P2 Flashcards
Glacial Weathering and Erosion:
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Physical weathering processes dominate e.g. freeze-thaw.
Líttle biological or chemical weathering is evident in glacial environments as both work best at high temperatures.
Processes of Glacial Erosion:
Abrasion:
Plucking:
Rotational Movement:
Abrasion:
- angular material is embedded in the glacier as it rubs against the valley sides and floor, gradually wearing it away.
- The scratching and scraping action may leave striations (elongated grooves) as well as generally smooth, gently sloping landforms.
Plucking:
- occurs where the ice freezes onto rock outcrops, after which ice movement pulls away masses of rock.
- The pressure of overlying ice generated by frictional contact may cause partial melting of ice on the upstream side of obstructions, and then the removal of pressure on the downside causes regelation (refreezing) helping attach material.
- It is generally previously loosened material that is removed.
- Jagged-features landscapes are formed.
Rotational Movement:
is the downhill movement of ice pivoting around a central point of rotation.
Corries are created by this rotational scouring of depressions and this process is most effective where temperatures fluctuate around OC (as with plucking) to allow freeze thaw to operate, particularly in areas of jointed rocks where weaknesses may be exploited.
Corries:
Description:
- Armchair Shaped Hollow
- Found in glaciated upland areas
- Steep back wall - can lead to Arête or Pyramidal peak formation
- Over deepened basin with rock lip made of moraine
- Often contains a small lake called a Tarn
- Usually faces between North and East in the Northern hemisphere
- Evidence of frost shattering on back wall in the form of scree.
- May be striations too
- Examples are Easedale tarn at Easedale or Red Tarn at Hellvellyn
Corries:
Formation:
Pre-glacially:
- Climate worsens and becomes subarctic, below OC with constant frost and heavy snow.
- Snow collects in hollows of NE facing slopes. The depth of snow increases over winter.
- In summer it melts and meltwater seeps into ‘nooks and crannies’ leading to freeze thaw action. The rubble created is removed by solifluction or meltwater streams.
- This process is repeated many times and the hollow soon deepens. (process is nivation)
Corries
- Corries form when snow continues to build up in a depression or nivation hollow, eventually compacting to form a glacier.
- The glacier becomes trapped within the hollow, meaning the only way it can move is through rotational slip.
- The back wall is eroded through plucking and frost shattering, and the hollow is deepened through rotational abrasion.
- Water can fill corries to make tarns (lakes).
- As corries are eroded rocks, they last a long time and are minimally affected by erosion.
- This is why corries have lasted thousands of years.
Corries: formations Postglacially:
Ice melts leaving a hollow containing a small lake.
• There is still evidence of winter freeze-thaw from the scree on the back wall.
There may be fluvial erosion due to the meltwater streams flowing from the tarn.
Arêtes:
A knife-edged ridge formed between two corries (when the two steep back walls meet). If three meet, they create a point called a pyramidal peak.
Similar to corries, arêtes last a long time.
* E.G. Striding edge above Red Tarn in the Lake District (850m)
* E.G. the Minarets - Sierra Nevada, California (3735m high, 338m prominence)
Pyramidal Peaks:
• Where three or more corries erode back toward each other or 3-4 arêtes radiating from a central point.
• A very steep, sharp mountain peak.
It often has near-vertical sides.
• E.G the Matterhorn in Zermatt, Italy/Switzerland. (4478m)
• E.G. Mont Blanc in Chamonix, Eastern France (4810m)
Glacial Troughs:
Description:
- A u-shaped valley formed by a glacier bulldozing and eroding through a river (v-shaped) valley.
- The glacier has enough force to erode away a river’s interlocking spurs.
- This leaves smooth but steep truncated spurs on the valley sides and a wide, flat valley floor.
- The river that originally flowed through the valley will continue to flow, giving it the name misfit stream due to its small size in comparison to the surroundings.
- U shaped valleys last for a long amount of time.
