Changing Earth’s Surface Mass Movement

RATES OF WEATHERING Factors of Rates of Weathering 1. Parent Material (The rocks themselves) - Igneous and metamorphic most resistant, sedimentary least resistant because of pores. 2. Surface Area 3. Climate - Warm and wet = chemical weathering Cold and Dry = mechanical weathering

Risk factors to increase likelihood of mass movement Gravity - hill slopes more vulnerable (on top of a hill, on the slope, or at the bottom of a hill), modified slopes (road cut, cut flat area to build on, coastal erosion, etc.) Water - risk is higher when ground is saturated and/or during heavy rains, El Niño events Earth Materials - loose soils (particularly clay-rich) or fractured rock, and old landslides pose greater risk Triggering Events - heavy rain during storm, rain after big storms or fires, earthquakes (when ground is saturated?)…are all triggers

EROSION EROSION - The breakup and removal of rock by moving natural agents (rivers, glaciers, wind, water)

Mass Movements General term used for rock fragments moving down a slope What is causing this erosion to occur? Can happen either fast or slow.

Toppling Failure TOPPLES: Toppling failures are distinguished by the forward rotation of a unit or units about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks

Rockfall - Rocks falling from a steep cliff Rockfall - Rocks falling from a steep cliff. This is the most rapid type of mass movement.

Talus Slopes - Large piles of rock that break off and pile up at the bottom of a hill

Debris flows or mud flows Mass movements that behave like fluids Unlike slides, flows are not controlled by a failure surface, but instead are dominated by internal movements

Debris flows or mud flows Factors that allow the force of gravity to overcome the resistance of earth material to landslide movement include: saturation by water steepening of slopes by erosion or construction alternate freezing or thawing earthquake shaking volcanic eruptions

Landslides Sudden movement of masses of loose rocks. Triggered by: Earthquakes Rainfall Thaws Volcanoes

magnitude 7.1? West Yellowstone 11:37 am on August 17, 1959 magnitude 7.1? West Yellowstone Produced waves in Hebgen Lake that swept over dam Triggered landslide of 85 million tons of rock Sped downslope at 150 km/hr and produced hurricane force winds • cars blown into air • valley floor covered by 45 m of rubble • 28 people (campers) were killed Madison Canyon slide

Mudflow - Fast movement of large masses of mud. Occur in dry, mountainous regions during fast, heavy rainfalls or volcanic eruptions

Creep - very slow down hill movement of weathered rock material. Usually goes unnoticed unless it hits a building or something. Creeps can include rocks, plants, buildings, etc.

MASS MOVEMENTS Speed of movement and water flow. On left have mass movement speed versus moisture content. On right have rates of travel for mass movements

water: two effects 1) small amounts of water • glues particles by surface tension--”sand castles” 2) excessive amounts of water • reduces friction between surface material and underlying rock • counteracts normal component of gravity …water pushed upward…

Water decreases rock/soil cohesion

Water decreases rock/soil cohesion

Water decreases rock/soil cohesion Water circulating underground can dissolve cements that hold sedimentary rocks together

The Weight of Water Sedimentary rocks commonly have porosities of 10 - 30% If pore spaces fill with water, the weight of the material is increased substantially, creating instability

SOILS Soils - soil science = PEDOLOGY Most important result of weathering and erosion is soil Two general classes of soil: A. residual soil - soil made from local bedrock B. Transport soil - soil that was moved by wind, or glacier

Factors determining soil makeup and Formation 1. Climate - Rainfall amount and temperature (most important) 2. Parent Rock - Type and chemical composition 3. Vegetation - Roots and Leaves 4. Topography - hilliness, etc. 5. Time - how long to develop

vegetation …removal (by fire or clear-cutting) leads to mass movement roots stabilize loose, unconsolidated material …removal (by fire or clear-cutting) leads to mass movement

prevention water, weight of house, road cut

improve drainage -- leads to less creep

modify slopes (where layering dips into roadway)