Presentation on theme: "“stuff rolls downhill”"— Presentation transcript:
1 “stuff rolls downhill” Ch. 15 Mass Wasting“stuff rolls downhill”
2 Mass WastingThe downslope movement of rock, regolith, and soil under the direct influence of gravity.Does not require a transporting medium.It is the process that takes place between weathering and erosion.Combined effects of mass wasting and running water produce stream valleysFrom slow imperceptible creep to fast moving avalanches.
3 Why is mass wasting important? MW processes represent a significant hazard to people and propertyNeed to identify where and under what conditions these occurAvoid construction in areas prone to mass wastingAttempt to prevent mass wasting
4 Mass Wasting and landform development For mass wasting to occur, there must be a slope angleMost rapid events occur in areas of rugged, geologically young mountainsAs a landscape ages, less dramatic downslope movements occur
5 Controls on Mass Wasting Gravity is the controlling force.Water is a factor.Destroys cohesion or internal resistance between particles.Creates buoyancy for masses of regolith and soil, thereby reducing the frictional coupling with the underlying substrate.Adds considerable weight to the mass of material.Changes the properties of clay; clay becomes "slick" when wetted.
6 Controls on Mass Wasting Adding material to the top of the slope or undercutting the slope at its base can increase the angle of repose.Oversteepening of slopes is a factor.Rock debris is stable at slope angles less than the angle of repose.Angles of repose vary between 25 and 40 degrees depending on the materials.
9 Classification is based on: Type of materialUnconsolidated vs. consolidated (e.g., bedrock)Dry vs. water saturatedType of motionFall: Free-fall on steep slopes. Forms talus slopesSlide: Movement along well-defined surface; material remains fairly coherent.Flow: Material moves as a viscous fluid, usually when saturated with water.Rate of movement
11 SlumpDownward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface.Slumped material does not travel very fast or very far.Crescent-shaped scarps are formed.Water percolating downward and along the curved surface may promote further instability through lubrication and buoyancy.Commonly occurs on slopes that have been oversteepened.
15 Rockslide or debris slide Downward sliding of blocks of bedrock that have broken loose.Among the fastest and potentially most destructive of the mass wasting processes.Often occurs in areas where the rocks are highly fractured, particularly if the fracture surfaces or bedding planes dip downslope.Often triggered by an earthquake.Examples - Madison River and Gros Ventre rockslides
17 MudflowRapid type of mass wasting that involves a flowage of debris containing a large amount of water.Most characteristic of semiarid mountainous regions.Tend to follow canyons and gullies.Lahars are mudflows on the slopes of volcanoes, often accompanying eruptions. E.g., Mount St. Helens.
19 Earth flowDownslope movement of water-saturated soil on hillsides in areas of deep weathering.Form tongue-shaped masses with well-defined head scarps.Moves relatively slowly and may be active for periods ranging from days to years.
22 Creep Imperceptibly slow downslope movement of soil and regolith. Can take place on even gentle slopes and is extremely widespread.A primary cause is the alternate expansion and contraction of surface materials caused by freezing and thawing or wetting and drying.
39 What causes debris flows in LA? San Gabriel Mtnsdeeply fractured due to stresses on the rocks caused by faultsrapidly uplifting and weatheringVery steep slopesFiresStrip vegetation from the slopesCombustion of chaparral plants leaves wax-like substance about 1 cm below soil surface. This prevents infiltration of rain and increases runoffRainLA averages ~ 15 in/yr.San Gabriels can get extreme rainfall eventsJan >44 ins. in 9 daysFeb – 1.5 ins. In 25 minutesApril 5, 1926 – 1 in. in 1 minute