9.2: mass movement Flashcards
mass movement
mass movements are large scale movement of Earth’s surface not accompanied by a moving agent such as a river, glacier or ocean wave
weathering
the decomposition and disintegration of rocks in situ
aggradation
to build up a surface or soil through the addition of material such as mass wasting deposits
geology
the study of rocks
degradation
the wearing down of the land by the erosive action of water wind or ice
geological structure
the arrangements of rocks in the Earth’s crust: strata, bedding planes, joints etc
regolith
the layer of unconsolidated material covering the bedrock of a planet (soil)
aspect
the direction in which a slope faces
name three factors which can reduce shear strength
- water –> loss of grain to grain contact, reducing the cohesion of soil
- earthquakes and the movement of trees cause vibrational shock
- burrowing of animals and other biological weathering of roots
name three factors which can increase shear stress
- weathering processes such as freeze thaw, dilatation and hydration can increase stress through lateral pressure
- water –> increase in pore water pressure as well as adding to the weight of the slope
- increasing weight by adding vegetation or accumulation of debris eg slag heaps when mining
what are slumps
weaker rocks such as clays ‘slump’ with a rotational movement along a curved slip plane
explain the key characteristics of a slump
clay saturated with water and minerals expand. the lubrication of slip plane causes a series of material slumps, along curved slip plane. Commonly occurs if the base has been undercut. This can be gradual or sudden separate, jerky events. This can leave a crescent shape scar on the slope.
what is a flow
more continuous, smoother form of slump. Occurs in deeply weathered clay, and if particle size is the same or smaller than a grain of sand
define a mudflow
fine grains mixed with water, slowing down a channel. Accumulates material down the slope. Faster than earthflows (which are deeper and thicker)
define debris flow
loose mud, sand and soil saturated with water. Larger than mudflows, accumulating material as it travels. Its higher water content allows flows to occur on shallow slopes and can occur on saturated toes of a landslide or as a separate event. This can create a scar, flowtrack and toe lobe
explain the key characteristics of falls
occurs on steep slopes greater over 40 degrees with bare rock faces and exposed joints. rocks detach and fall freely under gravity. Short fall = straight scree; otherwise a concave scree is produced. Significantly contribute to the retreat of steep rock faces and providing debris for scree/talus slopes. This can create a steep back wall and a lower scree slope
define a slide
when an entire mass of material moves along a slip plane - internal structure is maintained. Water infiltrates through limestone and adds weight to material on the slope. Water reaches impermeable clay and lubricates the slip plane. Blocky limestone slides down plane towards the valley base. Can rock/landslides or a rotational slide. Material holds its shape until hitting the slope bottom
what are the conditions needed for a slide
weak rocks, steep slope, active undercutting, intensly cold comditions or sudden change in water content
how can slides affect the shape of the plane
rotational slides cause stepped profile with upper steep scarp, main mass and lower lobe
What are avalanches
they are fast (40-200km/h) mass movements of ice and snow along a slip plane, often where the slope is steeper than 22. Often triggered by skiiing when new snow falls off old snow layer or where lack of sun means snow does not stabilise on the slope
what are lahars
lahars are a type of mudflow composed of pyroclastic material and rock debris with water. This flows down the side of a volcano, along a channel.
