Mountain Lecture 3- Moving dirt: large scale patterns of erosion

Question 1

What is weathering?

Answer 1

The (chemical/mechanical) breakdown of soil, rock or dissolved material.

Question 2

What is erosion?

Answer 2

The removal and transport of soil, rock or dissolved material from one location on the Earth’s crust to another location where it is deposited.

Question 3

What is mountain building dictated by?

Answer 3

The balance between the tectonic flux of material into a mountain range and the erosional flux of material out of it.

Question 4

What does denudation of mountains lead to?

Answer 4

Isostatic rock uplift/rebound.

Question 5

Can erosion cause surface uplift and build mountains?

Answer 5

Yes

Question 6

What happens if erosion is non uniform?

Answer 6

Isostatic rock uplift can cause the peaks to rise higher than their original elevation, even without tectonic activity.

Question 7

How can erosion change tectonic processes?

Answer 7

By removing material from the mountain range and creating space for more.

Question 8

How do tectonics and erosion interact?

Answer 8

Tectonics cause the rocks to deform and move to the surface. Through erosion, the rocks are removed from the mountain range.

Question 9

What matters for how a mountain range evolves?

Answer 9

Direction of precipitation delivery.

Question 10

Does topography influence precipitation?

Answer 10

Yes

Question 11

How does erosion and precipitation interact?

Answer 11

If erosion is somehow a function of precipitation, then it matters where it rains.
Erosion ‘pulls’ rock to the surface in areas where precipitation can be the most intense.
Most erosion at the flanks of the mountain.

Question 12

What happens if precipitation is from the pro-side?

Answer 12

Erosion concentrated on pro-side which pulls material up from shallow depths.
Precipitation does not access retro-side.
All tectonic activity on pro-side.

Question 13

What happens if precipitation is from the retro-side?

Answer 13

Erosion concentrated on retro-side which pulls material through the mountain range to the back side.
Most tectonic activity on retro-side.
Little faulting and almost no erosion on pro-side.

Question 14

Does topography influence atmospheric circulation?

Answer 14

Yes

Question 15

What determines the strength of the South Asian monsoon?

Answer 15

Tibetan plateau

Question 16

What happens within the Tibetan Plateau in summer?

Answer 16

Hot air rises over Tibetan Plateau, forms low pressure; draws in moist air from Arabian Sea/Bay of Bengal.
Rain in N India

Question 17

What happens within the Tibetan Plateau in winter?

Answer 17

Cold, cry air over Tibetan Plateau forms high, blocks moist air masses to South.
Dry in N India.

Question 18

Has the uplift of the Tibetan Plateau had a big affect on regional and global climate over time?

Answer 18

Yes

Question 19

How can we estimate the importance of mountains for generating precipitation?

Answer 19

Comparing global climate model results with and without topography.

Question 20

Is there difference between models showing precipitation and mountain interactions?

Answer 20

Yes.
Large differences identified over SE Asia.

Question 21

What can serve as proxies for erosion?

Answer 21

Measures of sediment transport or accumulation.

Question 22

What are the four main ways sediment is removed from mountains?

Answer 22

Suspended load
Solute load
Bed load
Aeolian transport

Question 23

What is suspended load?

Answer 23

Fine particles (silt, clay) that are transported in the river flow and slowly settle out in the oceanic water column.
Easy to measure.

Question 24

What is solute load?

Answer 24

Ions and dissolved components (around 80% consists of HCO3, SO4, Ca and SiO2).
Globally around 20% of the suspended load.

Question 25

What is bed load?

Answer 25

Coarse particles (sand-boulders) that are transported along the river bed. Around 10-50% of suspended load? 10% of less for large rivers?

Question 26

What is aeolian transport?

Answer 26

Sand, silt, clay transported by wind.

Question 27

Where is aeolian transport common?

Answer 27

From Sahara to Europe.

Question 28

What is sediment load?

