Lecture 6

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Published on 30 Oct 2010
Environmental Science
of 2
13th Feb 2009
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1. Gravity driven processes (mass wasting)
2. Transport by ice
3. Transport by water: rivers and waves
4. Transport by wind
How gravity affects slopes
- Stress: applied force
o Depends on rock type, climate, amount of water
- Strain: response to stress
- Normal stress: an applied force acting at a right angle to the slope
- Shear stress: an applied force acting parallel to the slope
- Shear strength: the ability to resist stress
o Failure: when slope collapses (e.g. slope sliding t moving rocks down slope)
- Slope angle t the steeper the slope, the more likely to fail; depends on the sediment of rock
Gravity driven processes (mass wasting)
- Flow: turns from a solid sediment to a liquid
o E.g. thawing of ice in soil after winter
- Fall
o E.g. Niagara falls
- Slide
o Translational slide
Strata: layers in the rock parallel with the slope
Because the shear strength is low, they will slide
Happens when slopes are steep (e.g. rocky mountains)
o Rotational slide(slump)
Sediments often fluidize and turn into flow
- This is how mountains erode and slopes flatten
- Often mudslides in tropical countries because of rain and deforestation
- Earthflow: soft bedrock moving the whole hillside
o Creep: slow movement of soft rock down a slope
- These areas have remained cold for thousands of years
- Soil and sediment becomes frozen from the water in it
- Only the top layer of the permafrost will thaw in summer
- Discontinuous zone: areas of permafrost are discontinuous t pockets of permafrost
- Continuous zone: continuous areas of permafrost
- Problems with building roads and houses because of the freezing and thawing of permafrost
- Beaufort sea t submarine permafrost; frozen sea floor
o During the last ice age the ocean floor was exposed because of the fall of sea level when
glaciers are fed with water
o After the ice age, sea level rose and covered the ocean floor
- Landslides can be created by thawing of soil and sediment t effects of climate warming
- Debris torrent: rock and sediment that is slumping and moving rapidly down slope
o E.g. British Columbia t sea to sky highway
- Gulley: concentrates debris flow down one
- Rockfall moving on an air cushion
o When glacial ice from last ice age melted, the walls expand
o Air acts as a support for the fall
o E.g. European alps
- Classic bedding plane slide
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o Layers of paleozoic limestones and shales parallel with the slope t very unstable
- Tiltmeters
o Used in volcanoes because before it erupts, the volcano inflates
o Installed on turtle mountain
- Rock bolts
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o Increase shear strength of the rock
Underwater landslides
- Happens at divergent boundaries
- Can cause tsunamis
Strength of material
- Pore water: water that is trapped within sand grains (looks like ground water)
- Sand is cohesive if there are films of water, not wet sand
2. Transport of sediment by ice
Movement of sediment below large ice sheets
- Glacial ice on the ground
- Sediment below the glacial ice
- Ice creeps and cracks surface
- E.g. Laurentide ice sheet moves sediments down towards the states
- Striation: glacial ice scratching bedrocks leaves striations
- Ice moving sediment, removing bedrock
- Fiords: deep coastal valleys (e.g. of land cutting into ice sheets
- Only glaciers can erode below sea-level called over-deepening
o Therefore a lot of deep freshwater
- Glacial till: rocks moved under glacial ice
- Erratic boulders: rocks brought down by ice (from e.g. Labrador) that are far away from their
Quickclay failures
- Ice sheets left thick deposits of silt (mostly along the St. Lawrence valley)
- So much pore water pressure in the silt makes solid into fluid
o Rapid transformation: matter of minutes
The concept of graded
- Rivers cannot cut below sea level
o Not all rivers are graded to sea level
o Can be their own base level because of dryness
o Unlike ice sheets that can cut and grade to sea level
- E.g. Lake Ontario graded 75m above sea level
- Death valley t below sea level ; hot and dry
- Dead sea is also below sea level