The term mass movement describes all downhill movements of weathered material, including soil, loose stones and rocks, in response to gravity

It excludes movements where the material is carried by ice, water, or wind.

When gravitational forces exceed forces of resistance, slope failure occurs and material starts to move downwards

A slope is an dynamic open system affected by biotic, climatic, gravitational, groundwater, and tectonic inputs which vary in scale and time

The amount, rate and type of movement depend upon the degree of slope failure (see fig 2.12 of handout A given to you with this lecture)

Two ways of classifying mass movements: The first type is the Varnes (1978) classification

system which is self explanatory (see handout B figure 5.2

The second type is the Carson and Kirby classification of mass movement which is also self explanatory (see handout B figure 5.1)

Their diagram shows, each type of mass movement can be plotted on the triangle according to its rate, measured along the base of the triangle, and its moisture content, shown along the right-hand side of the triangle, thus the HEAVES are invariably slow process, flows are wet processes, and slides are dry.

Three things happen: Initial failure of slopeTransportation of slide materialsDeposition of slide materials

The operation of mass movement processes relies on the development of instability on a slope

If instability occurs then mass movement will begin

Some mass movements are fast while others can be very slow which depends on whether the stress placed on the material on the slope exceeds its shear strength greatly or minimaly

When a slope is unstable it is less resistant to gravity then and the following factors will determine the slope’s ability to withstand gravity

the energy that is exerted by gravity on a load is determined by the following factors:1. Steepness of a slope2. Shape and size of particles3. Nature of material that forms a slope4. Depth of the weathered material5. Nature of the ground cover6. Stability of the ground7. Water content of the soil A word I will use often this lesson is

regolith the weathered bedrock from which soils are formed

STEEPNESS The steeper a slope is the greater the

effect gravity has on it which creates a higher level of instability

SHAPE AND SIZE OF PARTICLES– A slope made up of large angular rocks is

more stable than a slope made up of small circular rocks

This is because larger rocks have a greater ANGLE OF REPOSE

NATURE OF MATERIAL THAT FORMS A SLOPE

This refers to whether the material is clay or snow for example

Snow is very unstable and is composed of air and water in both solid and liquid form, it can go from stable to unstable in seconds

Clay can hold water which allows it to be heavy and cause it to collapse at once

DEPTH OF WEATHERED MATERIAL:

The deeper the soil the higher the chance it will slide down slope

Volcanic debris is layered so deep in Hawaii a massive chunk of the Island can slide into the ocean with just a small earthquake

GROUND COVER: The roots of

vegetation hold loose material together

Means transport is less likely around areas where there is a lot of plants

Clear cutting slopes increases the chance of slope failure while grass is planted along the sloping gradient along highways

GROUND STABILTY:

The lack of ground or tectonic stability can cause slope failure to begin

Earthquakes, tremors, natural vibrations, explosions, loud noises, even heavy traffic

WATR CONTENT OF SOIL:

Water fills in the spaces between particles and lubricates them thus allowing it to slide better

Heavy storms will cause landlsides

Water that freezes in the ground casues SOLIFLUCTION – upper section of ground thaws while underneath stays frozen causing the upper layer to slide on the lower layer

HUMAN CASUES: Humans quite often

cause slope failure Clear cut logging

removes vegetation cover allowing raindrops to hit bare soil and weaken it along with the removal of roots holding loose soil

Roads undermine slopes causeing them to fail

Humans building on slopes add weight on them

Urban development increases run-off

Rock bolts are fastened deep into the rock to keep the slope intact

Thin layer of concrete sprayed on the surface of

the rock to hold it together

Put a wired mesh to stop rocks from falling

Pipes inserted deep into slopes to drain excess water to stop

saturation

Retaining walls stop rock from falling and reduce debris

Terracing reduces slope

Snow sheds are built in areas where there is large amounts of

snow fall

Tunnels are made in places where gradient is

too steep

A great graph showing moisture and speed for all types of slides listed in this powerpoint is shown on page 145 of This Earth fig. 2.5.8

CREEPThis is the slowest of the downhill

movements and difficult to measure since it moves at a rate of less than 1cm per year

Almost a contineous processOccurs mainly in humid climates where

there is a vegetation coverOften reffered to as SOIL CREEP ussually occurs on a 5 degree slopeProduces TERRACETTES

there are 2 major causes of soil creep, both resulting from repeat expansion and contraction1. WET-DRY PERIODS – during times of heavy

rainfall moisture increases the volume and weight of soil, causing expansion and allowing the regolith to move down hill under gravity. Drying occurs which will then cause the soil (especially clay) to contract

2. FREEZE-THAW – when the regolith freezes the presence of ice crystals increases the volume of the soil by 9%. As soil expands, particles are lifted at right angles the slope in a process called HEAVE. When the ground thaws and the regolith contract, less particles fall back vertically under the influence of gravity and so move down slope

terracettes

Soil creep

Solifluction: Means soil flow Slightly faster movement between 5cm and 1m

per year Usually takes place under periglacial conditions

where vegetation is minimal During winter the bedrock and regolith are

frozen In the summer the surface layer thaws but

underlying layer remains frozen and acts like impermiable rock

Surface melt water cannot infiltrate downwards which will saturate the surface layer and will flow above the frozen layer

solifluction

FLOW MOVEMENTS:Flows occur in three main forms

mudflows, earthflows, debris flowMUDFLOWS – are rapid movements,

occurring on steeper slopes. Most likely to occur during times of heavy rainfall, when both volume and weight are added to the soil (can contain 60% water)

EARTHFLOWS – when a regolith on slopes of 5-15 degrees becomes saturated with water (not as much as mudflows), begins to flow downhill at a rate varying between 1 and 15 km per year

FLOW MOVEMENTS:DEBRIS FLOW – is the rapid movement of

relatively course material. They commonly occur in mountain regions where the surface is made of debris unconsolidated by any layers of vegetation. Heavy rain can mobalize it as a slurry that looks like cement. They move VERY fast and can carry cars and boulders

mudflow

earthflow

Debris flow

RAPID MOVEMENTS:Slides are different from flows because they

are a dry mass movementThey rely on a weakness already present in

their slope as their causative mechanism In a pure slide, failure occurs along a well

defined SHEAR PLANE (which is a plane that occurs when to parts of the slope slide past each other (see handout D)

A TRANSITIONAL SLIDE is a slide where the shear plane is parallel to the surface of the slope (handout D fig 5.3b)

RAPID MOVEMENTS: ROTATIONAL

SLIDES or SLUMPS (handout D fig 5.3c) have a curved concave shear plane

Material moves from the higher parts of the slope to the lower and is pushed outward

Very Rapid Movements: Rockfalls are spontaneous and

rare debris movements on slopes which exceed 40 degrees

They may result from extreme physical or chemical weathering in mountains, pressure release, storm wave action on sea cliffs, or earthquakes

Material once broken from the surface will either bounce or fall vertically from the scree at the foot of a slope

(handout C helps you visulaize all of this)

http://ca.youtube.com/watch?v=AiczROnfGbc&feature=related (Japanese landslide)

http://ca.youtube.com/watch?v=F31ywRvtNkY&feature=related (Japanese Landslide)

http://ca.youtube.com/watch?v=H6Ma0SVjMHA&feature=related

http://ca.youtube.com/watch?v=51C7vEAVbxk

Characteristics of mass wastage worksheets

Case studies (in class)