mass movement
Mass Movement
based on JAS West's excellent web page
http://members.tripod.com/~Jenn2000/main.html
see also contents of entire Earth and Beyond Course
Landslides
General mechanism of a landslide
Slumping
Pore Water Pressure
Soil Creep
Debris Flow
Mud flows and Lahars
Solifluction
Rock Falls and Rock Slides
Resources
Landslides
California landslide. Image courtesy of USGS
....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.
Slump blocks and scar in Guatemala: General Diagram of a slump
General mechanism of a landslide
Pore Water Pressure
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 the freeze/thaw.
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.
Debris Flows
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, Canada
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).
Mud flows and Lahars
....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 the
flow.
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.
Solifluction
...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 layer. 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 Division)
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 contributor. 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 falling rocks:
Resources
Terrain
Sciences Division Landforms Page
Terrain
Sciences Division Geohazards: Landslides and Snow Avalanche Page
USGS
Landslide Website
Oregon
State University's Geology Dept. Index
USGS Geological
Hazard Team Website
California
Real-Time Landslide Monitoring Project