GP1: Erosion & Transport Processes Flashcards
4 variables of erosion
Glaciological, substratum, topographic, temporal
what are substratum characteritstics?
the material below the glacier
What are temporal variables?
duration of glaciation or other processes in other variables of erosion
What are 3 glaciological variables?
basal shear/driving stress
subglacial water pressures and drainage configuration
thermal regime (erosion can only happen if the glacier is warm enough to move)
What affects the substratum of the glacier foundation?
structure, lithology, joint distribution, degree of weathering
What are the two types of material that can underlay a glacier?
sediments or bedrock
If sediment are underneath a glacier then what can affect its strength?
thickness, composition, structure and permeability
What is more likely to be eroded - sediments or crystaline bedrock?
bedrock
What are the two scales of topographic variables?
Small-scale = roughness of the bed at a specific point Large-scale = locations of large ice masses, valley shape and size, depth an overall accumulation rate
What are the two types of subglacial erosion?
Abrasion
Plucking/quarrying
What is evidence of abrasion happening?
Presence of striae on rocks.
How are striae caused through abrasion?
scoring of particle debris embedded in a glacier upon a specific rock
What can striae also indicate regarding the glacier?
ice flow-axis direction
What can happen to striae as you move downglacier and how?
They become wider and smoother because they are reworked that makes them blunter which then translates in to wider and smoother striae.
What can sand do to the surface of a rock it flows over?
It can smooth it through a sandpaper effect
What does plucking/quarrying involve?
Water at the bed can seep in to fractures in the bedrock where it can refreeze which can the pluck bits of the bedrock out of the bed as the glacier moves and then carry it with the glacier as it moves down glacier.
What can we use today to see how this would have happened in the past?
Downslope rock faces that have clearly had parts of them ripped out
What happens to erosion if the glacier bed is frozen to its base?
No abrasion or plucking can happen which means erosion is close to zero. However, plucking can still occur just at a much slower rate due tot he force generated by internal deformation as the glacier continues to move internally.
What happens when unlithified/soft sediment beds are eroded?
quick process whereby depositon and erosion happen almost instantaneously to each other. This happens because the bed is easily deformable. The process dominated a lot of the glacial processes in Great Britain and has now created many relic cirque glaciers
How can we determine how material was transported in the past?
By looking at its morphology today
What are the supraglacial debris entrainment mechanisms for valley glaciers?
Avalanches and freeze-thaw weathering can cause material to fall off on to the glacier
What are the supraglacial debris entrainment mechanisms for ice caps and ice sheets?
Nunataks and volcanic ash
What are nunataks and how can they contribute to debris entrainment?
Ridges that stick out the top of ice bodies
What is the process of frost-wedging?
freezing and then melting cycles of moisture in between rocks. The freezing causes the rock to expand 9% that places stress upon the rock that may cause it to break
What are two requirements for freeze-thaw weathering to happen?
temperate regions so that the cycle of temperatures can happen and enough saturation within rock to supply enough moisture
What differentiates debris at the base of a glacier to anywhere else in the glacier?
At the base there is as much as 75% debris content
What are 3 implications of the high debris content at the base of a glacier?
- ice is slowed down in motion as the rheology has been changed
- erosion rates increase
- increases the chances that material will be transported large distances from original position which can provide interesting information about transport (e.g. saunton sands)
How is regelation an entrainment mechanism?
this is where the glacier is forced to melt on the upside of an obstacle due to increased pressure against it and then refreeze on the leeside. During this process ice can infiltrate in to subglacial sediments where it can then form part of the glacier
How is englacial thrusting an entrainment mechanism?
Where basal debris can be elevated to englacial positoin by compressive deformation near the glacier margins.
When could englacial thrusting happen?
- during rapid glacier readvances
- where wet based ice decelerates against cold-based ice
- where ice flows against a topographic barrier that favours debris being forced in to ice
How can a nunatak cause entrainment?
material is transported down on the downslope side (relative to ice flow direction) and as the glacier flows around a nunatak it will accumulate material from the side and bed. The material collected from all sources will then accumulate as a wedge entrained within the ice
What is active transport?
basal transport zone where abrasion and crushing can happen and thereby rework sediments
What is passive transport?
supraglacial and englacial transport of particles with little to no modification. This means the end material remains angular.
What type of moraine can occur as a result of passive transport? and how?
Medial moraines - these are moraines that form on the surface or within the valley
What are the 3 types of medial moraine can occur as a result of passive transport?
Lateral moraine - confluences of ice streams that meet together and
Ablation dominant - ice melts and material therefore gets concentrated then transported on the surface
Avalanche - transient, isolated concentrations of debris from large, low-frequency rockfalls
