Download Types of Mass Wasting

Types of Mass Wasting
Mass-wasting events come in many shapes, sizes and speeds. Typically, the steeper the angle
of a slope, the faster will be the down-slope movement of rock and sediment. Also, water can
play a significant role in mass wasting, sometimes acting as the key component to a masswasting event, or serving as a lubricant within a mass of sediment and rock, enabling it to travel
faster and further than it would otherwise. It is important to understand that one type of mass
wasting can evolve into another type of mass wasting as the body of sediment/rock moves
down a slope. This can make it difficult to classify a single event as being one type of mass
wasting or another. Below is a simple classification of the different types of mass wasting, with
each type having sub types.
4 Main Categories of Mass Wasting:
Falls, Slide, Flow, and Creep
Falls:
This type of mass wasting can involve a single rock or thousands of rocks. For a masswasting event to be classified as a fall, it must travel at a high rate of speed down a very steep slope. If
the slope is vertical or overhung, then the rock(s) will drop straight downward, fragmenting when they hit
the base of the slope. Over time, this forms a body of angular rubble called talus, a distinctive transition
from the steep slope to flatter ground. Below are descriptions, diagrams and photographs of the two
basic types of falls.
1) Rock Fall- A rock fall consists of one or maybe a few rocks that detach from the high part of a steep
slope, dropping and perhaps bouncing a few times as they move very rapidly down slope. Rock falls are
very dangerous because they can occur without warning, and because the rocks are traveling at high
velocity. You can usually tell where rock falls are common by identifying talus at the base of steep
slopes. If you are out hiking or camping in mountains or canyons, avoid talus slopes and the rocks that
fall onto them!
Diagram 1 illustrates a very steep slope with the potential for a rock-fall event. The actual cause of such an event
might be an earthquake, the movement or weight of an animal, or the freezing and thawing of water. Obviously,
hanging out at the base of such a slope is not a good idea.
Diagram 2 shows the rock in the process of falling. This usually occurs without warning, and is rarely witnessed.
Sometimes a hiker may hear the fall off in the distance, but upon closer inspection will see only a pile of rock rubble
at the base of the slope.
Diagram 3 illustrates the rock debris, or talus, that forms at the base of a steep slope as rock fall and break apart on
contact with the base of a slope. The more rocks that fall, the greater will be the buildup of talus.
2) Rock Avalanche: This type of fall usually forms when a massive rock fall explodes apart on contact
with a slope. As this occurs, thousands of rocks continue their flying trajectories down slope, colliding
with each other and the slope itself, overwhelming anything in their paths. A rock avalanche is a
transitional sort of mass wasting event, changing from a pure rock fall to something more like a rapid flow
of material as the material moves further from the base of a slope. Therefore, some geologists classify
rock avalanches as flows. Whatever the classification, rock avalanches are extremely dangerous, and
you should
Diagram 1 shows a small town a short distance from a tall, steep-faced mountain. Such a setting is fairly common in
mountain ranges on all continents, with the mountain providing a spectacular backdrop for the town. Unfortunately,
this situation can also be a recipe for a natural disaster. be wary of locations where they occur with frequency in
mountainous regions.
Diagram 2 shows what can happen if an earthquake vigorously shakes a tall, steep-faced mountain. Here, several
massive blocks of the mountainside have peeled and fallen away, traveling at high speed toward the base of the
mountain. Anything or anyone directly in the path of such a huge rock fall will be obliterated.
Diagram 3 paints a very bleak picture for the town. As the massive rock fall contacts the base of the mountain, it
breaks into thousands of fragments that continue tumbling down slope at high velocity. The great energy of such a
large mass can enable the rock avalanche to travel much further from the base of the mountain than one would
expect, in this case destroying and burying the town. Fortunately, such an extreme rock avalanche is a rare event.
This mountain ridge in the San Gabriel Mountains of California shows the pathways for rock avalanches, both past
and future. For mountain climbers these straight, barren areas provide quick access to higher elevation, but they are
exceptionally dangerous during an earthquake, or when water is freezing or melting within rock fractures.
Slides:
There are two versions of slides, but what all slides have in common is that the mass of
sediment/rock sticks together as a coherent block as it travels down slope along a tilted plane or surface
of weakness. Typically, this surface of weakness coincides with the tilt angle of the slope that mass
wastes. Ultimately, as the moving slide mass comes to a sudden stop, it may break apart and continue
down slope as a type of flow. Below are the two basic types of slides.
1) Rock Slide- This type of slide occurs where there is a tilted, pre-existing plane of weakness within a
slope which serves as a slide surface for overlying sediment/rock to move downward. Such planes of
weakness are either flat sedimentary surfaces (usually where one layer of sediment or sedimentary rock
is in contact with another layer), planes of cleavage (determined by mineral foliation) within metamorphic
rocks, or a fracture (fault or joint) within a body of rock. Rock slides can be massive, occasionally
involving an entire mountainside, making them a real hazard in areas where a surface of weakness tilts in
the same direction as the surface of the slope. Rock slides can be triggered by earthquakes or by the
saturation of a slope with water. The addition of water to a slope increases its mass, and therefore
increases the pull of gravity on the slope. In addition, water can lubricate a layer of clay or shale within a
slope, which then serves as a slide surface for the rock above it.
Diagram 1 shows layers of rock tilted downward to the right. The topmost rock layer is prone to sliding because it
lacks support at the base of the slope.
Here, in diagram 2, gravity finally overcame the friction between the topmost rock layer and the rock beneath it.
Once this occurs, the topmost rock layer slides downward as a block. As it comes to a sudden stop the slide block
may break apart and continue moving for some distance as a rock avalanche or debris flow.
