Mass movements are large-scale movements of earth that are not caused by rivers, glaciers, or waves. They include slow movements like soil creep and fast movements like avalanches. Mass movements are caused by a reduction in the internal strength of a slope or an increase in downward forces. Common types of mass movements are falls, slides, slumps, flows, and avalanches.

1. ASGEOGRAPHY
ROCKSANDWEATHERING
3.3SLOPEPROCESSESAND
DEVELOPMENT

2. MASS MOVEMENTS
Mass movements are large-scale movements
of the Earth’s surface that are not
accompanied by a moving agent such as a
river, glacier or ocean wave.
Mass movements (Figure 3.5) include:
• very slow movements, such as soil creep
• fast movements, such as avalanches
• dry movements, such as rock falls
• fluid movements, such as mud flows

3. CAUSESOFMASSMOVEMENT
The likelihood of a
slope failing can be
expressed by its safety
factor.
This is the relative
strength or resistance
of the slope, compared
with the force that is
trying to move it.
The most important
factors that determine
movement are gravity,
slope angle and pore
pressure.

4. GRAVITYEFFECTS
Gravity has two effects:
• It acts to move the material
downslope (a slide component).
• It acts to stick the particle to the
slope (a stick component).
The downslope movement is
proportional to the weight of the
particle and slope angle.
Water lubricates particles and in
some cases fills the spaces between
the particles, which forces them
apart under pressure.

5. CAUSESOFSLOPEFAILURE
Slope failure is caused by
caused by two factors:
• a reduction in the internal
resistance, or shear
strength, of the slope
• an increase in shear stress,
that is, the forces attempting
to pull a mass downslope.
Both can occur at the same
time.

6. TYPESOFMASSMOVEMENTHEAVE
Heave or creep is a
slow, small-scale
process, which occurs
mostly in winter.
It is one of the most
important slope process
in environments where
flows and slides are not
common.
Talus creep is the slow
movement of
fragments on a scree
slope.

7. HEAVEPROCESSES
Individual soil particles are pushed or heaved to the surface by:
(a) Wetting,
(b) Heating,
(c) Freezing of water.
About 75% of the soil creep movement is induced by moisture changes and
associated volume change.
Freeze–thaw and normal temperature-controlled expansion and contraction
are also important in periglacial and tropical climates.
Particles move at right-angles to the surface as it is the path of least
resistance. They then fall under the influence of gravity when the particles
have dried or cooled, or when the water has thawed.
Net movement is downslope. Heave forms terracettes.

8. FALLS Falls occur on steep slopes (>40°),
especially bare rock faces where joints are
exposed.
The initial cause of the fall may be
weathering, such as freeze–thaw or
disintegration, or erosion prising open
lines of weakness.
Once the rocks are detached they fall
under the influence of gravity (Figure 3.6).
If the fall is short it produces a relatively
straight scree. If it is long, it forms a
concave scree.
Falls are significant in causing the retreat
of steep rock faces and in providing debris
for scree slopes and talus slopes.

9. SLIDES
Slides occur when an entire
mass of material moves along
a slip plane.
They include:
• rockslides and landslides
of any material, rock, or
regolith
• rotational slides, which
produce a series of
massive steps or terraces

10. SLIDESWHENTHEYOCCUR
Slides commonly occur where there is a
combination of weak rocks, steep slopes and
active undercutting.
They are often caused by a change in the
water content of a slope or by very cold
conditions.
As the mass moves along the slip plane it
tends to retain its shape and structure until it
impacts at the bottom of a slope (Figure 3.7).
Slides range from small-scale slides close to
roads, to largescale movements killing
thousands of people.

11. SLIPPLANES
Slip planes occur:
• at the junction of two layers
• at a fault line
• where there is a joint
• along a bedding plane
• at the point beneath the surface where the shear stress becomes
greater than the shear strength

12. LOOSEROCKSTONESSOIL
Loose rock, stones and soil all have a tendency to move downslope.
They will do so whenever the downslope force exceeds the resistance
produced by friction and cohesion.
When the material moves downslope as a result of shear failure at the
boundary of the moving mass the term landslide is applied.
This may include a flowing movement as well as straightforward sliding.
Landslides are very sensitive to water content. This reduces the strength of
the material by increasing the internal pressure.
This effectively pushes particles apart thereby weakening the links between
them. Moreover, water adds weight to the mass, increasing the downslope
force.

13. EXPERTTIP
Weak rocks such as clay have little shear strength to start with and are
particularly vulnerable to the development of slip planes.
The slip plane is typically a concave curve and as the slide occurs the mass
will be rotated backwards.

14. SLUMPANDFLOWS Slumps occur on weaker rocks,
especially clay, and have a
rotational movement along a
curved slip plane (Figure 3.8).
They have a higher water
content than landslides, and
involve smaller particles. Clay
absorbs water, becomes
saturated, and exceeds its liquid
limit. It then flows along a slip
plane.
Frequently the base of a cliff
has been undercut and
weakened by erosion thereby
reducing its strength.

15. FLOWSMATERIALS
By contrast, flows are more continuous, less
jerky, and are more likely to contort the mass
into a new form (Figure 3.9).
The material involved is predominantly made
up of fine particles, such as deeply weathered
clay.

16. THESPEEDOFFLOWS
The speed of a flow varies – mudflows are faster and more fluid that
earthflows, which tend to be thicker and deeper.
A higher water content will enable material to flow across more gentle
angles.

17. AVALANCHES
Avalanches are rapid movements of snow, ice, rock or earth down a slope.
Snow and ice may pick up rocks and/or earth. In steep mountainous areas, a
rock avalanche suggests a large-scale movement of material, whereas a rock
fall could be of individual rocks.
• They are common in mountainous areas: newly fallen snow may fall off
older snow, especially in winter (a dry avalanche), while in spring partially
melted snow can move (a wet avalanche), often triggered by skiing (Figure
3.10).
• Avalanches frequently occur on steep slopes over 22°, especially on north-
facing slopes where the lack of sun inhibits the stabilisation of snow.
• Debris avalanches are rapid mass movements of sediments, often
associated with saturated ground conditions.

19. HOMEWORK
1. State one difference between a rockslide and a mudflow.
2. Define the term mass movement.
3. Suggest how mass movements can be classified.
4. Define the terms shear strength and shear stress.
5. Compare and contrast the characteristics of falls and slides.
6. How does a slump differ from a slide?
7. Explain the terms rotational slide and avalanche.
8. Outline how avalanches are formed.