Why should we care about sedimentary rocks?
Understand environments
Processes that are currently happening
Processes that have happened in the past
They are the only rock type that has a good fossil record
They are the only rocks to contain fossil fuels
Oil, natural gas, coal
Regolith
Weather rock material at Earth’s surface
Forms sedimentary rocks
Creates surfaces for landslides or mass wasting
Mass Wasting
Any movement of loose weathered material (regolith) down a slope by gravity
without the direct aid of water or wind
This doesn’t mean that water isn’t important though….
Requirements for mass wasting
Slope of some kind
Reported as a number (no units) or as an angle from the horizontal
90o is straight up and 0o is horizontal
Natural angle of repose
Angle of a slope at which equilibrium is established between the forces of friction
and gravity
Will all materials have the same angle of repose?
Why or why not?
Sand dune: ~34o
Talus slope: 34o-40o
Cinder cone: ~40o
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 ?
Water
reduces coefficient of friction
causes soil particles to move around
increases weight of regolith
IMPORTANT: In mass wasting water is not the transport agent...gravity is
Reason for mass wasting?
Gravitational forces exceed frictional forces
What is a gravitational force?
What are frictional forces?
NOTE: Gravitational force is the driving mechanism of mass wasting
What is force
Push or Pull
Defined as (mass) x (acceleration) (more on this in a minute)
F = ma
Units of Force: Newtons (Kg m/s2)
Directly related to laws of motion
Force changes the state of motion of an object
Newton’s 3 laws of motion
Laws that describe motion of things in the universe
Laws are illustrated with equations
First Law:
A moving object will continue moving in a straight line at a constant speed, and a
stationary object will remain at rest, unless acted on by an unbalanced force
What does this mean?
Second Law:
The acceleration produced on a body by a force is proportional to the magnitude of
the force and inversely proportional to the mass of the object
What does this mean?
F = ma or if we rearrange equation for acceleration:
a = f/m
Large force applied to a small mass--> lots of acceleration
Small force applied to a large mass--> little acceleration
What is acceleration?
Acceleration
a measurement of the rate of change of velocity
Velocity is a rate ŕ m/s
Acceleration is the rate velocity changes ŕ m/s2
Gravitational acceleration and free fall (g= 9.8m/s2)
In free fall, every second something falls, velocity increases
1s= 9.8m/s
2s=19.6m/s (9.8 x 2)
3s= 29.4m/s (9.8 x 3)
What is terminal velocity
Free Fall
air resistance is negligible
Non-Free fall
acceleration is less than g (9.8 m/s2)
force of air resistance balances with weight (force)
How do you calculate acceleration along a straight line?
Ex// a vehicle is accelerating from 0-100 m/sec and it takes 8 seconds?
If the mass of the above vehicle is 750 Kg, what is the force required to achieve this
acceleration?
Third Law
For every action there is and equal and opposite reaction
says forces act in pairs
Types of forces and Mass Wasting
Weight: Fw = mg
Normal force: perpendicular to a surface
Shear force
Frictional force: often opposes normal force
Normal force (s)
perpendicular to the surface
helps prevent movement down slope
portion of the weight of the object (mg(cos q))
mg=weight…
Wt = 150 g
q = 26o
Shear force (t)
causes movement down slope
portion of the weight (mg(sinq))
mg=weight…
Wt = 150 g
q = 26o
Frictional force (opposes the shear force)
Dependent on material
Mass wasting occurs when the shear force is greater than ALL other forces.
Frictional forces
Dependent on coefficient of friction within regolith
Coefficient of friction between surface and regolith
Normal force
Components other than water that contribute to the potential for mass wasting?
Rock type
Bedrock is generally stable
Shale has a low coefficient of friction (creates slip surface)
Heavily fractured or jointed rocks—break apart easy
Vegetation
helps stabilize slopes
absorbs water
helps water run off
What are some things that trigger mass wasting events?
Earthquakes
Overloading a slope
Water, houses, snow, etc
Undercutting or cutting the toe of a slope
Water or people
Over steepening a slope
Removing vegetation
Styles or types of Mass Wasting
There are many different types/styles of mass wasting.
Mudflows
Mudslide
Slump
Rock fall
Avalanche
Ect…
Why are each classified as a different type?
4 things that are used to name mass wasting events
Material involved (mud, rock clay etc…)
Rate of movement (fast to slow)
Amount of water (high water to low water)
Type of movement (slide, fall, rotation, slump etc)
Types of mass wasting
Creep: Very slow movement. ALL slopes will have creep occurring
Slump: rotational motion, material moves as a cohesive mass. Leaves an offset
Mudflows: High water content.
On a volcano these are known as lahars
Follow stream valleys and travel long distances
Rock falls
Rock Slides:
Turtle mountain 1903 (Canada)
Solifluction:
Creates Lobate structures
frozen top surface thaws and flows over frozen lower layer
Local landslides and landslide potential
What is the potential for landslides in the northwest?
Why is it so high?
Volcanic activity
River deposits
High Rainfall
Steep topography
Problem areas?
Coast Range and foothill areas
Gorge
Bridge of the Gods Landslide
Kelso Washington
What are some ways mass wasting can be prevented or minimized?
What are some ways mass wasting can be prevented or minimized?