2) slump- Slumps are fairly small when compared to rock slides. Slumps form where the base of a slope
is removed by natural processes (stream or wave erosion) or by human efforts (road or building
construction). Removal of the lower part of a slope effectively removes physical support for the upper
part of a slope, causing the formation of a new fracture in the sediment/rock comprising the slope. Soon
thereafter, the slope will begin sliding downward, often rotating along the curved surface of rupture. The
development of a slump is illustrated in the three diagrams below.
In diagram 1, the residents of the house have a fine ocean view, with stable rocks below them.
In diagram 2, ocean waves have removed the base of the slope beneath the house. Once this support is gone, a
fracture will form, angling from the new base of the slope to the cliff top above. Residents of the house will see a
widening crack cutting across their lawn, a warning of bad things to come. Usually, people will abandon their homes
at this stage, removing as many belongings as possible.
Diagram 3 shows that the slump block has slid downward along the surface of rupture, what was originally the new
fracture formed due to the erosion of the base of the slope. Since the fracture's geometry was curved, so too is the
surface of rupture, which causes the slump block to rotate outward as it moves downward. Buildings tend to collapse
as this occurs. Note that the end of the slump block often breaks apart forming an earth flow which continues to
move slowly outward and away from the block.
Flows: Of the three basic types of mass wasting, flows are the most complex, both in terms of how
they originate and how they move. Unlike slides, in which the material sticks together as a coherent
mass as it moves down slope, flows are characterized by internal movements of individual grains (tiny like
silt or sand up to large boulders and small blocks of crust) within the flow itself. The internal flow
movements of individual grains can be fast and chaotic if the flow originates from a steep slope, or if it
contains a lot of water. Or, grain movements can be very slow and somewhat predictable if the slope
surface is very gradual in its angle.
1)Rock Avalanche- This type of mass wasting is transitional, usually originating as a massive rock fall which breaks
apart upon contact with the ground at the base of a steep slope. Initially, the rocks continue to bounce and fly down
slope, still behaving much like falling rock. As the avalanching rocks begin to slow and lose energy, the internal
behavior of the mass becomes more like a fluid, with individual rock fragments moving randomly and rapidly within
the mass. As the rock fragments bang into each other and Earth's surface beneath the flow, the mass will slow down
and eventually cease movement.
2) Debris Flow- As the name implies, this type of flow contains a variety of particles or fragments, mainly
small to large rock fragments but also trees, animal carcasses, cars and buildings. Debris flows usually
contain a high water content which enables them to travel at fairly high velocity for some distance from
where they originated. Debris flows tend to follow the paths of pre-existing stream channels and valleys,
but debris flows are much denser than water, so they can destroy anything in their paths such as houses,
bridges, or highways. Debris flows tend to originate from denuded slopes receiving heavy rainfall, but
they also evolve from leading edges of large rock avalanches and fast-moving slumps. In volcanically
active regions such as the Cascade Mountains of North America, the Andes Mountains of South America,
or the islands of Indonesia, ash on the slopes of volcanoes can readily mix with water from rainfall or
snowmelt. When this occurs, a low-viscosity debris flow, called by the Indonesian term lahar, can form
and move very rapidly down slope.
3) Earth Flow- Earth flows typically develop at the low end of a large slump, where the slump block
breaks apart and material continues moving down slope. This down-slope movement can be rapid and
short-lived, as a debris flow (example: the La Conchita event of 2005), or the movement can be slow and
variable, and prolonged over a long period of time (example: the Portuguese Bend earthflow). The speed
of an earth flow can be controlled by several factors, the most important being the amount of water
introduced into the earth flow - the more water, the faster it will move. Other factors that can speed the
movement of an earthflow include the shaking from an earthquake or the removal of the toe of an earth
flow due to erosion or human activity. Earth flows can move up to 100 feet per day, or not at all
depending on local conditions. Large earth flows can be complex structures with individual blocks moving
at different speeds, and with slumps, and fissures.
4) Creep: This is the slowest type of mass wasting, requiring years of gradual movement to have a
pronounced effect on a slope. Slopes creep due to the expansion and contraction of surface sediment,
and the pull of gravity. The pull of gravity is a constant, but the forces causing expansion and contraction
of sediment are not. The presence of water is generally required, but in a desert lacking vegetative
ground cover even dry sediment will creep due to daily heating and cooling of surface sediment grains.
The two primary factors causing active creep of a slope are the freeze-thaw cycle and wet-dry cycle.
Even when one of these processes occurs on a daily basis, the down-slope movement of grains is very slow - a few
inches to several feet per year. Although creep is not a life-threatening form of mass wasting, it can damage the
foundation of a building, eventually leading to expensive repairs or even abandonment of the structure.
How to identify a creeping slope:
(1) Slopes that experience creep can usually be identified by trees that have an unusual bend near the bases of their
trunks. This results from the active creep of surface sediment that occurs as the roots of a young tree begin to
penetrate deeply enough underground into bedrock, anchoring the tree to that location. If creep continues, it will
cause the top of the tree to tilt down slope. As the top of the tree grows upward, and creep keeps tilting the tree
down slope, a pronounced bend may develop in the tree's trunk.
(2) The tops of telephone poles and even fence posts will tilt down slope if their bases are sunk deeply enough into
non-moving sediment or rock, with creep of the surface sediment pushing the pole or post over. So, wherever you
see tilted telephone poles or fence posts, think "creep".
(3) Almost any human structure (building foundation or retaining wall) can suffer from the effects of a creeping slope.
It's not the speed of the down-slope movement so much as the weight of the creeping sediment that does the
damage, exerting tremendous force on construction materials (metal, wood, or concrete), eventually causing them to
fail.