Sunday, 12 June 2011

Glaciers - A summary

I think I am in danger of making quite an interesting topic sound quite boring and so, for the moment, I think I will leave it here. I thought I would do a little summary of the basics what I have (hoped to have!) covered so far.........

Glaciers are an open system with inputs, outputs, processes and stores.
  • INPUTS = Precipitation (primarily snow), Avalanches and the Zone of Accumulation
  • STORES = Ice
  • PROCESSES = Plucking, Abrasion, Flows (all the different types), Ablation, Accumulation and Freeze-thaw
  • OUTPUTS = Meltwate, Moraine, Zone of Ablation
Glaciers originate from heavy, prolonged snowfall and are made when loose snow, that settles, becomes more and more dense.
  1. Loose snow settles
  2. As it becomes more dense it turns into Firn (which is also known as Neve) and then further alterations in weather (as seasons change etc) provoke freezing and thawing which converts the loose snow into icy granules
  3. As more snow falls, it get more and more compacted. This creates pressure between the individual granules and therefore initates pressure melting
  4. Eventually this turns into a dull, white, structureless mass that is less permeable than fresh snow
  5. Air is squeezed out of the ice particels as they are forced to fuse together after further compression by the extra weight of additional ice and snow. This process is known as sintering.
  6. The resulting glacier takes a blue tinge and contain very few air spaces, making it impermeable to water
Snowflakes -----> Granular snow -----> Firn (or Neve) -----> Glacier ice

Glaciers grow and retreat......
  • When the rate of accumulation is greater than that of ablation the glacier advances = positive regime
  • When the rate of accumulation is less than that of ablation the glacier retreats = negative regime
Glaciers can be warm or cold.......
  • In cold glaciers, the ice is very hard and frozen, right to the bedrock. This is because temperatures stay below freezing point all year round. The absence of meltwater greatly restricts movement and the erosive ability of the glacier.
  • In warm glaciers, during summer, some of the ice melts and, via crevasses, finds its way to the base of the glacier where, the meltwater, acts as a lubricant and thereby allows the glacier to move freely. This encouragement of movement greatly increases the erosive ability of the glacier.  
Freeze-thaw is a really important process of weathering in glaciers.......

  •  Produces jagged, rough rocks
  • During the day water enters cracks in the rocks. Overnight, as the temperature drops, the water freezes. As it freezes it expands by 9% thus placing pressure on the rock which, over time, causes the rock to crack and split. This is a form of block disintergration.
  • Freeze-thaw leaves the rocks in an ideal condition for plucking
Abrasion is another dominant erosive process.....
  • Produces a smooth surface as it acts a bit like sandpaper
  • Glaciers carry a large amount of moraine and some of these sharp boulders are embedded in the bottom of the glacier and act as erosive agents. These rocks scour the valley floor and leave behind grooves which are known as striations
  • Like freeze-thaw, it leaves behind sharp, jagged rocks
  • It is considered to be the main erosive process in operation in glaciers
  • As the glacier moves along the valley the ice melts slightly around large boulders, and other obstacles, before refreezing around them. As it then advances further the boulders are literally ripped out of the ground. From this point onwards, the boulders ofen act as agents of abrasion.
From this, it is clear to see that despite the forms of erosin being seperate and slightly different to each other they are all interlinked and help to increase the rate at which each one occurs.

There are three main ways in which glacial debris is transported......

- On the surface ---> supra-glacial
Derived from forst shattering of peaks above and lateral moraines meeting after tributary glaciers join.

- Within the ice ---> englacial
Debris falls down through cracks and crevasses in the ice.

- At the base ---> subglacial
This is a mixture of material scarped up from below the glacier and that which has made its way downwards through crevasses, with or without the help of meltwater streams within ice.

Glacial sediment is classified according to its mode of deposition but the collective name for all the sediment and debris deposited under glacial conditions is Glacial Drift. Sediment that is depostied by melting ice or glacial streams is known as Fluvio-Glacial whereas that deposited directly by the glacier, such as moraine and intra-glacial debris dropped in site by retreating ice, is known as Glacial Till.

Well, that it most of what I would consider to be the real basics of glaciers. There is still so much more surrounding this topic and so I think I will pick it back up during the summer holdiays as I have yet to cover topics including ice sheets and their processes and landforms (which will also include lots more on depositional landforms which can also be found in valley glaciers but the size and extent of them are far greater from ice sheets), the effects of meltwater on landscapes, periglacial processes and the resulting landforms and a bit about permafrost.

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