Weathering is the decomposition of rocks, soils and their minerals through direct contact with the Earth’s atmosphere. Weathering occurs in situ, or “with no movement”, and this is how it differs from erosion, which involves the movement and disintegration of rocks and minerals by agents such as water, ice, wind and gravity. There are two important classifications of weathering processes; physical weathering involves the breakdown of rocks and soils through direct contact with elements heat, water, ice and pressure.
However Chemical weathering, involves the direct effect of atmospheric chemicals, or biologically produced chemicals, in the breakdown of rocks, soils and minerals. There are also two different types of erosion, classified into the same two categories; mechanical erosion and chemical erosion. Each of these has a different effect on the environment. Mechanical erosion would include water, wind, sun, ice, natural disasters such as earthquakes and shoreline erosion. Chemical erosion would be acid rain, over use of fertilizer, human land use, deforestation and overgrazing.
With regards to glacial regions weathering processes, such as Dilatation and Freeze-Thaw can provide the debris, or rock weaknesses that are required for glacial erosion to occur successfully. Weathering can also occur after glacial erosion, possibly in de-glaciated areas, where the processes decompose and wear away the landscape and features left behind after glaciation. Glacial erosion is the processes whereby the glacial ice itself alters the underlying bedrock beneath the glacier. The two main processes of glacial erosion are Abrasion and Plucking. Examine the impact of glacial erosion on the evolution of landscapes:
Many regions today have been shaped by glacial erosion, whether or not they are currently experiencing glaciation, or whether they are post-glacial regions. Erosional processes occur as glaciers move forward; advance downhill due to gravity. This movement and subsequent erosion can have impacts on a wide variety of scales, and varying implications can be seen over short and long periods of time, or repeated cycles of glaciations. This essay will attempt to describe and explain the processes of glacial erosion and the significance of the impacts on landscapes and their development.
Examples will be drawn upon with reference to areas currently undergoing glaciations as well as areas that were once, but are no longer glacial regions. There are two main types of glacial erosion; Plucking and Abrasion. Plucking occurs when sub-glacial melt-water enters weaknesses in the rock, mostly on the valley floors, and the water then re-freezes and attaches itself to the ice above (on the actual glacier). Ice continues moving downwards due to gravity and pulls along the attached melt-water, and thus the rock too.
The melt-water is initially produced due to the temperature reaching the pressure melting point of the ice, but after a short period of time falling again to cause the re-freezing of the water. Due to the particular conditions and temperature changes required for this to happen this process occurs in very specific areas. Abrasion is where the ice, or glacier, wears the rock away gradually, similar to the effect of sand paper on wood. As well as the ice itself eroding the bedrock, the debris within the basal layer of the glacier also erodes.
Evidence for this process includes the polished appearance and texture of the rock in post-glacial regions. These processes are dependent upon various factors. For example the geology of the underlying rock; the less resistant the rock type the greater the rate of erosion and therefore the greater the impact on the landscape. The rate or success of plucking is quite heavily influenced by the structure, or amount of jointing present in the rock, as greater jointing creates greater opportunity for the process to occur; thus greater rates of erosion and greater impacts on the landscape.
The size of the debris within the basal layer of the glacier determine their individual affect on the rate of erosion, larger more angular particles will tend to be more affective erosive material than smaller, smoother debris. The type of glacier also plays a role in determining what type of erosion occurs and the extent to which it does. Abrasion is most dominant in Temperate Alpine glaciers, as they provide the best conditions for basal sliding to occur, thus more movement and better situations for abrasive erosion.
However there is a critical amount of melt water that can be produced, after which hydrostatic uplift can occur, and result in no contact between the bedrock and glacier, and thus no friction and no erosion. Ice thickness therefore also plays a role, as this determines ice pressure, and thus dictates when the pressure melting point is reached, and when basal sliding occurs. Also, prior to the pressure melting point being reached, the greater the thickness of the ice the greater it’s mass, and therefore the greater the downward force onto the underlying bedrock; causing higher rates of erosion.
Another factor that influences the rate of glacial erosion is the direction of the slopes of the underlying bedrock. If the beds lie parallel to the rock surface then they will more resistant to potential erosion than if the beds lie at an angle to the surface, with the weaknesses exposed. Weathering processes can also help and play a role in the erosion of glaciers as the processes produce the debris required for abrasion to successfully occur, and creates, or enlarges pre-existing joints and weaknesses in the rock which are then exploited via plucking.
