Topic 2 - EQ3 Flashcards
How can glacier size and type affect glacial erosion?
There is great variation in the intensity of glacial erosion, with perhaps the most important single factor determining its efficiency of erosion being the glacier itself.
It’s size (which determines ice thickness), and it’s thermal regime, also determine the importance and intensity of the erosional process. A general rule is that all types of glacial erosion operate much more effectively when glacier ice is warm based. This also allows regelation (where water melts under pressure and freezes again when pressure is reduced), one of the key elements of the plucking process, can also occur.
What factors other than glacier size and glacier type affect glacial erosion?
Other important factors include the glacier’s velocity across the bed, the ice thickness and, hence, the power of the glacier to cause shattering. Quantity and shape of rock debris is also important. Subaerial processes of freeze-thaw combine with extensive mass movement from scree slopes to supply the tools for glacial erosion. The bedrock characteristics, such as jointing and hardness, are also significant in influencing compressional and extensional flow. Erosion is more intense when bedrock is relatively weak.
What factors influence rates of abrasion?
- Quantity of debris available
- Angularity of debris or clasts
- relative hardness of clasts in bedrock
- Lithology (amount of jointing in rock and overall hardness)
-Basal water sliding moves debris faster
-Semi-permeability of bedrock leads to meltwater at glacier-bedrock face.
What factors influence rates of plucking?
-Thermal regime of glacier
-Thickness of ice
-Presence of basal meltwater
-Higher velocity creates greater sheer stress
-Highly fractured and jointed rock promotes dilation and freeze-thaw
-Semi-permeability of bedrock leads to meltwater at glacier-bedrock face.
What are the issues with features and landforms of glacial erosion?
Processes do not operate at a constant rate during time, and the landforms have to continually adjust, especially after glacial retreat in the short paraglacial period and then post-glacially when the landforms shaped by glaciation are reshaped by water, weathering and mass movement.
A further complication is that most present-day landscapes resulting from glacials are products of many successive advances of glacier ice because of the alternating ice-house-greenhouse conditions during the quaternary period. As the last glacial period ended (relatively) recently, only around 11,500 years ago in the Loch Lomond Stadial), the mountain areas of the UK provide clear examples of glacial erosional landforms e.g the Lake District.
How can features and landforms created by glacier erosion be classified?
Many different features and landforms are produced by glacial erosion, and they can be classified in different ways based on scale/size range, relative altitude or even the dominant erosion processes that formed them.
What are macro-scale features?
These are around 1km or larger in size and form the major elements in a glaciated highland landscape. They also contain many of the meso- and mice-scale erosional features, as well as depositional landforms.
How are cirques formed?
Also know as corries or cwms, they are armchair/bowl-shaped depressions in the mountainside. They have a steep back wall and a rock lip.
The initial stage of formation is for snow to accumulate in a sheltered mountain side location. In the northern hemisphere, cirques most commonly form on the NE side of mountains, in the Lee from prevailing westerly winds, and in shadier sites protected from insolation.
Once a sheltered area has accumulated snow, nivation or snow patch erosion begins, enlarging the hollow by a combination of freeze-thaw weathering to loosen the rock, and in summer meltwater from melting snow transports the rock debris away, thus enlarging the hollow. Once a nivation hollow is established, positive feedback occurs: the enlarged hollow traps additional snow and gradually enlarges to provide a site for glacial ice formation.
The process of plucking and abrasion then combine to develop the cirque. The glacier ice may expand in areas and move down valley during the glacial period.
What is an arête?
A macro-scale landform which is a narrow ridge between two cirques. It forms as two cirques are enlarged back to back, and the ridge between them becomes increasingly narrowed.
Plucking and abrasion are the two dominant processes in this landform, with freeze thaw weathering also playing a key role.
What is a pyramidal peak?
This macro scale landform is a sharp, pointed mountain peak. As three or more cirques are enlarged the hilltop between them becomes increasingly narrowed, with the dominant process being the plucking of the backwall of the cirque.
What is a trough (U-shaped valley)?
Occurs as a result of the powerful plucking and abrasion of the glacier ice.
