Mountain Lecture 3- Moving dirt: large scale patterns of erosion Flashcards
What is weathering?
The (chemical/mechanical) breakdown of soil, rock or dissolved material.
What is erosion?
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.
What is mountain building dictated by?
The balance between the tectonic flux of material into a mountain range and the erosional flux of material out of it.
What does denudation of mountains lead to?
Isostatic rock uplift/rebound.
Can erosion cause surface uplift and build mountains?
Yes
What happens if erosion is non uniform?
Isostatic rock uplift can cause the peaks to rise higher than their original elevation, even without tectonic activity.
How can erosion change tectonic processes?
By removing material from the mountain range and creating space for more.
How do tectonics and erosion interact?
Tectonics cause the rocks to deform and move to the surface. Through erosion, the rocks are removed from the mountain range.
What matters for how a mountain range evolves?
Direction of precipitation delivery.
Does topography influence precipitation?
Yes
How does erosion and precipitation interact?
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.
What happens if precipitation is from the pro-side?
Erosion concentrated on pro-side which pulls material up from shallow depths.
Precipitation does not access retro-side.
All tectonic activity on pro-side.
What happens if precipitation is from the retro-side?
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.
Does topography influence atmospheric circulation?
Yes
What determines the strength of the South Asian monsoon?
Tibetan plateau
What happens within the Tibetan Plateau in summer?
Hot air rises over Tibetan Plateau, forms low pressure; draws in moist air from Arabian Sea/Bay of Bengal.
Rain in N India
What happens within the Tibetan Plateau in winter?
Cold, cry air over Tibetan Plateau forms high, blocks moist air masses to South.
Dry in N India.
Has the uplift of the Tibetan Plateau had a big affect on regional and global climate over time?
Yes
How can we estimate the importance of mountains for generating precipitation?
Comparing global climate model results with and without topography.
Is there difference between models showing precipitation and mountain interactions?
Yes.
Large differences identified over SE Asia.
What can serve as proxies for erosion?
Measures of sediment transport or accumulation.
What are the four main ways sediment is removed from mountains?
Suspended load
Solute load
Bed load
Aeolian transport
What is suspended load?
Fine particles (silt, clay) that are transported in the river flow and slowly settle out in the oceanic water column.
Easy to measure.
What is solute load?
Ions and dissolved components (around 80% consists of HCO3, SO4, Ca and SiO2).
Globally around 20% of the suspended load.
What is bed load?
Coarse particles (sand-boulders) that are transported along the river bed.
Around 10-50% of suspended load? 10% of less for large rivers?
What is aeolian transport?
Sand, silt, clay transported by wind.
Where is aeolian transport common?
From Sahara to Europe.
What is sediment load?
Mass of sediment leaving a catchment per unit time (measured from river samples, lake basins, or accumulation in deltas)
What is sediment yield?
The load, divided by the catchment area A. It provides a way of comparing basins of different sizes.
Where are the largest sediment loads?
India-Asia collision
Where are the largest sediment yields?
Small, wet, steep places
(Taiwan, NZ, Papua New Guinea)
What happens to discharge when the basin is larger?
Smaller discharge
What happens to load when the basin is larger?
Greater load
Larger rivers carry larger loads
What happens to yield when the basin is larger?
Smaller
Do mountain river generate a lot of sediment?
Yes
What do mountain rivers carry high ratios of?
Suspended (silt-clay) to solute (dissolved ions) loads.
Why is there PROPORTIONALLY more sediment in rivers in mountain landscapes?
Steeper slopes and rapid erosion such as landslides.
In lowlands, shllow slopes result in lower sediment yields.
What would knowing sediment load/yield from a drainage basin tell us?
Rates of erosion in the catchment
Rates of sediment transfer
Baseline for understanding effects of climate change
Baseline for understanding anthropogenic effects
What did Petellier (2012) propose?
Global model of suspended-sediment discharge (yield)
What does sediment yield correlate to in terms of discharge?
Topographic slope
Steep slopes, high yield
Rainfall and vegetation are also important but do not have same variation as slope.
Apply Pelletier (2012) to Amazon Basin.
Sediment sources are in the basin headwaters (Andes)
Sediment yields decrease progressively as you move downstream - little source and increasing storage.
What timescales are mountain ranges built over?
Geological
What timescales are measurements of sediment transport/erosion over?
Human
What can dramatically change sediment load?
Earthquakes
Why is it hard to know if sediment transport/erosion samples in moutain landscapes are representative of long-term scales?
Rare events
How much did suspended sediment concentrations increase after the M7/9 Wenchuan earthquake?
3-7 times.
How do we make longer term measurements of erosion rates?
Geochemical techniques such as cosmogenic radionuclides
Offshore records from sedimentary basins
What is the difference between rock exhumation and erosion?
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).
What is the difference between sediment load and sediment yield?
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.
Why do mountain rivers have higher sediment yields for a given runoff than lowland rivers?
Mountain slopes are steep and erode quickly (landslides!) – proportionally more sediment is being moved from the hillslopes into the rivers.
Lowlands have shallow slopes.
How and why might an earthquake affect a catchment’s sediment load?
Earthquake events can increase sediment loads – through earthquake-triggered landslides moving more sediment from the hillslope into the river.
What do measurements of suspended sediment load or yield tell us about the landscape? Provide one example.
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)