The weathering processes that occur include Freeze-Thaw, or frost shattering. This is when water seeps into joints or cracks in the rock, and as temperatures fall it freezes. As it does it expands by 9-10%, and when the ice melts again the pressure is released and the rock is left weakened further and debris falls away. This process requires diurnal and seasonal variations in temperature above and below 0i??C. Dilatation is another process that occurs when the ice (pressure) above the overlying rock is removed, usually during a period of glacial retreat, and this allows the rock to expand creating fracture on the surface.
These fractures and joints can then be exploited via plucking during a period of re-glaciation; glacial advance. The impacts on the landscape of the processes of glacial erosion vary greatly in size and scale, and occur over a variation of time periods; dependent on the cyclic nature of glaciers with seasonal advance and retreat, as well as glacial periods. The largest feature of glacial erosion is most certainly the U-shaped valley, or glacial trough. This is a smooth valley with steep sides, flat floor and a fairly straight channel.
The Great Langdale Valley in the Lake District National Park was formed via abrasion where a glacier fed into a pre-existing river valley during the last glacial period in the UK. The glacial ice scoured out, flattened and further deepened the river valley. The feature seen today is a result of long-run successive cycles, formed over 100,000s of years. The interlocking spurs that lined the Great Langdale valley prior to glacial erosion were also subject to abrasion by the ice.
This leaves steep, vertical ends of the previous features, known as truncated spurs. The smaller feeder glaciers are separated by these features, and as they are much smaller they erode less, and help the truncated spurs stand out further as their valley bases are much higher than that of the glacial trough they feed into. This leaves a Hanging Valley, a drop to the bottom of the main valley floor from the end of the tributary glacier. This feature is formed specifically due to varying rates of erosion from glacier to glacier.
Stickle Tarn is a small lake that sits in a previous corrie feeder glacier above the Great Langdale Valley, and sits over 120 metres above the main valley floor and is an excellent example of a truncated spur and hanging valley. Another way in which varying rates of erosion can create features is when there are previous Igneous Intrusions that protrude upwards into the glaciers path. This can be seen in Grand Teton Nation Park in Wyoming USA where the resistant igneous intrusions cause the glacier to move over the obstructions.
The ice is stretched moving over the first; extended flow, and then compressed moving between the two, where downward pressure is placed on the normal bedrock between, which as a result is over deepened, creating hollows. In Grand Teton National Park this has allowed the Jackson and Jenny Lakes to form. Cirque, or Corrie glaciers are another feature created from glacial erosion. These are the result of previous small feeder glaciers and can give rise to many other features that occur between and around the cirque glacier.
Corries are armchair shaped hollows found on North-facing slopes. They have very steep back walls and a scoured out over deepened basin, a lip on the front rock or moraine. They are always wider and longer than they are tall, and are usually only 100s of metre long but can be longer; the Cirque de Gavarnie in the French Alps is 2 kilometres long. The corrie feature formed just off the Hauslabjoch glacier in the Otztaler Alps in Germany was formed as a result of accumulation of snow over many years, which was then compacted, forming nevee and then ice.
At a critical mass, movement occurred in a rotational manner, subjecting the back wall to plucking and the basin to over deepening via abrasion. A crevasse can form at the back of the glacier, known as a Bergschrund, which is the deepest and largest crevasse formed when the rest of the glacier moves downwards due to gravity but the ice next to the back wall remains stuck there, thus creating a crevasse between, which is further widened through the constant movement of the ice downwards.
Another type of crevasse that can form is a Randkluft; between the glacier and the back wall itself, caused by melting of the ice due to the warmer temperature of the rock face on the back wall. Post-glaciation corries are often seen in the form of small lakes. Stickle Tarn and Red Tarn in the Lake District are small lakes at high altitudes situated in hollows formed from erosion by corrie glaciers during the last glacial period in the UK. Corrrie glaciers also give rise to the formation of other distinct features of glacial erosion.
The summit of Snowdon, known as Crib Goch in Snowdonia National Park, North Wales provides a perfect example of an Arete. A sinuous knife edged ridge found between two corries which are back to back either side of the mountain peak. The plucking of the back walls leaves a very steep sided narrow apex. In areas such as Snowdonia, which are no longer glaciated, these features can be subject to large amounts of weathering, which can further alter the appearance of the landscape, and make them appear different to the same type of feature in a currently glaciated region, such as Iceland.
The Matterhorn summit, which lies on the border between Switzerland and Italy in the Alps, is a pyramidal peak, where 3 or more corries lie back to back, creating a very thin peak. The Matterhorn has four faces, facing the four compass points, the faces are steep, and only small patches of snow and ice cling to them; regular avalanches send the snow down to accumulate on the glaciers at the base of each face. All of the back walls are retreating backwards due to glacial erosion, and thus this feature could eventually change drastically if it continues to be subject to further erosion and weathering processes.
