a) In what ways do glaciers erode? (8)
The almighty glacier can be very powerful when it moves, eroding anything that gets in its way. The glacier erodes in a number of ways, be it by abrasion, plucking or dilation, their paths of erosion rarely go unnoticed.
Abrasion occurs when there are debris between the bedrock and the icesheet or in the lower layer of ice. This layer of debris is pressed against the ice and grinds away material from the bedrock as the glacier moves. The rate of abrasion depends on a number of factors. The speed of the ices movement is an important factor, since if the ice were to move slowly, the rate of abrasion would be low. Another pivotal factor is the amount of debris there is. Obviously clean ice or one with very little debris will have little effect. The angularity of the debris is also important, as jagged debris will increase the rate of abrasion. The relative hardness of the bedrock to the debris is crucial since if the bedrock were to be much more resistant or harder than the debris then the debris will be worn down, as opposed to the bedrock being eroded. A more obscure but vital factor is the rate of removal of the debris. If the ice were already full of debris hence the rate of removal low, th!
e debris will form a quasi-protective layer shielding the bedrock from any abrasive action. The thickness of ice is the final factor that plays a role in abrasion rates. The graph below shows how the rate of abrasion changes with increasing ice thickness. At a), there is increasing friction as pressure increases hence increased abrasion. At b), however, increased pressure leads to pressure melting at the glacier base. Water reduces the friction, therefore reduces the abrasion. At c), the pressure increases to a point where friction is so great that material does not move and lodgment occurs.
Plucking, or quarrying, occurs when glaciers freeze around loosened rock and pluck them out when the glacier moves. The rock may be have been loosened due to a variety of reasons, for example, freeze thaw weathering within the glacier. This is known as regelation. Plucking may also take place away from a valley wall, e.g., when the glacier is upstream of a large bolder, pressure will build up behind the bolder. This will cause pressure melting hence allows the glacier to flow around the bolder and refreeze. Now the bolder is more susceptible to plucking.
Dilatation or pressure release occurs when ice takes the place of the rock it previously eroded. Since the ice is less dense than the previous rock, the rock is allowed to expand and release the pressure that it was once subjected to, hence the name: pressure release. This expansion will cause cracks to form in the rock that will encourage future plucking and abrasion.
Frost shattering takes place where the water enters cracks and freezes. Since ice expands, this causes the joint to be widened and makes it more vulnerable to future erosional processes.
Rock fracture is a very rare form of erosion, which occurs when the sheer force of the ice slamming into the bedrock erodes it. Since ice rarely moves at such a speed, this form of erosion is rarely seen.
Rotational movement occurs when large amounts of snow fall on the ice. The weight of the snow makes the ice readjust its mass. The rotational slumping that occurs is thought to be responsible for the over-deepening of corrie floors.
b) What landscape evidence would indicate that glacial erosion has occurred. (17)
Since abrasion erodes the bedrock, a polished surface tends to be the result, but deep scratches known as striations may be cut into the bedrock. These striations indicate the direction of the ice movement. The polishing of surfaces by abrasion also produces rock flour. These are tiny rock particles.
A cirque or corrie is developed as snow accumulates in the hollows. A series of processes, collectively known as nivation, including freeze-thaw, solifluction and chemical weathering operate under the snow patches and these would have gradually disintegrated the bedrock. The resulting debris would then have been removed by meltwater to leave an enlarged hollow. This process of over-deepening would probably have taken many years to produce a fully developed cirque. Once the over-deepened hollow is formed, a number of processes interact to produce the characteristic cirque. The steep headwall (A, fig. 1) results from freeze-thaw activity above the level of the ice, and plucking beneath the ice. The plucking is hastened by the meltwater, which seeps to the base of the glacier along the backwall, and via the bergschrund. Dilatation also weakens the rock, making it more vulnerable to plucking. The over-deepened floor (B, fig. 1) results from rotational movement and from abrasion. A!
brasion is particularly effective as the ice contains angular debris from the plucking and frost shattering of the headwall. The lip (C, fig. 1) develops where erosion decreases. This is due, in part, to the increase in pressure causing more pressure melting, and leading to a reduction in friction. It is also thought to be due to the effect of compressive flow as the ice starts to flow uphill. This produces slip planes within the ice that tend to lift the sub-glacial debris away from the floor of the glacier, so reducing its ability to erode.
When two adjacent cirques erode backwards or sideways towards each other, a narrow, steep-sided ridge called an ar*te. One of the best known examples is Striding Edge in the English Lake District. If three or more cirques develop on the sides of a mountain then a pyramidal peak may be formed. The Matterhorn in the Alps is an example of this landform, with steep sides and several ar*tes radiating from the central peak. (See picture below).
A glacial trough is where a glacier occupied a former V-shaped river valley, and erosion led to the formation of a characteristic U-shaped glacial valley. These have steep sides and flat floors. The widening and deepening of the valley is due to the fact that ice has more erosive power than a river, which only occupies a small part of the valley floor. Examples of glacial troughs include Wast Water in the English Lake District and Yosemite in California.
In its upper course the pre-glacial river would have flowed around inter-locking spurs. These would have been eroded by the glacier to leave steep-sided truncated spurs, e.g., El Capitan in Yosemite. Hanging valleys result from the different rates of erosion between the main valley glacier and its smaller tributaries. The latter would have eroded downwards much more slowly, so that the valley now lies above the main valley, and the tributary rivers must reach the main river by means of waterfalls, e.g., Bridal Veil Falls in Yosemite, California.
More evidence of glacial erosion, is the depositional landforms that are produced downstream of glaciers, e.g., morraines, as a result of the debris eroded from higher altitudes.