Answer 28

Mass of sediment leaving a catchment per unit time (measured from river samples, lake basins, or accumulation in deltas)

Question 29

What is sediment yield?

Answer 29

The load, divided by the catchment area A. It provides a way of comparing basins of different sizes.

Question 30

Where are the largest sediment loads?

Answer 30

India-Asia collision

Question 31

Where are the largest sediment yields?

Answer 31

Small, wet, steep places (Taiwan, NZ, Papua New Guinea)

Question 32

What happens to discharge when the basin is larger?

Answer 32

Smaller discharge

Question 33

What happens to load when the basin is larger?

Answer 33

Greater load Larger rivers carry larger loads

Question 34

What happens to yield when the basin is larger?

Answer 34

Smaller

Question 35

Do mountain river generate a lot of sediment?

Answer 35

Yes

Question 36

What do mountain rivers carry high ratios of?

Answer 36

Suspended (silt-clay) to solute (dissolved ions) loads.

Question 37

Why is there PROPORTIONALLY more sediment in rivers in mountain landscapes?

Answer 37

Steeper slopes and rapid erosion such as landslides. In lowlands, shllow slopes result in lower sediment yields.

Question 38

What would knowing sediment load/yield from a drainage basin tell us?

Answer 38

Rates of erosion in the catchment Rates of sediment transfer Baseline for understanding effects of climate change Baseline for understanding anthropogenic effects

Question 39

What did Petellier (2012) propose?

Answer 39

Global model of suspended-sediment discharge (yield)

Question 40

What does sediment yield correlate to in terms of discharge?

Answer 40

Topographic slope Steep slopes, high yield Rainfall and vegetation are also important but do not have same variation as slope.

Question 41

Apply Pelletier (2012) to Amazon Basin.

Answer 41

Sediment sources are in the basin headwaters (Andes) Sediment yields decrease progressively as you move downstream - little source and increasing storage.

Question 42

What timescales are mountain ranges built over?

Answer 42

Geological

Question 43

What timescales are measurements of sediment transport/erosion over?

Answer 43

Human

Question 44

What can dramatically change sediment load?

Answer 44

Earthquakes

Question 45

Why is it hard to know if sediment transport/erosion samples in moutain landscapes are representative of long-term scales?

Answer 45

Rare events

Question 46

How much did suspended sediment concentrations increase after the M7/9 Wenchuan earthquake?

Answer 46

3-7 times.

Question 47

How do we make longer term measurements of erosion rates?

Answer 47

Geochemical techniques such as cosmogenic radionuclides Offshore records from sedimentary basins

Question 48

What is the difference between rock exhumation and erosion?

Answer 48

Exhumation is the movement of rock to the Earth’s surface from depth. Erosion is the process the moves/removes rock from the Earth’s surface (allows for rocks to be exhumed to the surface).

Question 49

What is the difference between sediment load and sediment yield?

Answer 49

Sediment load (or discharge) is the mass of sediment leaving a catchment per unit time (measured from river samples, lake basins, or accumulation in deltas). Sediment yield is the load, divided by the catchment area A. It provides a way of comparing basins of different sizes.

Question 50

Why do mountain rivers have higher sediment yields for a given runoff than lowland rivers?

Answer 50

Mountain slopes are steep and erode quickly (landslides!) – proportionally more sediment is being moved from the hillslopes into the rivers. Lowlands have shallow slopes.

Question 51

How and why might an earthquake affect a catchment’s sediment load?

Answer 51

Earthquake events can increase sediment loads – through earthquake-triggered landslides moving more sediment from the hillslope into the river.

Question 52

What do measurements of suspended sediment load or yield tell us about the landscape? Provide one example.

Answer 52

Rates of erosion in the catchment; rates of sediment transfer (e.g. reservoir infilling); baseline for understanding effects of climate change; baseline for understanding anthropogenic effects (e.g. agriculture)