Features of glacial erosion are also smaller in size, proving that the impacts of glacial erosion are prevalent on many scales, and over a variety of time periods, as larger features, such as glacial troughs are the result of many successive glacial cycles, but some of the smaller feature about to be discussed can occur after a number of decades of even years of glacial activity. A Roche Moutonnee, found on the Cascade Mountains near Cle Elum in Washington USA was formed due to glacial regolation; obstructions in the glacial channel (trough).
The obstructions are asymmetric in shape due to the processes they are subject to. the upslope side is much shallower and smoother due to higher pressure as the ice is forced to move over and around the obstruction, this results in the ice ‘polishing’ the rock surface. Whereas the downslope side is steep and angular as there is low pressure downslope, as this side of the obstruction is facing the opposite direction of the flow of the ice. As a result of the high pressure on the upslope side, melt water is produced, and some starts to percolate into the rock obstruction, which then allows plucking to occur on the down slope side, ripping pieces of rock off as the glacier moves downhill, creating the jagged surface.
Some of the smallest impacts of glacial erosion can be seen in the form of striations and p-form features. These are formed on a very local scale, but can still alter the appearance of the landscape significantly. The Glacial grooves in the Kelley’s Island State Park are the largest and most easily accessible remains of glacial striations in the world and are produced from the debris in the basal layers of the glacier gouging into the bedrock, creating straight parallel grooves due to abrasion. These particular striations are 120m long, 11m wide and 3m deep.
It can be said that the impacts of glacial erosion are similar across the globe with regards to the processes that occur, however the physical outcomes that result can be very different. As mentioned before, abrasion can occur at greater rates in temperate glaciers, as greater rates of basal sliding can occur. However, this excess melt water can cause hydrostatic uplift, and cause much lower rates of erosion, creating completely different landscapes, than those in regions dominated by cold based glaciers; higher latitudes.
Alaska is dominated by cold based tidewater glaciers; which consist of recurring periods of advance alternating with rapid retreat and punctuated by periods of stability. During portions of its cycle, a tidewater glacier is relatively insensitive to climate change, making it very different to the glacial climate in lower latitudes, at higher altitudes; previous periods of glaciation in the UK. It can be argued that the impacts of glacial erosion in currently glaciated regions such as Alaska; covered by the Bering glacier complex; over 5827 km2, cannot be compared to the impacts seen and studied in post glacial regions such as the UK uplands.
As with the Arctic circles, the real impacts cannot possibly be seen in the same way as those in de glaciated regions, especially in area like Greenland covered by ice sheets, where the entire landscape’s appearance in dictated by glacial erosion. The impacts in the UK only occur in upland areas, restricting the extent of the impacts, places such as the Lake District, which have also been altered since the last glacial period by severe weathering processes, fluvial erosion as well as human activity.
Therefore the impacts of glacial erosion vary immensely in scale across various areas of the world, and in many respects cannot be compared, and are very dependent on local factors that influence the rate of erosion over specific time periods, daily, seasonally, throughout a glacial period or over many cycles. However, on a very general scale, glacial troughs can be seen as the greatest impact of glacial erosion as they are the single largest feature to be produced by glacial erosion. Glaciation is a very dynamic concept, which makes the impacts even harder to assess as changes are constantly occurring.
Arguably the largest influence, on a wide scale, on changes in glacial regions, and thus the impacts of glacial erosion, is global climate change. On the one hand, in areas of high latitude, this process is having a negative effect on glaciation, and causing increases in temperatures, and overall glacial retreat. The Bering glacial complex which dominates Alaska has thinned by 100s of meters over the past century, and since 1900 the terminus of the main glacial channel has retreated over 12km. Across the Greenland Ice Sheet in Antarctica, huge changes can be seen, causing concern across the world.
Icebergs breaking off from the ice cap can be seen in huge numbers, with breakage occurring daily, at Cape York off Greenland. In the region, the area that is experiencing melting has increased 16% since 1979, and shrinkage is projected to increase in the future. A rate of melting of 239km3/year has been recorded by the US Space Station. This melting in Polar Regions in predicted to severely affect the ocean current circulation, especially in the northern Atlantic, where the Gulf Stream provides a regulator on the climate of the UK and northern Europe, keeping the climate moderate.
There is concern surrounding the vast amounts of colder, fresh water being added to the ocean due to ice melting, and predictions that this could shut down the Gulf Stream, give rise to theories that this could cause the climate in the UK to become much colder, and for it to be possible for another glacial period. This would cause glacial erosion in the UK uplands to resume and change the landscape again, showing that the impacts of glacial erosion are never stagnant, and the appearance of glacial landscapes is susceptible to change at any time, potentially even in post-glacial regions.