Download AICE ENV Day 2 Types of Mass Movements

Rock Falls
The most common type of mass wasting is falling This type of mass wasting can involve a single rock
or thousands of rocks. For a mass-wasting 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.
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.
Below are photographs of slopes prone to rock falls, and the talus that gathers at the bases of such slopes.
The pictures below show where rock falls have occurred from the faces of very steep slopes. The first three
are from the Escalante region of southern Utah, where each rock fall travelled only a short distance downward,
and the buildup of talus is only minor. The fourth photograph was taken outside of Las Vegas,
Nevada. Here, the rock fall shattered on impact, with large boulders rolling up to 100 feet from the base of the
slope. The fifth photograph was taken in the Sierra Nevada Mountains of California. Here the scale is much
grander than the preceding pictures, with the vertical cliff face that generates hundreds of rock falls each year
rising 2000 feet straight up! Needless to say, this is a very dangerous location to stand!
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Both of these photographs are from the Goosenecks area of the San Juan River in southern Utah. The first
shows the layer of limestone that forms the rim rock along the canyon, with large blocks that have separated
from the rock outcrop. The second picture shows one of these blocks that fell and bounced all the way down
to the bottom of the canyon, into the river, a journey of over 800 vertical feet!
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The first two pictures below were taken in Colorado. The first is from Colorado National Monument, near the
Utah border. The second is from the Front Range region, near Pikes Peak. Both show the classic buildup of
talus at the base of very steep slopes. Picture 3 shows talus that is gradually covering a roadway in the San
Bernardino Mountains of southern California.
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These photographs were taken along a mountain highway in central Colorado. Here, steep slopes composed
of fractured sedimentary rocks are prone to rock falls which can shut down the highway for days at a
time. The retaining wall next to the highway provides only minimal protection to mass wasting here.
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These two pictures both show steep slopes that have been undercut by wave action along the Pacific Ocean
coast of southern California. So, both slopes are extremely prone to rock fall!
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Landslides / Rock 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.
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.
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.
The two pictures below were taken in the Sierra Nevada Mountains of California, near Mt.
Whitney. Over a period of millions of years, the Sierras have been uplifted along the Sierra Nevada
Fault. As a result, the sedimentary and metamorphic rocks that used to overly the igneous rocks of
the Sierras have been stripped away. As a result, the igneous rocks have expanded, forming cracks
(called joints) along which huge slabs of rock peel away (exfoliate) and slide down slope. The first
picture shows numerous joints tilting downward to the left, and freshly exposed surfaces along which
rocks have slid. In the second picture, a large slab of rock that was part of a recent rock slide event
rests on top of talus generated by past rock fall events.
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In addition to the joints formed due to the unloading of igneous (and other) rocks, rock
slides also occur along pre-existing weaknesses in metamorphic rocks where flat
minerals are aligned parallel to each other. Such planes of foliation are responsible
for the small rock slides that have occurred along the Alaskan road in this picture,
where Diane Kawahata provides a scale for the viewer.
Rock slides also result from slippage of tilted sedimentary layers along the contacts
between layers, called bedding planes. This is especially true if a clay-rich layer
becomes wet and slippery. The picture to the left shows where tilted sedimentary
rocks have slid into the Pacific Ocean near Gaviota, California, exposing the planes
along which they slid.
As a slide progresses down a mountain slope, it can pick up tremendous speed, and energy. Some
slides have been reported to travel at speeds approaching 200 miles per hour. The resulting winds
can be so forceful, that they are known to strip the leaves off of surrounding trees. The momentum of
falling material has been known to cause some of the materials to roll several hundred feet back up
the other side of a valley.
The amount of material moved in a landslide can be tremendous. In some cases this material is so
substantial, that it is measured in cubic miles. This much material falling across a stream, can be the
cause for the formation or a new natural lake.
Earth Slumps
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
earthflow which continues to
move slowly outward and away
from the block.
Below are a series of photographs that illustrate some of the variety and features of
slumps. Although most of these pictures are from locations in California, slumps can be found most
anywhere there is a slope.
This picture, taken in El Moro Canyon near Laguna
Beach, California, shows a new slump. The key features
of this slump are labeled, and the outline of the slump
block is highlighted with dashed lines. Slumping, along
with the natural processes of weathering and erosion by
water, causes mountains to become flattened over a
period of time.
Point Sal, California
The three pictures below were taken at Point Sal, along the coast of central
California. Here, a small slump is narrowing the roadway. Picture 1 shows the surface of rupture
and slight tilting of the top of the slump block. Pictures 2 and 3 are reverse angle views of the same
slump, and the small fissure that can develop as the slump block separates and slides downward
along the surface of rupture.
