WEEK 9 DYNAMIC PLANET

Question 1

What are erosional landscapes?

Answer 1

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

Question 2

What are some reasons why land surface may be uplifted?

Answer 2

via isostatic processes, lithospheric flexure, dyanamic effects

also via the formation of volcanoes

(the newly available rock is weakened through weathering)

Question 3

What are the controls off weathering rates?

Answer 3

1.climate controlled
2. slope controlled

Question 4

How are rates of weathering climate controlled?

Answer 4

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

Question 5

How are rates of weathering slope controlled?

Answer 5

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

Question 6

What are weathering limited slopes?

Answer 6

transport processes are so efficient that debris is removed more quickly than it can be generated by further weathering

characterized by= thin regolith

Question 7

What are transport limited slopes? What are they characterised by?

Answer 7

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

Question 8

Where are the highest rates of weathering?

Answer 8

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

Question 9

What is erosion?

Answer 9

Erosion is the process of removal of the regolith that was generated through weathering, and its transport to a new location.

Question 10

What are the 2 main mechanisms for erosion?

Answer 10

1. 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.
2. Fluid flow= exerts shear stress which moves material from one place to the next

Question 11

What are the 3 types of fluid flow?

Answer 11

1. flow of water in streams and rivers (fluvial processes), which entrains sediment (gravity driven)
2. flow of ice (glacial processes), which entrains sediment (gravity driven)
3. flow of air (wind, or aeolian processes) from areas of high to low atmospheric pressure, which entrains sediment.

Question 12

What is mass wasting?

Answer 12

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

Question 13

What are the types of mass wasting/movement?

Answer 13

The following are listed in descending order based on strength of the sediment/regolith and gradient required

1. Rock fall
2. Slide
   (these are brittle)
3. Slump (ductile)
4. debris flow
5. turbidity and pyroclastic flows
   (both fluid)

Question 14

What occurs during rock fall? Where is it common?

Answer 14

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.

Question 15

What are debris flows?

Answer 15

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.

Question 15

What occurs during a slump?

Answer 15

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

Question 15

What occurs during a slide?

Answer 15

detachment and movement occurs along one or several surfaces of shear (a scar and fault), with limited brittle internal deformation (fracturing or faulting).

Question 16

What is the relationship between hillslope processes and gradient?

Answer 16

-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.

Question 17

What is fluvial erosion?

Answer 17

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

Question 18

Where does deposition and erosion occur in a river/water flows?

Answer 18

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

Question 19

What occurs in fluvial environments when rocks are soft and weathering (regolith development) is high?

Answer 19

-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.

Question 20

What occurs in fluvial environments when rocks are hard and weathering (regolith development) is low?

Answer 20

-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”

Question 21

How does landscape evolution occur in fluvial environments?

Answer 21

Following the establishment of a fluvial erosional system, if erosion>uplift, it will “denude” the landscape= landscape evolution.

Question 22

What are the characteristics of young rivers?

Answer 22

-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

Question 23

What are the characteristics of intermediate rivers?

Answer 23

-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.

Question 24

What are the characteristics of mature ‘old’ stage?

Answer 24

-Widens its flood plain further
-Slopes retreat until they meet, forming a flat featureless landscape.
-This surface is called apeneplain.

Question 25

What is glacial erosion? Why does it occur?

Answer 25

-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.

Question 26

What are the ways in which the glacier can erode and transport any grainsize?

Answer 26

-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.

Question 27

How do glaciers modify a pre-existing valley?

Answer 27

deepen and widen it modifying its morphology
-base of the glacier is relatively flat and the sides are relatively steep

Question 28

What happens to the transported material at the front of the glacier?

Answer 28

melted out in situ
all the sediment is “dumped” in an unsorted manner in moraines.

Question 29

What occurs to geomorphology during ice retreat?

Answer 29

1. Over-deepened trunk valleys, with flat bottoms and steep sides (the famous “U” shape
   -Typically with a “misfit” river.
2. Hanging valleys where tributary glaciers eroded down only to level of the top of the valley glacier
3. Amphitheatre-like “cirques” often filled with and quasi-circular lakes (“tarns”) at the headwalls of valley glaciers.
4. Extremely narrow, steep ridges, called arretes, and steep “horns” (or “pyramidal peaks”, where the steep margins of glacial valleys have retreated towards one another.