What is mass wasting

-refers to the downslope movement of rock, regolith, and soil under the direct influence of gravity. 

      How does mass wasting differ from erosion?

-It does not require a transporting medium such as water, wind, or glacial ice. 

What is the ultimate destination for sediment?

The Sea

Why are stream valleys much wider than the streams creating them (example:  Grand Canyon)?

-The combined effects of mass wasting and running water produce stream valleys, which are most common and conspicuous of Earth’s landforms. 

If streams alone were responsible for creating the valleys in which they flow, the valleys would be very narrow features.  However, the fact that most river valleys are much wider than they are deep is a strong indication of the significance of mass-wasting processes in supplying material to streams.

What must exist for mass wasting to occur?

Slopes that rock, soil, and regolith can move down.

    How is this constantly being created?

-rapid events occur on young mountain ranges.

   -young mountains posses steep unstable slopes

-Here, massive mass wasting can occur.

    -over time, the areas elevation is lowered giving away.

-Earth’s mountain building and volcanic processes that produce these slopes through sporadic changes in the elevation of landmasses and the oceans floor. 

Where does mass wasting occur?

In areas of rugged, geologically young mountains.  

What causes a slope to reach its point of instability?

- If dynamic internal processes did not continually produce regions having higher elevations, the system that moves debris to lower elevations would gradually slow and eventually cease.

-long before mass wasting occurs, various processes weaken the slope making it more susceptible to gravity

-Eventually the slope is weakened to the point of instability.

What is a trigger?

-an event that initiates downslope movement.

-remember that the trigger is not the sole cause of the mass-wasting event but just the last of many causes. 

What controls mass wasting events?

1. Water

2. Oversteepened Slopes

3. Removal of Vegetation

4. Earthquakes 

Water

-mass wasting is sometimes triggered when heavy rains or periods of snow-melt saturate surface materials. When the pores in the sediment become filled with water, the cohesion among particles is destroyed, allowing them to slide past one another with relative ease. 

-for example, when sand is slightly moist, it sticks together quite well. However, if enough water is added to fill the openings between the grains, the sand will ooze out in all directions.  Thus, saturation reduces the internal resistance of materials, which are the easily set in motion by the force of gravity

  What is cohesion?

-Cohesion- shear strength of a rock/sediment

-when pores space becomes filled with water (saturated), the cohesion between grains is destroyed allowing them to slide past one another.

Oversteepened Slopes

 – present in a variety of locations:

-a stream undercutting a valley wall.

-waves pounding against the base of a cliff.

-humans lent to create over steepened slopes that later prime locations for mass wasting.

       What is the angle of repose?

-gravity is the controlling force of mass wasting, but several factors play an important role.

Removal of Vegetation - 

-plants deter erosion and contribute to slope stability.

-their root system binds soil together.

-plants also shield thee slopes surface from raindrop impact.

-when vegetation is lacking, mass wasting is enhanced.

   What effect do wildfires have on an area?

-during summer, wildfires are common occurences.

-wild fires result in the loss of anchoring vegetation.

-this accelerates the mass wasting process.

Earthquakes - 

conditions favoring mass wasting may exist for a long time without an event occurring.

-one of the more important triggers are earthquakes.

-also, if the sediment is saturated, liquefication will occur.

With all that being said, can mass wasting events occur without a trigger?

-yes.

-sometimes, mass-wasting events can occur without any distinguishable trigger.

Where is the greatest risk for mass wasting events?  Why?

-In the US, all states experience some damage from mass wasting events.

-greatest risk occurs in mountainous areas.

  1-heavy rains

    2-melting snow

                  3-many mountainous regions are prone to earthquakes.

What are the different types of mass wasting events classified by?

                  1) Type of Material

                  2) Type of motion

                  3) Rate of Movement

          1) Type of Material

-did the descending mass begin as unconsolidated material or as bedrock?

-if unconsolidated, the terms debris, mud, or earth are used.

-if bedrock, the world rock is used.

        2) Type of motion

-How did the descending mass move downslope?

 Fall – when movement involves free fall of individual pieces, it is termed a fall.

-fall is common on very steep slopes where loose material cannot remain stable

What is a talus slope?

-rock falls are responsible for building and maintaining talus slopes. Rock fragments lying at base of a steep slope from which they originated.

     Slide – Mass movements where there is a distinct zone of weakness separating the slide material from the stable underlying material.

  Example?  Slideona Playground

Flow – occurs when material moves downslope as viscous fluid.

-most flows are saturated with water and move as lobes or tongues. 

  3) Rate of Movement

-most mass wasting is imperceptibly slow.

-each site of mass wasting behaves according to its environment.

  -rates of movement can be rapid of extremely gradual

  What is the fastest?

-most rapid type is a rock avalanche.

-can travel at speeds of 12MPH (or more).

-at this speed, rock literally “floats in air” as they travel downslope.

-during avalanches, the debris traps and compresses the air beneath it.

Make some notes about the rapid mass wasting events:

1. Slumps

     2)  Rockslides

1)  Slumps

-downward sliding of rock or unconsolidated material moving as a unit along a curved surface.

-the slumped material does not travel fast nor very far.

-a very common type of mass wasting, especially in clay.

-commonly occurs on an over steepened slope.

-the upper portion is held in place by the material at the bottom.

-as the bottom is removed, the upper material becomes unstable.

-can also occur due to undercutting at the slope’s bottom by wave action or river flow.

 2)  Rockslides

-occurs when blocks of bedrock break loose and slide down slope.

-one of the fastest and most destructive mass wasting events.

Triggers for rock slides:

-usually occur where rock layers are inclined or where joints exist parallel to the slope

Triggers: undercutting of the base.

                -water lubricating underlying rock layers  

-earthquakes

3)  Debris Flows

-very rapid mass wasting events involving the flow of highly saturated unconsolidated materials.

-sometimes called mudflows when the material is fine grained.

-tend to flow canyon sand streams channels.

 -debris flows are categorized into the following:

A.) Debris flows in semi arid conditions

       -semi arid regions sporadically receive cloud bursts and/or rapidly melting snow, creating a sudden flood

       -the end product is a flowing tongue of melting mixed mud, soil, rock, and water. 

B.) Lahars

 -debris flows composed of volcanic materials on the flanks of volcanoes.

   -historically, Lahars are one of the deadliest volcanic hazards.

What is an alluvial fan?

  -the rate of flow depends on slope and water content.

  -when reaching the end of a canyon or river channel, it spreads out into a fan-like deposit called an alluvial fan. 

4.)    Earthflows

-typically quite viscous.

    -move slower rates than debris flows

    -movement is typically faster during wet periods. -when water saturates a hillside, material may detach, forming a lobe/tear drop shaped mass flowing downslope.

  -Most often form in humid areas during times of heavy precipitation or snow melt.

    -earthflows are not confined to stream or canyon channels.

Slow mass wasting events:

1. Solifluction

2. Creep

Creep

– gradual downhill movement of material.

-expansion and contraction of surface material due to freezing and thawing contributes to creep.

    -observing creep- creep imperceptibly slow and cannot be observed in action.

  -creep accelerates during any distribution to the slope.

 - acceleration also occurs when the ground becomes saturated with water

Solifluction

-when a slope is saturated, it may flow downslope at a rate of a few millimeters per day.                 

 -common whenever water cannot escape from the saturated surface layer by percolating deeper.