A steep-sided, flat-floored, straight valley and it is macro scale. A pre-glacial river valley is widened and deepened by erosion from an advancing glacier.
What is a truncated spur?
This macroscale landform is a steep and possible rocky section on the side of a trough. The pre-glacial interlocking spurs of the river valley are eroded by the much more powerful glacier.
What is a hanging valley?
A small tributary valley high above the floor of the trough, often with a waterfall. Tributary glaciers with small amounts of ice did not erode their valley floor as deeply as the main glacier and so are left at a higher altitude.
What is a Roche moutonnée?
This is a meso-scale landform which is asymmetrical, bare rock outcrop with a gently sloping side facing up valley. It is formed as ice crosses a resistant rock outcrop, the regelation creep causes the increased localised pressure which causes melting and basal sliding and the up valley side is smoothed by abrasion. On the leeward side pressure is reduced, re-freezing occurs and plucking takes place, causing a steep, jagged slope.
What are striations?
These are micro scale and look like grooves which form in expose rock. They are formed by the abrasion by debris which is embedded in the base of the glacier as it is passed over bare rock. They can indicate the direction of ice movement.
What are glacial troughs?
This is when glacier ice moves through mountain valleys, it straightens, widens and deepens them, changing the valley from V to U shaped. The ‘more correct’ terminology is as a parabolic shape. They can vary in length from 5km (like in Snowdonia), to incredibly spectacular features like the Yosemite Valley in the Yosemite National park, California.
What do glacial troughs show about erosion?
Along their lengths, many glacial troughs have a stepped profile, reflecting differential erosion as a result of both irregularities in the underlying bedrock and variations in intensity of erosion. For example, when several cirque glaciers meet at the head of a valley the enlarged glacier erodes very deeply to form a trough end to the valley. After deglaciation, successive rock basins down a glacial through are separated by rock steps. Longer and deeper basins make contain a feature known as a ribbon lake. Post-glacial weathering and mass movement has led to infill of glacial troughs, which are now commonly occupied by misfit streams.
What has happened to many coastal glacial troughs?
With relative sea level rises at the end of the last glacial period, many coastal glacial troughs were flooded by the sea to form sea lochs.
How do hanging valleys occur?
They occur where a small side tributary glacier meets a larger main valley glacier. During the glacial phase the surface ice elevation of the tributary and main valley glaciers is the same but, because of the rate of erosion beneath the main valley glacier is much greater, once the glaciers have retreated the tributary valley is left hanging hundreds of metres above. Often with a waterfall plunging from the hanging valley to the main valley below.
What is the link between glacial troughs and truncated spurs?
Many glacial troughs show truncated spurs, marked by very steep, almost vertical, side walls where original interlocking spurs have been cut away or truncated by glacial erosion, because of the inflexibility of glaciers moving down the valley (e.g Lauterbrunnen Valley in Switzerland or Yosemite in California).
Where are meso-scale features often found?
They are found largely within marco features, for example whalebacks or roches moutonnées found on the floor of the Yosemite glacier troughs. These intermediate-scale landforms can range from about 10m to 1km in length. Streamlined bedrock features such as whalebacks are most common, where a glacier moves over a resistant rock knoll, so abrading it.
How are Roches Moutonnèes formed?
They are stoss and Lee features. Abrasion smooths and polishes the up-glacier, stoss-side of bedrock knoll, while glacial plucking makes the down-glacier, Lee-side rugged and rough. This then produced a asymmetric. These features can be roughly 300m in length and 30m in height.
What are micro-scale features?
Micro-scale features of glacial erosion are those that are a few metres in size or less. They include striations, which are scratches on hard bedrock caused by debris being dragged across the surface during abrasion, almost like chisel marks. They tend to be parallel to the direction of ice movement, with the deepest part of the scratch at the initial point of impact, and are therefore useful for tracking the direction of past glacier movement.
What are chatter marks?
Chatter marks are irregular chips and fractures in the rock, whereas cresentic gouges have a more regular pattern and are usually concave up-glacier. Look out for these micro features on abraded surfaces.
What are micro-features useful for?
