| Landslides, Set 1 |
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This set of slides graphically illustrates the potential danger that major earthquakes
pose to school structures and to the children and adults who happen to be inside at
the time of the earthquake. It includes pictures from 1886 to 1988. The slide set
includes nine destructive earthquakes that occurred in the U.S. and eight earthquakes
that occurred in foreign countries. The slopes above streams and rivers are subjected
to a variety of processes that cause them to recede and retreat from the river or
stream channel. These processes, collectively called mass wasting, can be classified
according to rapidity of movement and according to the type of materials that are
transported. Gravity is the force behind all such downslope movement. Factors that
enable the force of gravity to overcome the resistance of inertia and friction to
move more material downslope include: saturation by water which acts as a lubricant,
steepening of slopes by streams, waves, or road construction, alternate freezing and
thawing, and earthquake vibrations. Mass wasting of surface material is widespread
process that can be found in high mountains, desert hillsides, deep ocean shelves,
steep ocean shores and even on the moon and other rocky planets. The major methods
of mass movement include: Rockfalls: Large or small amounts of rock material break
away from the face of a cliff as a result of weathering, and in the most rapid type
of mass movement, free fall or bounce along an irregular slope to the base of the
cliff forming talus. Rockslide: Rock material slides along a plane of structural weakness
such as a bedding plane. Although they are most common on steep slopes, they can even
occur on slopes of 15 degrees. Millions of tons of rock may plunge down slope at speeds
greater than 160 km (100 miles per hour in what is often the most catastrophic form
of mass wasting. Debris slide: Dry to moderately-wet, loose rock fragments and soil
move rapidly over the surface of underlying bedrock. The interface of moving material
and undrlying bedrock is dry in a debris slide. Debris avalanche: Loose earth on a
steep slope becomes wet and slides to the bottom of the slope. Snow avalanche: Unstable
snow breaks loose and plunges down slope carrying rock and debris, carving avalanche
chutes. Debris flow: Rock fragments, mud, and water flow downslope as a thick viscous
fluid. Debris flows may begin as slumps and continue as flows. Movement may be as
slow as that of freshly poured concrete or as rapid as that of a river. Mudflow: Silt
and clay particles with water content as high as thirty percent follow stream valleys
until the terrain flattens. Then they spread out as fans. Mudflows are sometimes over
100 m (330 ft) thick; they can float large boulders and move houses from their foundations.
The speed of movement depends on the slope and the water content of the flow. Landslide:
Unconsolidated rock material and even the bedrock itself may be involved in what is
usually a rapid movement of material beginning with the slumping of stream banks or
sea cliffs, or the sliding of mountain sides. Landslides move as a unit or series
of units along a definite plane (in contrast to debris flows which move as viscous
fluids). The material moves downward and outward along a curved plane. Eventually
the material breaks into fragments that slide over uneven ground at the base until
friction overcomes the force of motion. Slumping: A resistant rock overlies a weaker
rock layer. The weaker rock is eroded undermining the resistant rock and producing
an unstable condition. Slump blocks can be as much as 5 km (3 mi) long and 150 m (495
ft) thick. They may move in a matter of seconds or gradually slip over a period of
several weeks. SolifluctionThe upper zone of saturated soil flows slowly down even
the most gentle slopes in arctic and subarctic regions where an impermeable permafrost
area exists. Water can not percolate into this permafrost area so the thawed surface
remains saturated and flows as a viscous fluid.Rock GlacierAngular rock debris resembling
glaciers move as a body down slope at rates ranging from 3 cm (1.2 in) a day to 1
m (3.3 ft) a year. A considerable amount of ice exists in the pore spaces between
the rock fragments and is responsible for much of the movement. The increased weight
of rock fragments falling onto the flow cause the ice to flow. Steep cliffs and a
cold climate that keeps ice permanently frozen are conditions that most often result
in rock glaciers. A steep flow front, lobes, and concentric ridges on the flow are
evidence of rock glaciers.CreepThe mantle on a slope moves downward almost imperceptibly
under the constant pull of gravity. In areas subject to cold winters the water in
the layers of soil or clay freezes and increases in volume. This lifts the rocks upward
at right angles to the slope. However, when melting occurs the rocks fall vertically
and so are moved downhill. Wetting and drying has the same effect since moisture causes
expansion of clay materials. Burrowing organisms displace particles permitting the
force of gravity to move them. Growing plant roots and the tramping of animals also
force soil material downslope. Photograph credit: All photographs in this slide set
were provided by B. Bradley of the University of Colorado's Geology Department.
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