unit three a: processes of glacial weathering, erosion and the characteristics and the formation of associated landforms and landscapes Flashcards
what is weathering?
the breakdown of rock where its located
what are the 2 types of weathering?
- physical processes: freeze-thaw weathering
- chemical processes: carbonation
what is erosion?
the removal of rock by ice, water, wind or gravity
what are the 4 stages involved in freeze-thaw weathering?
- in glacial environments temperatures can fluctuate above and below 0 degrees
- water fills the crack, freezes and expands by 9%
- when the water freezes it exerts pressure on the crack
- repeated freeze-thaw weathering fractures the rock
what characteristics that are needed in the rock for freeze-thaw weathering to occur?
- rock has to be porous/permeable to allow water into cracks
- whether the rock is hard or soft can affect the rate at which it occurs
what type of climate is required for freeze-thaw weathering to occur?
temperature have to fluctuate above/below 0 degrees, water also must be present so precipitation is required
what is the altitude needed for freeze-thaw weathering to occur?
has to be high enough, to be cold enough for temps to drop below 0 degrees
what is the aspect needed for freeze-thaw weathering to occur?
north facing in northern hemisphere, as this again is most likely to provide temps below 0 degrees (shaded for longer periods of time)
what is dilation?
when rocks fracture under the pressure/release of pressure from glaciers. occurs when glacier is melting, as the glacier will lose weight
what is the process of dilation known as?
pressure release
what are the 4 different stages of dilation?
- glacial ice (known as the overburden) pushes down on the rock beneath, causes the bedrock beneath to become compressed
- glacier retreats/melts (overburden is eroded), reducing the pressure, causes the rock to relax upwards which creates cracks that are parallel to the surface
- pressure continues to be reduced (overburden continues to be eroded away), causes the fractures to expand, releasing pressure
- as the cracks form, material is created for erosion
how does abrasion work?
as a glacier moves, embedded bedrock at the base is scraped against bedrock, wears it away. the subglacial debris itself can be worn away and converted to rock flour
how do different sized rocks affect the bedrock?
large rock = scratches bedrock vs smaller (fine silt and sand) rock = smooths, polishes bedrock, sandpapering effect
what are the factors that affect the rate of abrasion?
- ice thickness
- subglacial meltwater
- speed of glacier movement
- supply of debris
- relative hardness of debris/rock
- shape of debris
how does ice thickness affect the rate of abrasion?
the thicker the ice, the more pressure applied (deeper scratches, eroded deeper)
how does subglacial meltwater affect the rate of abrasion?
the more meltwater = more lubrication and faster rate of movement
how does the speed of glacier movement affect the rate of abrasion?
faster the glacier moves = quicker abrasion, going to come in contact with debris
how does the supply of debris affect the rate of abrasion?
more debris = more abrasion
how does the relative hardness of debris affect the rate of abrasion?
- angular = sharp, scratches bedrock
- rounded = smooth, sandpaper affect
or sub angular/rounded
what is plucking also known as?
quarrying
how does plucking occur?
occurs when meltwater seeps into cracks, freeze - thaw weathering occurs and debris is trapped in ice
the and on the down - valley side of rock obstacles (regelation slip)
where does plucking occur?
effective at the base of glaciers due to the presence of meltwater from pressure melting
What are the six stages of plucking on a regelation slip?
1) glacier encounter obstacle as it moves downslope, increases pressure as glacier pushes against obstacle so pressure melting point is reached, base starts to melt
2) meltwater lubricates base, allows it to move over obstacle
3) meltwater flows into cracks in the bedrock below
4) once the base has moved over glacier, the pressure drops, means the base will refreeze as its no longer at pressure melting point
5) as it refreezes glacier exerts pressure on rock, eventually breaking pieces away
6) as the glacier refreezes to the bedrock beneath it, it ‘plucks’ away the broken rock as it continues to move downslope
how does sub-glacial meltwater erosion occur?
meltwater can get channeled beneath glacier, before emerging at the snout as a pro-glacial stream
how does increasing the velocity of meltwater affect the rate or subglacial erosion?
increase velocity = creates bigger meltwater channels
What are the 4 erosional processes?
- hydraulic action
- abrasion
- attrition/corrasion
- solution/corrosion
how does hydraulic action work?
water forces air to be trapped, pressured into cracks, this constant pressure eventually causes rocks to crack and break apart
how does abrasion work?
erosion where the sediment is grinded and pounded against a surface e.g. bedrock
how does corrasion (attrition) work?
the rocks and pebbles being carried by water collide against each other, weakening/breaking them down into smaller, rounded pebbles
how does corrosion (solution) work?
the chemical erosion of rocks in riverbank by the slightly acidic water - occurs in streams running through rocks such as chalk and limestone
why are subglacial streams more powerful than ordinary streams?
they have increased hydrostatic pressure.
this is because they are confined beneath the glacier so they’re under an immense amount of pressure
why do subglacial streams have different discharge patterns to normal streams?
- they’re dependent on temp fluctuations rather than precipitation patterns
- they flow fastest in the summer months, as temps are high so melting is more extreme
- flow slowest in winter, melting is vastly reduced
what are the three types of erosional landforms? (in terms of size)
> Macro = large
Meso = medium
Micro = small
what is a cirque/cwm/corrie?
it is a large amphitheater shaped hollow found at the source of a valley glacier
what are the two stages involved in cirque formation?