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Portuguese Bend, Palos Verdes Peninsula, California
The following pictures were taken at Palos
Verdes Peninsula, southern California, at the end of the Portuguese Bend Landslide. This complex
area of mass wasting is characterized by earth flow movement within the main body of the mass, and
slumping from the point of origin all the way to the end of the mass where it meets the Pacific
Ocean. Picture 1 shows the upper end of the slide complex. Pictures 2 through 5 present different
views of the end of Portuguese Bend Landslide, showing the relationship of this large landslide to the
Pacific Ocean waves which continue to remove the end of the slide mass. As a result, this unstable
portion of the California coastline crumbles and slumps into the ocean almost continuously. You
should be able to recognize slumps in all four of these photographs. Picture 6 show where a new
slump is forming next to the main road in this area, Palos Verdes Drive North.
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Soil Creep
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 fence posts 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.
Solifluction
Definition: Mass movement of soil and regolith affected by alternate freezing and thawing.
Characteristic of saturated soils in high latitudes, both within and beyond the permafrost zone.
A number of features of the Cairngorm environment
contribute to active solifluction:
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frequent freeze-thaw cycles
saturated soils and regolith, after snow melt and
heavy rainfall
frost-susceptible materials, with significant contents
of silt and clay, at least at depth
extensive regolith across a range of slope angles
Solifluction adds detail to the terrain underfoot. Small-scale, active landforms include lobes and
sheets (Sugden, 1971) and turf-banked terraces. The latter reflect also the action of wind in stripping
and shaping the vegetation mat and frequently occur in association with deflation surface (Gordon,
1993). Ongoing mass movement is also indicated by 'ploughing boulders' - large blocks that are
moving downslope, pushing a rampart ahead of them and leaving a furrow behind.
Much more striking, however, are the large boulder terraces and lobes that give crenulated patterns
to many granite slopes on the plateau. These steps terminate in stone banks up to 3 m high. These
forms are absent from within the limits of Loch Lomond Readvance glaciers (Sisson, 1979) and so
date from this or earlier periods. As delicate features such as tors have survived beneath ice covers,
it is possible that the larger solifluction terraces and lobes may be of considerable age.
Although solifluction deposits are not an obvious feature on the Cairngorm Granite, considerable
thicknesses of frost-shattered and soliflucted debris occur on Dalradian metamorphic rocks in the
Ladder Hills.
Mudflows
A mudflow or mud flow is a form of mass wasting involving "very rapid to extremely rapid surging
flow" of debris that has become partially or fully liquified by the addition of significant amounts of
water to the source material. Mudflows contain a significant proportion of clay, which makes them
more fluid than debris flows; thus, they are able to travel farther and across lower slope angles. Both
types are generally mixtures of various kinds of materials of different sizes, which are typically sorted
by size upon deposition. Mudflows are often called mudslides, a term applied indiscriminately by the
mass media to a variety of mass wasting events. Mudflows often start as slides, becoming flows as
water is entrained along the flow path; such events are often called flow slides. Other types of
mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and
jökulhlaups (outbursts from under glaciers or icecaps).
Heavy rainfall, snowmelt, or high levels of ground water flowing through cracked bedrock may trigger
a movement of soil or sediments. Floods and debris flows may also occur when strong rains on hill or
mountain slopes cause extensive erosion and/or what is known as "channel scour". The 2006
Sidoarjo mud flow may have been caused by rogue drilling.
Some broad mudflows are rather viscous and therefore slow; others begin very quickly and continue
like an avalanche. If large enough, they can devastate villages and countrysides. They are composed
of at least 50% silt and clay-sized materials and up to 30% water. Mudflows are common even in the
hills around Los Angeles, California, where they have destroyed many homes built on hillsides
without sufficient support after fires destroy vegetation holding the land.
The point where a muddy material begins to flow depends on its grain size and the water content.
Fine grainy material or soil has a smaller friction angle than a coarse sediment or a debris flow, but
falling rock pieces can trigger a material flow, too.
The Mameyes mudflow disaster, in barrio Tibes,
Ponce, Puerto Rico, was caused by heavy
rainfall from Tropical Storm Isabel in 1985. The
mudflow destroyed more than 100 homes and
claimed an estimated 300 lives.
