glaciation Flashcards
Abrasion
After plucking large amount of rock debris is contained within the base of the glacier which continues to move downhill
This rock debris then scours, scrapes and grinds against the valley sides and floor. This greatly deepens (wears away) and smooths the underlying rock surface
As the process continues, the larger rocks create scratches in the rock surface known as striations
Plucking
Ice flowing over bump (piece of rock) on the bedrock below experience higher pressure on the uphill side of the object and this creates melting, this creates melting
This melt water flows downhill and refreezes attaching itself to the bump in the rock.
As the glacier moves onward, the rock is pulled out of the bedrock below and incorporated within the ice at the base of the glacier
Freeze thaw
Water or snow falls into existing cracks in the rock
At night, this water freezes and consequently expands by 9%. During the day, the ice thaws and contracts
The repeated freezing and thawing process “stresses” the surrounding rock (by applying and releasing pressure) until the rock eventually cracks and breaks
The loose rock then rolls down hill and can become part of a scree or talus slope at the bottom
Corrie formation (8M question)
As the climate cools, snow accumulates in the north facing hollows then when more snow falls in the winter than melts in the summer (1m)
North facing hallows are more shaded allowing this snow to lie longer and eventually compact into neve, firn and eventually ice (1m)
Under the influence of gravity, the glacier begins to move downhill, putting the ice away from the back wall, forming a bergschrund crevasse.(1m) This ‘hole’ allows melt water and rock pieces to fall to the base of the glacier, increasing abrasion (1m)
plucking steepens the back wall.(1m) Due to the pressure of the ice pushing against the rock surface, melting occurs. These melt waters refreeze into rocky outcrops, and continuous downhill movement eventually pulls the rock away from the ice.(1m)
Abrasion deepens the base of the hallow (1m). This is when the angular rocks, combined with other debris in the glacier, are then used to wear away the underlying rock (1m)
The weight of the glacier results in rotational sliding which over-deepens the hollow in the centre (1m) Friction slows the ice movement at the front edge of the corrie triggering deposition of moraine, forming a rock lip (1m)
Once the ice melts, a deep, armchair shaped hallow is left behind with steep back wall and sides and an over deepened floor (1m)
As the glacier begins to melt, meltwaters are trapped behind the rocklip forming a lochan or a tarn (1m)
eg - red tarn beck in the lack district (1m)
Arete formation (8m)
An arete is a narrow, knife edged ridge formed where two or more corries or glacial troughs, are forming back to back or side by side.
As the climate cools, snow falls and collects in north facing hallows or valleys when more snow falls in winter than melts in summer (1) This snow compacts into, neve then firn and eventually into ice before moving downhill under gravity (1) Plucking steepens the back wall of the corrie (1) - ice becomes sticky due to pressure melting/re freezing and rocks become embedded in the glacier - they are ripped away from the surface as the ice moves. (1) Abrasion deepens the floor (1) by using the morain embedded the glacier to wear away the rock like sandpaper (1)
As the backwalls of the corrie continue to erode back towards each other, they make the rock in between much narrower, while freeze-thaw sharpens the edges of the rock - this is known as an arete (1)
An example is striding edge in the lake district (1)
Drumlins formation
drumlins are elongated depositional features consisting of glacial debris normally found in a “basket of eggs” formation
as a glacier moves downhill, it becomes overloaded with sediment due to plucking
drumlins often form where a small obstacle on the ground interrupts the flow of the glacier, this acts as a trigger for the glacier to deposit material - allowing the till and boulder clay to build up around the obstacle (1) and as it flows over the top of the debris, it streamlines it (1)
The steep, “stoss” side faces upstream and the gentler “lee” side faces down the valley (1)
drumlins can be reshaped by further ice movements after being deposited (1)
Esker formation
Eskers are long meandering, steep sided ridges of sand and gravel found on till at the end of a glacier (1) They are formed either by meltwater streams flowing through, or underneath the glacier where the ice is relatively thin flowing slowly - near the terminus (1)
As the glaciers melts, rivers and meltwater runs through tunnels at the base of the glacier - these are known as sub glacier streams (1) these streams carry a load of sediment composed of glacial debris and deposit this load on the channel bed below - forming an esker (1)
Eskers are fluvioglacial features (made by meltwater), so the material they are made form is sorted (1). The following water drops the larger, heavier stones first (to make up the base) and the lighter, smaller debris is dropped second (on top) (1)
The deposits have been carried by water, so rough edges are eroded by attrition (smoothed out) meaning the material inside an esker is rounded (1)
Depositional features
esker
kettle hole
durmlins
eerosional features
U-shaped valley hanging valley aretes corrie pyramidal peak
Formation of Roches Moutonnees
A roche moutonnees is formed when a glacier meets with a more resistant rock type, for example Granite. The glacier is forced up and over the rock, increasing the pressure within the ice.
This increased pressure causes the ice to meet its’ “pressure melting point” and causes the glacier to release some meltwater at the base of the glacier – this allows the base of the glacier to slide more than normal. Abrasion takes place more powerfully than normal here.
On the lee side of the slope, the pressure drops. The meltwater now runs into the space between the rock and the glacier and refreezes (due to lack of pressure). Some freeze-thaw may occur on the lee side of the slope, weakening the rock (makes plucking more effective).
Now that the glacier has frozen onto the rock again, it will pluck at the lee side of the slope as it continues to move downhill, leaving behind a jagged, rough lee surface.
Understanding the stoss and lee formation of roches moutonnees helps us to understand the direction a glacier flowed in…
Formation of Crag and Tail
A crag is formed from a resistant piece of bedrock (possibly granite or a volcanic plug) and the tail of the feature is made of less resistant rock and often deposited moraine.
As the glacier approaches the resistant piece of rock, it erodes the landscape surrounding it, but is deflected left and right of the resistant rock.
There is a much thinner piece of ice left to flow over the top of the ‘crag’ and hence the ‘crag’ is not eroded as much as the surrounding rocks and is left sticking up (largely because the rocks contained within the ice used for abrasion have less weight to press them down into the crag below).
As the glacier moves past the crag, the upwards motion of the ice allows some deposition of moraine into cavities (gaps) on the leeward side of the crag – this forms the tail.
E.G: Edinburgh castle is the crag and the royal mile is the tail
Formation of an Esker 6m
Eskers are long, meandering, steep sided ridges of sand and gravel found on till at the end of a glacier (1).
They are formed either by meltwater streams flowing through, or underneath the glacier where the ice is relatively thin flowing slowly – near the terminus. (1)
As the glacier melts, rivers of meltwater runs through tunnels at the base of the glacier – these are known as sub-glacial streams (1).
These streams carry a load of sediment composed of glacial debris and deposit this load on the channel bed below – forming an esker (1).
Eskers are fluvioglacial features (made by meltwater), so the material they are made from is sorted (1). The flowing water drops the larger, heavier stones first (to make up the base) and the lighter, smaller debris is dropped second (on top) (1).
The deposits have been carried by water, so rough edges are eroded by attrition (smoothed out), meaning the material inside an esker is rounded (1)
