WEEK 9 DYNAMIC PLANET Flashcards
What are erosional landscapes?
generated by uplift, weathering and erosion
-tend to have a topographic relief
- dominated surfaces covered in weathering products (“regolith”)
-or dominated by bare eroding bedrock
What are some reasons why land surface may be uplifted?
via isostatic processes, lithospheric flexure, dyanamic effects
also via the formation of volcanoes
(the newly available rock is weakened through weathering)
What are the controls off weathering rates?
1.climate controlled
2. slope controlled
How are rates of weathering climate controlled?
generally:
-areas of high rainfall provide the water necessary for chemical weathering
-areas that are too cold the water is in solid phase/ice so cannot mediate chemical reactions
-higher temperatures speed up the rate of chemical weathering reactions
How are rates of weathering slope controlled?
Regolith growth is self-limiting.
-reduction in grainsize and formation of clays reduced permeability, and limits access of water to for chemical weathering.
-so weathering rate decreases downward in the regolith profile.
However if regolith is readily removed then water can access the reaction front
so rapid (or continuous) removal of regolith increases weathering rates
and the rate of removal of regolith is related to slope gradient
Steep slope= instantaneous removal of regolith + high weathering rate
Moderate slope= moderate removal of regolith +weathering rate
Low gradient slope= low rate of removal regolith + weathering rate
What are weathering limited slopes?
transport processes are so efficient that debris is removed more quickly than it can be generated by further weathering
characterized by= thin regolith
What are transport limited slopes? What are they characterised by?
occur where weathering processes are efficient at producing debris but where transport processes are inefficient at removing it from the slope
characterised by= thick regolith
Where are the highest rates of weathering?
Is high in areas of abundant rainfall and high temperatures
Is high in areas of steep slope (and therefore topography)
Note the positive feedback because of orographic rainfall
What is erosion?
Erosion is the process of removal of the regolith that was generated through weathering, and its transport to a new location.
What are the 2 main mechanisms for erosion?
- Gravity= moves material from uplifted areas (mountains, hinterlands) to low areas (sedimentary basins)
aka- the movement of material under its own gravitational potential in areas of steep slope. - Fluid flow= exerts shear stress which moves material from one place to the next
What are the 3 types of fluid flow?
- flow of water in streams and rivers (fluvial processes), which entrains sediment (gravity driven)
- flow of ice (glacial processes), which entrains sediment (gravity driven)
- flow of air (wind, or aeolian processes) from areas of high to low atmospheric pressure, which entrains sediment.
What is mass wasting?
Secondary erosional processes
-caused as a result of steep surface topography (sometimes enhanced by fluvial and glacial erosion)
-represent hazards to life and infrastructure
-can also happen in submarine settings
What are the types of mass wasting/movement?
The following are listed in descending order based on strength of the sediment/regolith and gradient required
- Rock fall
- Slide
(these are brittle) - Slump (ductile)
- debris flow
- turbidity and pyroclastic flows
(both fluid)
What occurs during rock fall? Where is it common?
detachment occurs with little or no shear, movement occurs through free-fall, bouncing or rolling.
-clasts and piles= called talus
-Common in glacial, canyon and sea-cliff geomorphic terranes with steep valley walls.
What are debris flows?
form where water saturated soil, sediment or regolith, fails – usually via catastrophic (rather than creeping) slumping
The kinetic energy imparted by the mass movement, causes grains to collide and bounce apart, allowing to flow freely between the grains, so that the material (even though it is composed of >50% grains) behaves as a fluid.
-pose a significant threat to society and infrastructure.
What occurs during a slump?
detachment and movement occurs along one or several surfaces of shear, with brittle and ductile internal deformation (faulting and folding).
-leaves behind a slump scar (arcuate) and a slump mass
Example= Mam Tor
What occurs during a slide?
detachment and movement occurs along one or several surfaces of shear (a scar and fault), with limited brittle internal deformation (fracturing or faulting).
What is the relationship between hillslope processes and gradient?
