2.3 How do glacial landforms evolve over time as climate changes? Flashcards
2.3 How do glacial landforms evolve over time as climate changes?
Key idea ⮕ Glacis-fluvial landforms exist as a result of climate change at the end of glacial periods.
Glacio-fluvial landforms
Those produced by meltwater from glaciers. They can include both erosional and depositional landforms.
Glacial periods end
When global temperatures rise. They are followed by shorter inter-glacials lasting from 10,000 to 15,000 years. In the post-glacial period temperatures often increase gradually, with many fluctuations as part of a general warming trend.
Glacio-fluvial streams and river deposition
Glacio-fluvial streams and rivers deposit a distinctive sediment type known as outwash. In contrast to till, deposited directly by glacial ice, outwash tends to be:
-Generally smaller as meltwater streams typically have less energy than glaciers and so only carry finer material
-Smooth and rounded by contact with water and by attrition
-Sorted - horizontally, with the largest material found furthest up the valley and progressively finer material with distance down the valley due to the sequential nature of the deposition mechanism
-Stratified - vertically, with distinctive seasonal and annual layers of sediment accumulation in many of the landforms.
Landforms of glacio-fluvial deposition
-Kames
-Eskers
-Outwash plains
Landforms of glacio-fluvial deposition definition
Deposition of outwash results in the formation of distinctive glacio-fluvial landforms: kames, eskers and outwash plains.
Kames (Landforms of glacio-fluvial deposition)
A hill or hummock composed of stratified sand and gravel laid down by glacial meltwater. The two types of kame are Delta kames and Same terraces.
Delta kames (Kames (Landforms of glacio-fluvial deposition))
Delta kames form in different ways. Some are formed by en-glacial streams emerging at the snout of the glacier. They lose energy at the base of the glacier and deposit their load. Others are the result of supraglacial streams depositing material on entering ice-marginal lakes, losing energy as they enter the static body of water. Some also form as debris-filled crevasses collapse during ice retreat.
Kames are widespread in East Lothian, Scotland.
Kame terraces (Kames (Landforms of glacio-fluvial deposition))
Kame terraces are ridges of material running along the edge of the valley floor. Supraglacial streams on the edge of the glacier pick up and carry lateral moraine which is later deposited on the valley floor as the glacier retreats.
The streams form due to the melting of ice warmed in contact with the valley sides as a result of friction and the heat-retaining properties of the valley-side slopes. Although they may look similar to lateral moraines, unlike moraines they are composed of glacio-fluvial deposits that are more rounded and sorted.
In the Kingsdale valley of the Yorkshire Dales, a game terrace extends for about 2 km along the north side of the valley. It is approximately 2 m high for most of its length.
Esker (Landforms of glacio-fluvial deposition)
A long, sinuous ridge composed of stratified sand and gravel laid down by glacial meltwater.
Material is deposited in sub-glacial tunnels as the supply of meltwater decreases at the end of the glacial period. Sub-glacial streams may carry huge amounts of debris under pressure in confined tunnels at the base of the glacier.
The Trim esker near Dublin is one of a group of twelve in the area. It is 14.5 km long and between 4 and 15 m high.
Scientists’ views on Eskers (Landforms of glacio-fluvial deposition)
Some scientists argue that deposition occurs when the pressure is released and meltwater emerges at the glacier snout.
As the glacier snout retreats, the point of deposition will gradually move backwards. This may explain why some eskers are beaded - the ridge showing significant variations in height and width - with the beads of greater size representing periods when the rate of retreat slowed or halted. However, others argue that the beads are simply the result of the greater load carried by summer meltwater.
Outwash plains (Landforms of glacio-fluvial deposition)
(Also known as a sandur), a flat expense of sediment in the pro-glacial area. As meltwater streams gradually lose energy as they enter lowland areas beyond the ice front, they deposit their load. The largest material is deposited nearest the ice front and the finest further away. Outwash plains are typically drained by braided streams. These are river channels subdivided by numerous islets and channels.
Sub-glacial streams
Drain from the surface of the glacier and flows along its base.
Glacier snout
The lowest end of a glacier; also called glacier terminus or toe.
Braided streams
River or stream channels subdivided by numerous islets and channels, caused by accumulated sediments choking off the channel and the stream splitting into several smaller streams. Occurs when the load supplied to a stream exceeds capacity.
Stream channel
Narrow pathway carved into sediment or rock by the movement of surface water.
