Planet earth mass wasting notes

Prof. C.ValentiPlanet Earth Mass Wasting Notes 1

Introduction Weathering of rocks produces sediments, soil, rock fragments, all subject

to gravity, wind, water and ice. Mass wasting is the downslope movement of soil and rock due to gravity

without the aid of wind, water and ice. Mass-wasting is an important part of the erosional process, as it moves

material from higher elevations to lower elevations Mass-wasting causes regolith to move down-slope where sooner or later the loose particles will be picked up by another transporting agent and eventually moved to a site of deposition such as an ocean basin or lake bed.

The main driving force on mass wasting is the force of gravity. Mass-wasting processes are occurring continuously on all slopes; some

mass-wasting processes act very slowly, others occur very suddenly.

Planet Earth Mass Wasting Notes


Gravity Gravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface, parallel to the Earth's surface, the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity.

On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope referred to as the normal force, and a component acting parallel to the slope referred to as the shear force.

The perpendicular component of gravity, the normal force, helps to hold the object in place on the slope.

The parallel component of gravity, the shear force, causes a shear stress parallel to the slope and helps to move the object in the down-slope direction.

As the slope steepens, the shear stress component of gravity increases, and the normal force component of gravity decreases.

Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object.

When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope.

Thus, down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.



Mass Wasting ClassificationsAny perceptible down slope movement of rock or regolith is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement, the composition of the material being moved, and the velocity at which the movement occurs.

Types of mass wasting. Classification schemes are based on The type of material – Mass wasting processes are usually

distinguished on the basis of whether the descending mass started as bedrock or fine or coarse grained soil. The term earth refers to fine grained earth material while debris means any unconsolidated material at the earth’s surface, such as soil and rock fragments of any size.

The style of movement – flow, slip, or fall. Flow – the descending mass is moving downslope as a viscous fluid. Slip – the descending mass remains relatively coherent, moving along

one or more well-defined surfaces. In a slide, the descending mass moves along a plane approximately parallel to the slope of the surface. A slump involves movement along a curved surface, the upper part moving downward while the lower part moves outward.

Fall – the material free-falls or bounces down a cliff.

The velocity of movement.

Mechanism Material VelocityRock Fine-

grained soilCoarse-grained soil

Slide Slump Earth slump Debris slump

Slow

Block slide Earth slide Debris slide RapidFlow Rock

avalancheMudflow Avalanche

Debris flow, avalanche

Very rapid

Creep Creep, earthflow

Creep, earthflow

Extremely slow

Fall Rock fall Earth fall Debris fall Extremely rapid



TYPES OF MASS WASTING (FLOWS, SLIDES AND FALLS)

FLOW Movement resembles a viscous fluid. Fastest movement is at the surface, slowest at the failure surface

(friction). Internal turbulence determines the velocity (churning).

Increasing water, decreases viscosity, increases velocity and vice versa.

Finer grain sizes increase velocity by decreasing viscosity and vice versa.

Mud>Debris>Earth>Creep. Fastest has most water and most fines.

1. Creep. Very slow <1cm/yr. No threat to life, cosmetic damage. Downslope movement of soil and regolith caused by alternating wet/dry, freeze/thaw (growth of ice), thermal expansion and contraction. Sediments is shifted up vertically, gravity pulls particles downslope. Particle movement is perpendicular to the failure surface. Evidenced by bent trees, tilted fences, poles, cracks in walls, doors/windows stick.

2. Earthflow. Faster than creep, slow/sluggish, no threat to life (you can run away). Saturated soil on a steep slope from torrential rains triggers earth flows (associated with rainy seasons). Moves everything, large and small by churning.

3. Debris/Mudflow. Very fast, highly destructive and dangerous. Debris flow is poorly sorted. the fluidized fine grains flow turbulent and

can carry the coarse or large boulders which ‘float’ at the top. Mudflows are all fines. Mudflows have the lowest viscosity and are the

fastest of all flows. Mudflows may turn into debris flows by picking up stuff along the way.- Lahars (mudflow) are associated with volcanic eruptions, where large

quantities of ash becomes mixed with melt water at the top of the volcano.

- Mudflows may occur in arid environments or when heavy rains follow a drought. Local thunderstorms produce a lot of rain in a short period of time. With little vegetation, the result is a quick saturation of sediments.



