mass movement/wasting Flashcards
definition of mass movement
movement of a mass of surface material such as rocks debris and soil downslope due to gravity
movement may be slow – soil creep or fast – landslide
define soil creep
slowest of downhill movements so cannot be seen but over several years several indicators on slope make it obvious that its occurring
difficult to measure as it takes place at rate of approx 1 cm a year
where can soil creeps occur
on gentle slopes with gradients as slight as 2 degrees
but usually more on those over 5 degrees
gradual but constant process
most common in damp soils where moisture acts as a lubricant and so reduces friction
this increases volume and weight of the soil
between soil particles and dust thus allowing regolith to move downslope under gravity or in areas where freezing and thawing
define regolith
weathered materials
indicators on a slope
1 terracettes
2 leaning fences and trees with bent trunks in direction of slope
3 tilted electricity poles
thinner soil at top of slope
thicker soil at bottom of slope
broken roads
define terracettes
small step like features 20 to 50 cm in height roughly parallel to contour lines
develop because vegetation stretches and tears as soil slips downhill
define landslides
sudden and rapid downslope moment of large volume of soil and or rock
carries vegetation and buildings with it potentially causing huge disruption
in very wet conditions soil turns to mud and flows like thick liquid called mudflows
what is a lahar a type of
lahar that occurs on volcano is type of mudflow
difference between landslide and mudflows
landslide – whole land is still attached and moves as one succumbing to gravity
mudflow has water content and flows
what are all slopes affected by
gravity
loose material will eventually slip and fall
however as slope angles increase so does the risk of slide as well as its possible size
two types of landslides
rotational slides or slumping
translation or planar slides`
define rotational or slumping landslide
landslides occurring along limed or spoon shaped surface
usually composed of relatively loose unconsolidated material
define translational or planar slides
where ground slides with little rotation along flat plane such as fault joint or bedding plane parallel to surface
material in this slide may range from large unconsolidated material to expansive slabs of rock or both
list physical causes of landslides
1 unconsolidated or loose material on slope
like gravel or weathered rock material like regolith or volcanic ash
2 earthquakes – shaking trigger landslides on steep sloped primarily if there is unconsolidated mass
3 volcanic eruptions
4 heavy rainfall – rainfall leads to surface material becoming saturated which makes it heavier and water will also act as lubricant which decreases friction and helps move downslope
5 bedding planes beign roughly parallel to slopes surface which makes it easier for material to slip downwards along bedding planes
gravity exerts its force easily on rock beds which affects even greater if rock layer below is permeable
6 slippery rocks like shale and clay especially when wet
the wet clay acts as lubricant so rock beds above it slide down more easily
7 base of slope being undercut by river or by coastal erosion
list human causes of landslides
1 deforestation – removal of vegetation binds soil together and also veg uses up some of soil moisture
people remove trees for agriculture construction fuel mining and other economic activities
2 undercutting base of slope like building of roads
3 building an unstable slopes which increases weight on slope and adds to downward pull of gravity