- e.g nant ffrancon
Examples of Glacial Troughs:
Nant Ffrancon in Wales
In long profile, there is a basin and step formation;
The basin is made up of less resistant rock
• The step is more resistant rock
where Ice came from:of Glacial Troughs:
- A highland field - tongues of ice spilled out down valleys
- Corries - ice flowed over the lip down steep sides into river valleys
- Ice accumulated at the head of the valley itself and steepened the back wall forming a trough end
- Ice action steepened, widened and straightened pre-existing river valleys changing long and cross profiles.
Cross profile of glacial troughs
- the valley is overdeepened by the sheer mass of ice and erosion on base.
- It is straightened by removal of interlocking spurs.
- The ice thickness and velocity are greatest over the central part of the valley floor so erosion is greatest and deepest in the center.
Long profile: of glacial troughs
- A distinct trough end due to accumulation of ice at head of valley
- Irregular long profile due to extending and compressing flow
- Basins on the valley floor associated with greater erosion caused by compressing flow where a tributary glacier provides additional ice so the enlarged glacier can achieve greater downcutting (or an area deeply weathered prior to glaciations, or a band or less resistant rock or due to constriction of valley walls)
Often find a ribbon lake in the basin - glacial flow is compressional, the ice gains a rotational movement causing enhanced abrasion and deepening of the rock basin.
- Steps on the valley floor marked by the position of a more resistant band are the zone of extending flow - the ice is stretched, thins and so erodes less.
erosional processes of Glacial Troughs:
Plucking -
- ice loosens, picks up and removes masses of rock varying in size.
- Most effective in areas with well jointed rocks or permeable rocks where water produced by pressure melting percolates into cracks in the bedrock then freezes and shatters the rock.
Abrasion -
- using the material entrained in the ice from plucking, the glacier will smooth the truncated spurs and will overdeepen the valley floor.
Pressure Release -
- when a certain thickness of the bedrock is removed it is replaced by ice which is 1/3 the density of rock and so causes the uppermost layers of the rock to separate along the sheet joints.
- This weakening in the upper bedrock allows other erosive processes to operate rapidly.
Roche Moutonnees:
- A mound of rock shaped by a glacier flowing over it and eroding it.
- The glacier would be moving right (stoss side) to left (lee side) in the picture.
- The glacier hits an obstacle that is too large and hard to pluck, it must move over it.
- The glacier hitting the obstacle increases friction and pressure, therefore increasing
melting as the lower ice can reach the pressure melting point. - This meltwater allows the glacier to slide over the rock, and smaller rocks will abrade the stoss side
- When the glacier reaches the top of the obstacle, friction and pressure drop ●
- Meltwater refreezes
- Frozen rocks are plucked from the lee side
- These landforms last a long time as they are made of rock.
- E.g. Cairngorms in Scotland
Crag and Tail:
- Consists of a larger mass of resistant rock or crag and gently sloping tail of less resistant rock and/or sediment on one side.
- E.G. in Edinburgh. Castle sits on hard basaltic rock, the Royal Mile runs down softer sedimentary rocks.
Striations:
When glaciers move across exposures of rock, angular debris embedded within the ice may leave parallel scratches or grooves called striations.
Glacial Deposition:
Subdivided into:
- Till - all material deposited directly by the ice, largely unsorted in nature.
- Fluvioglacial material - sediments deposited by meltwater streams. These usually have been sorted with coarse material nearer the original glacier snout and finer particles carried further away by meltwaters.
Till (boulder clay):
• Unsorted mixture of rocks, clays and sands.
• Once carried as supraglacial debris and later deposited to form moraines, it was deposited during ice movement or glacial retreat.
• There is little rounding of debris and it tends to remain subangular in form.
Erratics:
- Fragments of glacial debris which range in size from pebbles to large boulders.
- They have been carried by glacier ice before being deposited.
- E.G. Big Rock in Alberta, Canada (16500 tonnes).
- They are said to be ex situ.
- They are usually distinguishable by their lithology - they are likely to be of a different rock type from the underlying rock and by their attitude, they do not lie in the same manner as the local strata.