SLIDES Earth moves coherently as one coherent mass downhill. No internal deformations as in flows. Well defined failure surfaces such as bedding planes or fractures in rock. Triggered by earthquakes.

1. Slump. The slump block rotates downward in one big piece along a curved surface. Caused by erosion at the base of the slope. Evidence is a scarp left behind showing the exposed failure surface. Near the toe, mixing may occur resembling an earthflow. May be terraced, the result of more than one failure surface. Objects on/in the moving block remain undisturbed, just relocated.

2. Landslides. Fast, rapid sliding mass of bedrock with soil on top. Downslope movement of a rock mass along a bedding plane or anywhere rock surfaces are inclined toward an open space (road cut). Triggered by earthquakes which fracture rock along weak bedding planes.

FALLS Steep slopes such as vertical cliffs or walls of rock. Totally under the influence of gravity (not water, grain size). Can be triggered by an earthquake. Fractures and joints in rock may undergo frost wedging, root wedging, or

thermal contraction/expansion breaking rocks into chunks teetering on the top.

Evidence is talus cones, fans of steep rock piles at the base of the cliff.

CONTROLS ON SLOPE STABILITYMass wasting is slope failure or any down slope movement of soil and rock under the influence of gravity. Objects on slopes are acted upon by forces that try to cause them to move down hill under the influence of gravity. They work against forces that resist downslope movement. An unstable slope is a slope where the forces driving material downhill

(shear force) exceed the forces that are resisting downslope movement (shear strength).

Stable slopes have resisting forces (shear strength) equal to or greater than driving forces (shear force).

DRIVING FORCES Fd(driving force) = w (weight) * sin theta. Weight = mass*gravity. Gravity. Always pulling objects down, towards the center of the earth at

9.8 m/sec2. Mass. since gravity is constant, increase the mass (therefore the weight)

of the material on the slope will increase the driving force. Increasing



mass increases the weight of an object on a slope, shifting the shear force component of gravity to increase (decreasing the normal component of gravity). Mass on slopes can be increased by constructing buildings, adding water to soil pore spaces, increasing vegetation.

Slope Steepness (angle theta). Zero degrees is horizontal, ninety degrees is vertical (cliff). Increasing the angle theta, causes gravity to effectively work towards moving things downslope. Increase the sin of theta (slope angle) will increase the driving force. Increasing angle theta increases the shear force component of gravity.

The steeper the slope, and the heavier the material, the greater the shear force and the greater the tendency for the block to slide.

Pore Water Pressure. Between each grain of sediment, there exists the force of friction keeping material on the slope. If the pore spaces between the grains is saturated with water, water pushes the soil particles away from each other, reducing the friction between the grains, driving material downslope. Friction between grains is lost and material fluidizes. Water also adds mass to the slope.

Regional Geology. Stratified rock layers increase downslope movement if layers dip parallel to the slopes. This is especially prevalent in rock layers that are underlain by weak materials. Weaker lower layers weather easier.

RESISTING FORCES There are a great number of forces acting on slope materials to keep them up (favor shear strength). Grain Size and Shape. Grain size and shape affects the amount of

friction between grains. Angular grains tend to interlock better, increasing friction between grains. Fine grain sized, clay particles actually 'stick' to one another.

Cohesion. Each clay mineral is charged, therefore there is an electric attraction to each other. Clay has high cohesion. Sand, well rounded, electrically neutral minerals are cohesionless. Angle of repose is directly related to the cohesiveness of the material. It defines the steepest slope at which a pile of material will be stable Sand has an



angle of repose of 32-35 degrees. When you try to make it steeper, material rolls down slope. Clays can practically cut a vertical wall it has a such a high angle of repose.

Pore Water Pressure. The presence or absence of water between grains

can influence the angle of repose. Adding a little water creates a film of surface tension (because water is a polar molecule), increasing the angle of repose and increasing shear strength of the material on the slope. Like building a sandcastle. Water can increase and decrease slope stability.

Vegetation. Vegetation can stabilize slopes by (1) reducing pore water pressure by keeping water in biomass, and removing excess water by transpiration. and (2) root systems increase the cohesion of sediment through water uptake, keeping soils unsaturated. Absence of vegetation on slope is an indicator of mass wasting.


Planet earth mass wasting notes

Technology

Transcript of Planet earth mass wasting notes