Landslides...a rapid downslope movement of rock or soil as a more or less
coherent mass. Landslides are characterized by a slippage plane that is
clearly defined. A landslide may turn into a flow at the bottom as the
blocks become tumbled over. Usually the material moves as a large block
known as a slump block. The scar above a landslide is easily visible.
Steep slopes of shale are susceptible to landslide activity. But
landslides occur everywhere on large or small scales. They can occur after
earthquakes or removal of part of the slope due to construction,
particularly in the construction of roads.
General mechanism of a landslide
Slump blocks and scar in Guatemala: General Diagram of a slump
Pore Water Pressure is the key to monitoring landslides.
Pore water pressure is the pressure that develops as water fills in the
pore spaces inbetween particles. Shear strength, a resisting force,
decreases and the weight, a driving force, increases. The safety factor
becomes less than one and a movement becomes possible.
Diagram of the apparratus used to monitor Pore Water Pressure
Equipment such as this measures the Deep Pore Pressure and Shallow Pore
Pressure. Data received from one of these monitors may look like this:
"Steady change in the extensometer output (vertical axis) indicates
downslope movement, brief changes that return to a constant value, such as
the spikes in the graph of E-4 (yellow), usually result from physical or
electrical disturbances and do not indicate movement."
Soil Creep is a very, very slow form of mass wasting. It's just a slow
adjustment of soil and rocks that is so hard to notice unless you can see
the effects of the movement. These effects would be things like fenceposts
shifted out of alignment, or telephone poles tipping downslope.
Another effect is the way a grass covered slope seems to ooze downhill
forming little bulges in the soil. This heaving of the soil occurs in
regions subjected to freeze-thaw conditions. The freeze lifts particles of
soil and rocks and when there is a thaw, the particles are set back down,
but not in the same place as before.
Gravity always causes the rocks and soil to settle just a little farther
downslope than where they started from. This is the slow movement that
defines creep. Creep can also be seen in areas that experience a constant
alternation of wetting and drying periods which work in the same way as
Monitoring is essentially done through observation of the effects of
creep. Since the process is so slow, it can only be monitored in terms of
flow over long periods of time.
An ancient debris flow showing just how big they can be are one of the
most dangerous of all mass wasting events. They can occur suddenly and
inundate entire towns in a matter of minutes.
Debris flows are made of exactly what the name suggests: debris. This
debris can include anything from the smallest mud particles to boulders,
trees, cars, and parts of buildings. Debris flows occur when rain water
begins to wash material from a slope or when water sheets off of a freshly
burned stretch of land. Chapparral land is especially susceptible to
debris flows after a fire. The rapidly moving water cascades down the
slopes, and into the canyons and valleys below. It picks up speed and some
debris as it descends the valley walls. In the valley itself, months of
dry ravel, loose soil and rocks that have rolled or slid off the slope,
begins to move with the water.
As the system gradually picks up speed, the flow takes on the
characteristics of a basic river system. The faster the water flow, the
more the water can pick up. As the water picks up more mud and rocks, it
begins to resemble a fast flowing river of concrete. This wall of debris
can move so rapidly that it can pluck boulders the size of cars from the
floors of the canyons and hurl them along the path of the flow. It's the
speed and enormity of carried particles that makes a debris flow so
dangerous. Boulders crash through homes and the mud-water mix fills in the
rooms sometime totally overtaking the house.
People have tried many methods for stopping or diverting debris flows. In
California, catch basins have ben constructed to "catch" the debris. Some
basins have special overflow ducts with screens to remove the water from
the flow and allow more room for the bigger items that may be washed in
and take up needed space.
Catch basin in British Columbia, Canada Debris Chute in British Columbia,
Debris flows happen so rapidly that there is really no way to monitor one
until it is on top of you. Instruments in catch basins and flow channels
can measure the rate or discharge of the flow by calculating the amount
(volume) of material per unit of time (usually seconds).
are special forms of debris flows that are mainly made of the smallest mud
and silt particles. Extremely heavy rain, or a sudden thaw can trigger
these types of flows. In the case of lahars, a sudden thaw of mountain
snow due to a volcanic eruption can send a torrent of mud, ash, and hot
water down the slope of the volcano and over neighboring towns.As can be
seen from this photograph of the unfortunate town of Armero, a lahar can
overtake a town far from a volcano. This lahar rushed down stream and
river valleys into the town and killed over 23,000 people. They had no
warning. The town was quickly buried by mud that later, as rescuers
attempted to find victims, dried and hardened like cement.
This map shows how the lahar found it's way to Armero and the extent of
In parts of Canada and Scandinavia, a special type of mudflow can occur.
Marine mud at the margins of a receding glacier can have a property known
as quick clay. There is a high water content in these marine muds due to
their relatively low compaction. These clays can change into a viscous
fluid with only the slightest disturbance. They become flows that can move
very rapidly even on a slight grade. Since these flows also give no
warning, they can be very destructive.
...a major type of mass movement in cold polar regions and some high
mountains. Solifluction is a special type of creep that occurs in areas of
permafrost. Permafrost refers to the layer of groundwater that fills in
the pore spaces of soil and rock that is permanently frozen. The
permafrost layer can be anywhere from a meter to several hundred meters
thick. It takes up about 20% of the world's land. In times of warm
weather, the ground will begin to thaw from the surface downward. All of
the freshly melted water cannot absorb into or move through the permafrost
This causes the upper layer of soil and regolith to become saturated and
flow down the slightest of slopes as it slips over the frozen ground
underneath. Another visible aspect of solifluction areas is cryoturbation
During cryoturbation, "small ridges and mounds of bare soil are produced
by the processes of frost churning (cryoturbation) and soil flow
(solifluction). Freeze-thaw generates a circular motion in the surface
material, heaving the soil to the surface (the light- coloured areas) and
dragging it down at the margins to form gutters (the darker, vegetated
areas). The process creates a network of circular patches which, on
slopes, are stretched into long stripes by an additional creeping
movement. Flowing water then deepens the gutters." (Terrain Sciences
Rock Falls and Rock Slides
Falls are usually the free-fall of pieces of rock from a mountain or cliff
face. The size of the piece(s) can range from tiny grains to blocks
weighing a ton or more. The debris and rock fragements from rock falls
collect at the base of the slope as talus. This talus protects the base of
the mountain from erosion. On mountains, ice wedging is the main
As water from snowmelt finds its way into the cracks and joints of the
rock face, it may refreeze and being to expand. This expansion widens the
cracks in the rock. Over time, the cracks have been widened enough so that
they are a point of structural weakness. Gravity takes over and the pieces
of rock fall from the face of the mountain.
Rock slides can travel a long way from the source of the initial fall
Slides are rapid downslope movements of blocks along a bedding plane,
joint, or area of weakness. The blocks tend to break up into smaller
pieces as the slide moves downslope and large pieces collide with each
other. These pieces can travel a great distance due to the force of the
falling rock. Road cuts are susceptible to rock falls and slides when the
base of a mountain is removed for the roadbed. So to make the roads safer
for motorists, some protective barriers have been designed to catch the