- development of nivation hollow
- enlargement of nivation hollow
How does a nivation hollow form?
- a north facing slope receives little insolation
- temp remains relatively cold, even during summer months (snow builds all year round), formation of firn occurs
- freeze-thaw weathering occurs, due to temp fluctuation, providing loose material
- the surface of glacier thaws in the summer, producing meltwater
- rock debris is removed by meltwater streams of solifluction (its the movement of water downslope) in summer
- process of removing rock debris and freeze thaw weathering, begins to enlarge hollow
- the ground beneath the glacier remains permanently frozen
How does a cirque form? (Stage 2)
1) further accumulation of ice adds to the weight, resulting in rotational movement of ice.
2) movement aided by meltwater, due to pressure melting point being reached
3) movement of glacial ice initiates the second stage
4) as it rotates it removes fragments from the back wall through process of plucking, this is why backwall is steep, jagged and angular
5) the fragments are transported by the rotating ice, which leads to abrasion at the base of the hollow (deepening it further)
6) rotation aided by surface meltwater, enters through a crevasse near the backwall (known as the bergshrund), this reaches from the surface to the base of the glacier
-> it forms when the ice is pulled away from the back wall of hollow due to stress as it rotates
7) as erosion enlarges the hollow, moves rotationally releasing pressure causes dilatation cracks to form in underlying rock, parallel to cirque. this weakens underlying rock, aiding further erosion
what is and how does an arete form?
a sharp “knife edge ridge” which has been produced by 2 cirques, eroding backwards towards each other
what is an example of an arete?
bristly ridge in snowdonia
what is and how does a pyramidal peak form?
where three of more corries have formed around a mountain, become sharper and steeper
what is an example of a pyramidal peak?
glyder fawr above cwm idwal
what is a glacial trough?
where glaciers flow into pre-existing river valleys
what is an example of a glacial trough/valley?
yosemite valley, california
how does a trunctated spur form?
- in a normal river valley, interlocking spurs are produced as the river erodes downwards into its bed and meanders side to side
- as the glacier moves down a river valley it erodes each spur, overtime the interlocking spurs become eroded back to become truncated spur
- often have a steep cliff face
what is and how do hanging valleys form?
frequently the smaller glacier will have its flow impaired by the larger one.
> this coupled with its smaller size, means it has less erosive potential, won’t carry out as much vertical erosion
> this results in a much smaller hanging valley compared to the much deeper main valley
what are the features of a ribbon lake?
long and narrow, finger shaped lake
where are ribbon lakes found?
in a glacial trough
how do ribbon lakes form?
- its formation begins when a glacier moves over an area containing alternate bands of hard and soft rock
- the glacier will erode the softer rock quicker (via abrasion), creating a hollow called the rock basin
- on either side of the rock basin, the more resistant rock is eroded less (these are known as rock bars), which act as a dome
- the rock bars allow rainwater to accumulate in the rock basin (when the glacier has retreated/melted)
what is an example of a ribbon lake?
windermere in the lake district
What type of glacial landforms are the following?
- cirque
- aretes and pyramidal peaks
- glacial troughs
- truncated spurs
- hanging valleys
- ribbon lakes
macro landforms
what are the features of a roche moutonee?
a piece of resistant rock on the floor of a glacial trough that has smooth up-valley side and a steep/jagged down-valley side
what is the rough size of a rochee moutonee?
they’re typically 1-5m in height and 5-20m in length
How do roche moutonees form?
- abrasion takes place on the up (stoss) side glacier, where high pressure pushes sub-glacial debris into obstacle and smooths it over
- pressure melting point occurs at high pressure as the base of the glacier which creates meltwater
- plucking takes place on the down (lee) side glacier, meltwater from pressure melting point seeps into cracks and refreezes due to pressure (ice expands by 9%, debris freeze at the base of it). this creates a jagged lee-side
what is an example of a roche moutnonee?
nant-ffrancon valley in snowdonia, north wales
what are the features of a crag and tail?
- formed when a glacier moves over a band of resistant rock surrounded by two bands of less resistant rock
- crags can reach meters above sea level
- tails can stretch from over a km
what is an example of a crag and tail?
edinburgh castle, scotland
how does a crag and tail form?
- glacial ice eroded the weak sedimentary rock of the volcanic crag, this left behind a steepstoss
- the more resistant volcanic rock isn’t eroded, therefore the weak rock on the lee-side is protected therefore less erosion occurs, creating a gentle slope
What is the crag part of the crag and tail made out of?
volcanic activity 325 million years ago created outcrops of volcanic rock, which are more resistant than surrounding sedimentary rock
what type of glacial landforms are the following:
- roche moutonee
- crag and tail
meso landforms
what are striations?
they are scratches on the hard bedrock produced by the process of glacial abrasion
how do striations form?
debris frozen in the basal ice is dragged across the bedrock to produce them, and therefore give a clear indication of the direction the ice was moving
what are chatter marks?
these are small intermittent chips/fractures/ gouges in the bedrock
how do chatter marks form?
formed when the basal debris within the ice is not in continuous contact with the bedrock
how does polished rock form?
ice can polish if sand and silt is found in basal ice layer, opposed to angular rock fragments