Channel
A type of landform consisting of the outline of a path of relatively shallow and narrow body of fluid, most commonly the confine of a river, river delta or strait. The word is related to canal, and sometimes takes this form, e.g. the Hood Canal.
Islets
A very small island
Modification of Landforms of glacio-fluvial deposition
As with glacial deposits, glacio-fluvial deposits are often difficult to identify in the field. Again, repeated advance and retreat modify and alter the appearance of landforms which are also subject to weathering, erosion and colonisation by vegetation in post-glacial times. As temperatures continue to rise, further melting and retreat of glaciers results in the production of more meltwater and thus a greater expanse and accumulation of outwash material in the pro-glacial zone. Kames and eskers will be exposed in greater number and of greater length during this continued retreat.
Modification of Landforms of glacio-fluvial deposition located example
Sòlheimajökull is presently retreating about 100 m/year, and within a couple of decades (given the present rate of retreat continuing) pro-glacial lake will probably form at its snout. As temperatures increase, so does the growing season for vegetation. Exposed outwash material tends to become colonised over time, first by mosses and lichens and then by grasses, flowering plants and shrubs.
Climate change and the effects on geomorphic processes
It is difficult to establish the precise climatic conditions in southern Britain during the last glacial period. However, evidence from contemporary periglacial environments suggests that mean annual temperatures would have been less than -6ºC with winter minimums much lower and only 10-30 days of summer temperatures above 0ºC.
Freeze-thaw weathering (Climate change and the effects on geomorphic processes)
Freeze-thaw weathering is a dominant process in periglacial environments. This is due to seasonal fluctuations in temperature around freezing. In truly glacial climates (e.g. Antarctica) temperatures are invariably below freezing throughout the year.
Frost heave (Climate change and the effects on geomorphic processes)
Frost heave is a sub-surface process that leads to a vertical sorting of material in the active layer. Stones within fine material heat up and cool down faster than their surroundings as they have a lower specific heat capacity.
As temperatures fall, water beneath the stones freezes and expands, pushing the stones upwards to the surface. Ground ice also pushes overlying, finer material upwards, producing a domed surface.
Ground ice development (Climate change and the effects on geomorphic processes)
The development of ground ice is also an important process. During summer melting periods, water percolates into the sub-surface geology where it accumulates below the water table. During the sub- zero winter months this water freezes and expands by between 9 and 10% of its volume. As this expansion occurs, so the ground surface is pushed upwards, as it is unable to extend downwards into the permafrost below.
Periglacial landforms
-Patterned ground
-Pingos
Periglacial landforms definition
The landforms of periglacial areas are very varied. Periglacial landforms are a feature of current periglacial environments, but they are also fossil features, widespread in more temperate regions today (e.g. northern Britain).
Patterned ground (Periglacial landforms)
The collective term for a number of fairly small-scale features of periglacial environments. We have seen that as a result of frost-heave, large stones eventually reach the surface and that the ground surface is domed. The stones then move radially, under gravity, down each domed surface to form a network of stone polygons 1-2 m in diameter.
A particularly distinctive example of patterned ground can be seen in the area around Barrow in Alaska. On slope angles of 3-50°, the larger stones move greater distances downslope and the polygons become elongated into stone garlands. On slopes of 60° and over, the polygons lose their shape and stone stripes develop.
Pingos (Periglacial landforms)
Rounded ice-cored hills that can be as much as 90 m in height and 800 m in diameter. They grow at rates of a couple of cm/year.
They are essentially formed by ground ice which develops during the winter months as temperatures fall. There are two types that are recognised:
-Open-system pingos (Formed in valley bottoms.)
-Closed-system pingos (Developed beneath lake beds.)
Modification of Periglacial landforms
Patterned ground is a relatively minor and small periglacial period, patterned ground was often colonised by vegetation, making it hard to find and identify. Over time mass movement by creep also degrades the scale feature. As temperatures rose at the end of the frost-heaved domes, making the landform less obvious.
Pingos collapse when temperatures rise and the ice core thaws. When this happens the top of the dome collapses leaving a rampart surrounding a circular depression called an ognip. Relict ognips can be found in Britain, although due to the thawing of the permafrost only the remains of the rampart may be seen.
Modification of Periglacial landforms located example
Patterned ground around Leedon Tor, Dartmoor is now mainly covered by a layer of soil and grasses.
A good example of a Pingo, about 15 m in diameter, can be seen at Llanberis in north Wales. ain where