-as gradient reduced by hillslope processes there is a general change in behaviour
-eventually, gradients are insufficient to move material by their gravitational potential alone so fluvial processes take over.
What is fluvial erosion?
Water is the most important agent of erosion
Rainfall when it lands on the soil flows downslope
Because water has viscosity, it can impart a shear stress on particles, and
-move grains as bedload
-move grains as suspended load,
-and as the universal solvent, dissolve and transport solute minerals.
Suspended load= small particles
Bed load= large particles, roll and bounce
Where does deposition and erosion occur in a river/water flows?
Tributive Zone= erosion
-As water flows down slope, rivulets and streams join together.
-River systems that are eroding have a characteristic “tributive” plan view, which contrasts with the lower depositional reaches of the system.
Distributive Zone= deposition
What occurs in fluvial environments when rocks are soft and weathering (regolith development) is high?
-through erosion rivers incise the uplifted land to form valleys
-when rocks soft and weathering high the steepened valley walls fail, via hillslope processes (slumps and debris flows) , in order to achieve at-equilibrium slopes.
-leads to the characteristic v-shape profile of fluvial environments.
What occurs in fluvial environments when rocks are hard and weathering (regolith development) is low?
-the steepend valley walls do not fail, and the valley walls remain steep, as the river carves ever more steeply.
-leads to the formation of canyon and gorge geomorphology.
-associated hillslope process: “falls”
How does landscape evolution occur in fluvial environments?
Following the establishment of a fluvial erosional system, if erosion>uplift, it will “denude” the landscape= landscape evolution.
What are the characteristics of young rivers?
-steep gradients (high elevation)
-river mainly erodes by downcutting and headward erosion (nickpoints)
-the divides between rivers are broad, and undisected, because tributaries have had little time to form.
Example: The Yellowstone river is one
What are the characteristics of intermediate rivers?
-Streams have fully disected the former highlands
-Whole area is covered by an extensive drainage network.
-River erodes through horizontal sweeping and develops a flood plain (and meanders).
-Slopes retreat, and interfluves become narrower
-Landscape dominated by rolling hills between the main distributaries.
What are the characteristics of mature ‘old’ stage?
-Widens its flood plain further
-Slopes retreat until they meet, forming a flat featureless landscape.
-This surface is called apeneplain.
What is glacial erosion? Why does it occur?
-Ice caps and associated glaciers form because an excess of snow falls (relative to melting).
-Compaction of the snow forms ice which behaves in a ductile to brittle manner, and can flow downhill, over timescales of years to 100s years.
-Ice is less dense than water, but it has a much higher viscosity, so its capacity to erode and transport material is enormous.
-Typically, valley glaciers form in, and re-excavate major fluvial trunk river valleys.
-These are fed by ice-caps, or more minor hanging, or “cirque” glaciers which form in the former fluvial tributaries.
What are the ways in which the glacier can erode and transport any grainsize?
-as material that has fallen via gravity processes on to the top of the glacier
-by melting and then freezing around, then “plucking” rocks from the interface of the glacier with bedrock.
-by pulverising rock at the interface of the glacier with bedrock.
-by pushing sediment that is in front of the glacier, during episodes of growth.
How do glaciers modify a pre-existing valley?
deepen and widen it modifying its morphology
-base of the glacier is relatively flat and the sides are relatively steep
What happens to the transported material at the front of the glacier?
melted out in situ
all the sediment is “dumped” in an unsorted manner in moraines.
What occurs to geomorphology during ice retreat?
- Over-deepened trunk valleys, with flat bottoms and steep sides (the famous “U” shape
-Typically with a “misfit” river. - Hanging valleys where tributary glaciers eroded down only to level of the top of the valley glacier
- Amphitheatre-like “cirques” often filled with and quasi-circular lakes (“tarns”) at the headwalls of valley glaciers.
- Extremely narrow, steep ridges, called arretes, and steep “horns” (or “pyramidal peaks”, where the steep margins of glacial valleys have retreated towards one another.
