Glasser and Bennet 2004 (6) Flashcards
What is glacial erosion?
The removal and transport of bedrock and/or sediment by glacial quarrying, glacial abrasion and glacial meltwater.
How are landforms of glacial quarrying (such as roches moutonnées, rock basins and zones of areal scouring) created?
When cavities form between an ice sheet and its bed.
This means that they are indicative of low effective basal pressures and high sliding velocities that are necessary for ice bed separation.
How can abrasion be achieved?
Bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice.
When is abrasion favoured?
In situations where effective basal pressures are greater than 1 MPa and where there are low sliding velocities
What is palaeoglaciology?
The reconstruction of ancient ice sheets and the spatial analysis of glacial landforms is an essential tool in this process.
Understanding glacial erosional processes and landforms and how they relate to former ice sheets is important for a number of reasons.
Over the timescales of a glacial cycle, glacial erosion is capable of modifying the bed of an ice sheet and therefore altering large-scale ice dynamics and mass balance.
Landforms of glacial erosion are used as palaeo-environmental indicators.
What is quarrying/ plucking?
The removal of large blocks from the bed.
What is abrasion?
The surface wear created by the passage of isolated clasts in ice or the sliding of sediment over the bed (Hindmarsh, 1996a).
What two processes are involved in quarrying/plucking?
(1) the fracturing or crushing of bedrock beneath the glacier; and (2) the entrainment of this fractured or crushed rock
When does the fracturing of bedrock take place?
When a glacier flowing over bedrock creates pressure differences in the underlying rock, causing stress fields that may be sufficient to induce rock fracture.
Evacuation of rock fragments along joints in the bed is possible where…
Localized basal freezing occurs.
Where is quarrying/ plucking favoured?
Beneath thin, fast-flowing ice (Hallet, 1996)
What are the dominant glaciological conditions required for quarrying?
Low effective basal pressures (0.1–1 MPa) and high sliding velocities because these conditions favour extensive ice/bed separation (subglacial cavity formation) and also concentrate stresses at points, such as the corners of bedrock ledges, where ice is in contact with the bed (Iverson, 1991a; Hallet, 1996).
How can abrasion be achieved?
By bodies of subglacial sediment sliding over bedrock (Cuffey and Alley, 1996; Hindmarsh, 1996a) or by individual clasts contained within ice (Hindmarsh, 1996b).
The effectiveness of meltwater as an agent of erosion depends on:
(1) the susceptibility of the bedrock involved, in particular the presence of structural weaknesses or its susceptibility to chemical attack; (2) the discharge regime, in particular the water velocity and the level of turbulent flow; and (3) the quantity of sediment in transport.
What are microscope landforms of glacial erosion?
Those below 1 m in size.
They are of low relief amplitude and are often found superimposed on larger landforms.
There are four types of micro-scale landforms, what are they?
Striae, micro-crag and tails, bedrock gouges and cracks, and p-forms and micro-channel network
What are Straie?
Striae are the lines or scratches on a rock surface produced by the process of glacial abrasion as debris or clasts entrained in basal ice are dragged over rock.
Striae can be placed in one of three morphological categories:
Type 1 striae become progressively wider and deeper down glacier until they end abruptly, often as deep steep walled gouges.
Type 2 striae start and terminate as faint, thin traces. They steadily broaden and deepen until they reach a maximum width and depth near their centre point.
Type 3 striae begin abruptly as deep gouges and then become progressively narrower and shallower down-glacier. They form where a striator point contacts and indents the bed. Clast rotation with little dis- placement along the bed produces a low ploughing angle, so that a gradual reduction in indentation depth occurs as sliding proceeds (Iverson, 1991b).
What are micro crag and tails?
Micro crag and tails are small tails of rock that are preferentially protected from glacial abrasion in the lee of resistant grains or mineral crystals on the surface of a rock.
What are bedrock gouges and cracks?
This heading comprises a family of small cracks, gouges and indentations created on bedrock surfaces
Gouges and cracks can provide information regarding…
The direction of forward dip of the fracture provided a good indication of ice movement direction (Harris, 1943).
The term ‘p-form’ was introduced by Dahl (1965) and is applied to…
a variety of forms including smooth sinuous depressions and grooves sculpted on glaciated bedrock surfaces.
The name given to these forms reflects the fact that their form was originally attributed to the plastic flow of ice around bedrock obstacles.
What are meso-scale landforms?
Those between 1 m and 1 km in size and comprise a family of landforms including streamlined bedrock features, stoss and lee forms, rock grooves and rock basins, and meltwater channels
