Mass wasting:

Any movement of loose weathered material (regolith) down a slope by gravity without the direct aid of water or wind

Reason for mass wasting?

         Gravitational forces exceed frictional forces

               What is a gravitational force?

               What are frictional forces?

Requirements

         Slope of some kind

               Reported as a number (no units) or as an angle from the horizontal

                     90^o is straight up and 0^o is horizontal

               Natural slopes over 40^o are rare due to natural angle of repose

angle of a slope at which equilibrium is established between the forces of friction and gravity

                     Angle of repose

                           Will all materials have the same angle of repose? 

                           Why or why not?

                           Each material has different properties (grain size, shape and sorting)

                                 Different materials have different coefficients of friction

changing the properties of the material changes the coefficient of friction and therefore the angle of repose

What are some natural processes that can change the properties of material found in our environment ?

Forces involved in mass wasting?

         Normal force

perpendicular to the surface

prevents movement down slope

portion of the weight of the object (mg(cosq))

         Shear force

causes movement down slope

portion of the weight (mg(sinq))

         Frictional force     

              (opposes the shear force)

Mass wasting occurs when the shear force is greater than the other forces.

Ex//  a rock that weighs 20 Kg is sitting on a hill that has a slope of 18o. 

What is the shear force for this rock? 

What is the normal force? 

If this is a frictionless surface, will the rock slide down the hill? 

What components contribute to the potential of mass wasting?

         Water

reduces coefficient of friction

causes soil particles to move around

increases weight of regolith

                     IMPORTANT:  Water is not the transport agent

         Rock type

bedrock is generally stable

shale has a low coefficient of friction (creates slip surface)

heavily fractured or jointed rocks

          Vegetation

helps stabilize slopes

absorbs water or helps water run off

What can trigger mass wasting?

What are different ways things move downhill?

What determines what type of mass wasting occurs?

         Material involved

         Rate of movement

         Amount of water

         Type of movement

What are some types of mass wasting?

Creep

         Expansion/contraction motion

Slump

         Rotational motion

Mudflow

         High water…flow

Rock slides/falls

         Slides move along a surface

         Falls…free motion from a height

Earth flow

         Slow motion but high water

Solifluction

         Permafrost regions

               Freeze thaw cycles