Geomorphology Flashcards
What is Geomorphology?
The branch of geography concerned with the structure, origin, and development of the topographical features of the Earth’s surface.
What is Tectonics?
The branch of geology relating to the structure of the Earth’s crust and the large-scale processes which take place within it.
Drivers of Erosion 1:
- Exogenic (external, i.e. solar) energy drives surface processes (e.g. precipitation, winds) which can break down and transport various components of Earth’s surface.
- Materials also have gravitational energy and materials on slopes want to move downhill.
Drivers of Erosion 2:
- Weathering (physical and chemical destruction of rock) is needed first.
- Erosion occurs as material is transported by flows of water, wind and ice.
- Weathering is highly temperature dependent.
Base Level
Base level is the lowest elevation to which a landscape can be eroded, usually being sea level. Erosion rates decline towards the base level because the transporting power also declines.
The Hjulstrom Curve
This curve defines river flow and what kind of river velocity you need to be able to transport certain types of rock size, depending on velocity and sediment size.
Process Geomophology: Process Response and Process Form
This explains the relationship between the landforms we see and the processes responsible. A change in process leads to a predictable response (i.e. change in form).
Process Geomorphology example:
Stream channel size is a balance between size, sediment supply, and discharge:
- Too much sediment for flow to carry = deposition.
- Deposition = smaller channel = faster flow.
Thresholds
Where we have a flow, it is not always able to transport the material afforded to it, there are different thresholds at which new processes can occur.
Dynamic Equilibrium
This is where the system fluctuates around a mean trend, e.g. glacier erosion during warming climate. A trend that has a certain direction due to some external factor.
Dynamic Metastate Equilibrium
This follows a trend e.g. influence by climate, that switches through different thresholds. E.g. sediment transport depends on river discharge, so may be different between a stormy period and a dry period.
Cyclic Time Memory
Geologic, 10^6 yrs+ - Climate, initial elevation etc. all depend on time.
Graded Time Memory
Modern, 10^2 yrs - short enough to observe a dynamic equilibrium
Steady Time Memory
Present, days - short enough to appear unchanging, e.g. channel width and depth.
Lag Time
The delay time between when a fundamental control changes and the landscape responds.
Relict landforms
Landforms created under previous climate have yet to respond to be in balance with the present climate.
Complex Response
Exceeding a particular threshold can lead to more than one geomorphic process responding.
Limits of Process Geomorphology
We are limited to what we can learn of landscapes. We need to work at observable levels and relate what’s happening to each process, and derive general normative rules. Observational limits are minutes to 100’s of years, beyond this (on cyclical time scales) we can hardly know ALL the processes occurring.
Conventional Time and Spatial Scales:
- Landform size/shape, rate of sediment transport etc. may be predicted with confidence using observed processes.
- Many examples from which to derive normative rules.
- Climate change and tectonic effects are relatively weak.
Larger Time and Spatial Scales
- Short term observations may be misleading.
- Past processes may dominate.
- Too few examples to derive general rules.
Define Aeolian
Pertaining to the action or effect of the wind, requiring reasonable wind-speeds at the surface and freely available sediment (loose or little vegetation).
Places with good conditions for Aeolian action
- Dry lake beds
- Sandy coasts
- River plains
- Margins of glaciers
- Some agricultural fields
What are Drylands?
These are arid, semi-arid and sub-humid regions of the Earth. They have a moisture deficit which means that the potential evapotranspiration exceeds precipitation. Resulting in a reduced moisture availability In the soil, limiting and stressing vegetation.
Dryland Distribution
Drylands occur globally and have diverse geomorphic characteristics and ecosystems, such as hot deserts and cold deserts.
There is a dense concentration of drylands in central Asia, Turkmenistan and Afghanistan, they are also found in Africa, Australia, S. Spain and Central USA.
Why are Dryland Important?
- Desert winds carry more sediment than any other geomorphological agent. (Cooke et al,1993).
- The Sahara desert produces 200 m t/yr dust whereas the river Niger only 15 m t/yr.
- Wind is the most powerful geomorphic agent on Mars.
Erosional Landscapes (Aeolian)
Wind erodes landforms by removing grains of sediment, producing erosion known as deflation and abrasion.
Depositional Landscapes (Aeolian
Net deposition forms sand sheets and bed forms: dunes.
How does Aeolian Transport Work?
There needs to be a suitable sediment source (sand, silt) and sufficient wind energy to mobilise sediment.
Entrainment Thresholds (Aeolian)
Wind has to provide enough lift, overcome drag and cohesive forces between particles.
Bagnold (1941) recognised a threshold shear velocity defined by the critical shear velocity (the velocity at which particles will start to move).
80% of sand movement is undertaken by winds blowing for 12% of the time.
How are grains actually transported?
Some grains go into suspension, but most bang along the ground known as creep, saltation is bouncing along the ground. For deposition, obstacles are needed to reduce wind velocity and entrainment capacity.