On December 14, 1999 in Vargas, Venezuela, a mudflow known as The Vargas tragedy significantly
altered more than 60 kilometers (37 mi) of the coastline. It was triggered by heavy rainfall and caused
estimated damages of US$1.79 to US$3.5 billion, killed between 10,000 and 30,000 people, forced
85,000 people to evacuate, and led to the complete collapse of the state's infrastructure.
The world's largest historic landslide (in terms of volume) occurred during the 1980 eruption of Mount
St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA. The volume
of material displaced was 2.8 km3 (0.67 cu mi). Directly in the path of the huge mudflow was Spirit
Lake. Normally a chilly 5 °C (41 °F), the lahar instantly raised the temperature to near 38 °C (100 °F).
Today the bottom of Spirit Lake is 100 ft (30 m) above the original surface, and it has two and a half
times more surface area than it did before the eruption.
The world's largest known prehistoric terrestrial landslide took place in southwestern Iran, and is
named the Saidmarreh landslide. The landslide was located on the Kabir Kuh anticline at 33.0°N,
47.65°E. The landslide had a volume of about 20 km3 (4.8 cu mi), a depth of 300 m (980 ft), a travel
distance of 14 km (8.7 mi), and a width of 5 km (3.1 mi). This means that about 50 billion tons of rock
moved in this single event.
The area most generally recognized as being at risk of a dangerous mudflow are:
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Areas where wildfires or human modification of the land have destroyed vegetation
Areas where landslides have occurred before
Steep slopes and areas at the bottom of slopes or canyons
Slopes that have been altered for construction of buildings and roads
Channels along streams and rivers
Areas where surface runoff is directed
Rotational slumping
A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated
materials or rock layers moves a short distance down a slope.[1] Movement is characterized by sliding
along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough
wetting, freezing and thawing, undercutting, and loading of a slope.
Rotational slumps occur when a slump block, composed of sediment or rock, slides along a concaveupward slip surface with rotation about an axis parallel to the slope.[3] Rotational movement causes
the original surface of the block to become less steep, and the top of the slump is rotated backward.
This results in internal deformation of the moving mass consisting chiefly of overturned folds called
sheath folds.
Slumps have several characteristic features. The cut which forms as the landmass breaks away from
the slope is called the scarp and is often cliff-like and concave. In rotational slumps, the main slump
block often breaks into a series of secondary slumps and associated scarps to form stairstep pattern
of displaced blocks.[4] The upper surface of the blocks are rotated backwards, forming depressions
which may accumulate water to create ponds or swampy areas. The surface of the detached mass
often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near
the toe of the slump. Addition of water and loss of sediment cohesion at the toe may transform
slumping material into an earthflow. Transverse cracks at the head scarp drain water, possibly killing
vegetation. Transverse ridges, transverse cracks and radial cracks form in displaced material on the
foot of the slump.
Slumped chalk slopes at Mupe Bay, Dorset
Slumps frequently form due to removal of a slope base, either from natural or manmade processes.
Stream or wave erosion, as well as road construction are common instigators for slumping. It is the
removal of the slope's physical support which provokes this mass wasting event. Thorough wetting is
a common cause, which explains why slumping is often associated with heavy rainfall, storm events
and earthflows. Rain provides lubrication for the material to slide, and increases the self-mass of the
material. Both factors increase the rate of slumping. Earthquakes also trigger massive slumps, such
as the fatal slumps of Turnagain Heights Subdivision in Anchorage, Alaska. This particular slump was
initiated by a magnitude 8.4 earthquake that resulted in liquefaction of the soil. Around 75 houses
were destroyed by the Turnagain Slump. Power lines, fences, roads, houses, and other manmade
structures may be damaged if in the path of a slump.
The speed of slump varies widely, ranging from meters per second, to meters per year. Sudden
slumps usually occur after earthquakes or heavy continuing rains, and can stabilize within a few
hours. Most slumps develop over comparatively longer periods, taking months or years to reach
stability. An example of a slow-moving slump is the Swift Creek Landslide, a deep-seated rotational
slump located on Sumas Mountain, Washington.
Slumps may also occur underwater along the margins of continents and islands, resulting from tidal
action or a large seismic event. These submarine slumps can generate disastrous tsunamis. The
underwater terrain which encompasses the Hawaiian Islands gains its unusual hummocky
topography from the many slumps that have taken place for millions of years.
One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank south of
Africa in the Pliocene or more recently. This so-called Agulhas Slump is 750 km (470 mi) long,
106 km (66 mi) wide, and has a volume of 20,000 km3 (4,800 cu mi). It is a composite slump with
proximal and distal allochthonous sediment masses separated by a large glide plane scar.[5]