Common in arid regions • Form at outlets of canyons in which masswasting has occurred

Lahars • Form as a result of volcanic eruptions • Water-saturated debris flows funnelled down stream channels

Permafrost Mass Wasting • Caused by melting of thin top-layer • Meltwater aids shallowslope motion

Mass Wasting Summary • Gravity-driven erosion • Typically occurs along pre -existing fractures • Variable process depending on: – Material type – Movement rate • Generally must have some triggering “event”

Mass Wasting Summary • Movement Types: – Heaves • Creep • Solifluction • Continuous Creep – Slides and Falls • • • Rockfalls Topples Translational Slides Rotational Slides Rockslides – Flows • • Debris Flows Mudflows Debris Avalanches Earthflows

Mass Wasting Summary • Movement Types:

Mass Wasting Summary • Typical Triggers – Intense rainfall – Water-Saturation – Undercutting – Oversteepening of slopes – Earthquakes – Removal of vegetation • Some landslides may have multiple triggers

Next Time… Earthquakes & Seismology • Stress vs. Strain • Causes and Evidence • Seismic Waves • Defining Locations • Magnitude vs. Intensity • Earthquake Prediction