Micro features aren’t only useful when helping glaciologists understand which direction the ice came from, but also for determining the maximum altitude of glacial erosion, where there are no micro features. In the Glyders you can see how block fields, scree and tors, clearly indicative of periglacial activity, supersede abraded, ice-scratched rocks.
What is glacial scouring?
When ice sheets and glaciers expand out beyond constrained mountain valleys, they erode large areas of lower relief by the process of areal scouring. Glacial scouring allows for the formation of Roche mountonees, crag and tails and Knock and Lochan topography
What is Knock and Lochan topography?
Results from glacial scouring.
In NW Scotland the topography is called ‘knock and lochan’. This is because the higher areas of resistant rock are interspersed with numerous small lakes in the rock basin (lochans). A chaotic drainage pattern has resulted often where patches of residual moraine interfere with the drainage. Other areas where landscapes of areal scour occur include Central Finland Lake Belt and the Canadian Shield. Both areas of ancient resistant igneous and metamorphic rocks where differential erosion is controlled by the density of jointing.
What is crag and tail?
This is an example of a meso-scale landform. They occur where the glacier ice is forced around a large and resistant rock obstacle, such as a volcanic plug, which protects the less-resistant material on the Lee-side causing the feature to taper into a tail in the down-glacier direction. A famous example of a crag and tail is Edinburgh Castle, with a steep-up-glacier stoss end and a long, gently sloping tail on the Lee side that runs down for 1.4km to the Scottish parliament.
What has work in Antarctica shown about ice sheet erosion?
It has confirmed that ice sheets did not create the overall landforms of the great shields areas, which has acquired their almost-level surfaces by denudation before the ice age. What these ice sheets did do, however, was to considerably modify the underlying surface over which they passed. This is confirmed as there is generally a low amplitude of relief with many meso- and micro-scale features.
What happens to supraglacial debris?
Supraglacial refers to material being transported along the surface of the ice. If rock debris from surrounding slopes falls on to the glacier in the accumulation zone it will become buried by new snow and become englacial. If it falls on the ablation zone area, it will most likely stay on top till it reaches the snout, which is often very dirty from accumulated debris. Supraglacial debris can also become englacial by falling into deep transverse crevasses opened up by extensional flow, or it can be carried downwards by meltwater in warm (wet) based glaciers.
What happens to sub-glacial debris?
It is transported beneath the glacier ice all the way to the snout, but it can be thrusted upwards during compressional flow.
What is the effect of the transportation and eventual deposition of debris by glaciers?
This is just as significant as glacial erosion in modifying the pre-glacial landscape, in this case by covering it over.
What are erratics?
These are large boulders known as glacial erratics, so called because they are of a different rock type to the bedrock they ‘sit’ on. This testifies the sheer scale of the ability of glaciers to transport enormous quantities and weight of rock debris over great distances. Huge erratics - boulders weighing up to 16,000 tonnes - were carried over 300km from Canadian Rockies to the plains of Alberta by the Cordilleran Ice Sheet. Some are actually dumped as perched boulders, for example Bowder Stone in Borrowdale in the Lake District.
How can erratics help with mapping glacial movement?
If the erratic is made of a rock source of a distinctive geology from a resurrected location - e.g Ailsa Craig granite from West Scotland - you can precisely map the direction of glacier ice movement.Erratics from Scandinavia have been found in the boulder clay of Northumberland, Durham and Yorkshire coasts of NE England. This confirms the presence of the continental ice sheets from Scandinavia.
What are the 4 main processes in glacial deposition?
-Lodgement
-Ablation
-Deformation
-Flow
What is lodgement?
This process occurs beneath the ice mass when subglacial debris that was being transported becomes ‘stuck’ or ‘lodged’ on the glacier bed. Lodgement occurs when the friction between the subglacial debris and the bed becomes greater than the drag of the ice moving over it. It is commonly associated with glaciers carrying huge loads of debris and where the glacier is very slow moving, if not static.
What is ablation?
This process refers to debris being dumped as the glacier melts and thaws. It can include supraglacial and englacial material, as well as sub-glacial material.
