Glaciation OCR alevel

Snout

The end area of the glacier

Terminus

The end of the glacier

Ablation

The loss of mass from a glacier (often due to melting or evaporation)

Accumulation

The increase/gain of ice into a glacier, primarily through precipitation

Glacial budget/mass balance

The difference between the amount of accumulation and ablation in a glacier in a one year period

Accumulation Zone

The upper reaches of the glacier, where accumulation exceeds ablation and majority of inputs occur

Ablation zone

The lower areas of the glacier, where ablation exceeds accumulation and majority of outputs occur

Equillibrium zone

The point in a glacier where accumulation is equal to ablation

System

a set of interrelated objects comprising of stores and flows - forming a working unit.

Inputs of a glacial system (material)

1) precipitation

2) sediment via deposition, weathering and mass movement

3) avalanches

Inputs of a glacial system (energy)

1. Gravitational potential energy

2. Solar radiation energy (Thermal energy)

3. Kinetic energy

4. Geothermal energy

Stores and flows (simplified) of a glacial system

• Snow

• Meltwater

• Accumulated debris

• ice movement (e.g internal defamation and basal sliding)

• water movement

• movement of debris via gravity

Outputs of a glacial system

Debris

Meltwater

Water vapour

heat energy

Dynamic equilibrium

The state achieved in a glacier when inputs are equal to outputs - and when disturbed, the glacier regulates itself to restore mass balance. - an example of a negative feedback loop

Firn line

The zone that seperates bare ice from snow at the end of the ablation season

How does precipitation impact mass balance of a glacier?

The greater the seasonal variation in precipitation, the more varied the mass balance will be

How does wind impact glacial landscapes

• able to carry out erosion, weathering and move material

• Shapes glacial landscapes

lithology

The physical and chemical composition of rocks.

How does lithology impact glacial landscapes

Rocks with weaker lithology (eg clay) are less resistant to erosion; weathering as the bonds between the particles in the rock are weaker

stronger lithology rocks (basalt) are resistant - likely to form landforms eg aretes and pyramidal peaks

Structure of rocks

The property of the individual rock - jointing, bedding and faulting

Also includes permeability of rock - pores in rock that can absorb and store water (primary permeability)

How can structure of rocks influence glacial landscapes

Structure includes the angle of dip on the rocks - influencing the valley side profiles

How do latitude and altitude influence glaciers

High latitude glaciated landscapes (eg Antarctica) have cold, dry climates with minimal precipitation - but also minimal temp. variation (staying below 0) - keeping ice sheets thick

High altitude landscapes have more seasonal variation in precipitation & temperature and therefore variation in mass balance, more meltwater.

Impact of aspect on a glacier

For glaciers in the northern hemisphere, if they are south facing they will recieve more sunlight, more solar radiation and so more melting and are generally smaller. Mass balance will most likely be negative in these areas - retreating.

Glacial retreat

The glacier loosing more mass than it gains (negative mass balance)

Glacial advance

The glacier gaining more mass than it looses (positive mass balance) - often have more erosive power.

Pressure melting point (PMP)

The temperature at which ice melts at a given pressure - above ground, 0C but at higher pressure it decreases.

Characteristics of a warm/based glacier (temperate glacier)

• High altitude

• Steep relief

• Basal temperatures above/at PMP

• Rapid rates of movement (20-200m/year)

• Varied mass balance year round

Characteristics of a cold based glacier (polar glacier)

• High latitude

• Low relief

• Basal temperatures below PMP - frozen to bedrock

• Very slow rates of yearly movement - a couple of metres/year

• Minimal seasonal variation

Why is movement limited in polar glaciers/why is there lots of movement in temperate?

Ice at PMP deforms more easily than ice below it (if PMP is at the base/PMP is the coldest), then it can produce meltwater and move freely. Polar glaciers have temperatures lower than PMP and so movement is limited.

Ice sheets…

Are the largest accumulations of ice, extending for more than 50,000km. They posses 96% of the worlds ice, even though only two exist. At its thickest, it is over 4700m deep in Antarctica.

Valley glaciers…

Much smaller - as they are confined to valley sides. They follow the course of existing river valleys/corridors of low ground. Usually, 10 to 30km in length

Formation of glacial ice (step one)

1. Precipitation - Snowfall in accumulation zones

Snow begins to fall and settle in cold, high altitude areas - especially in north facing areas/ shaded hollows

Formation of glacial ice (step two)

2. Nivation occurs, helping to hollow out the landscape and prepares it for long-term snow buildup

Nivation

A combination of freeze-thaw, solifluction and chemical weathering occurring underneath snow patches

Formation of glacial ice (step 3)

3. over time, more layers of snow landing on top and each new fall compresses & compacts the layer beneath - expelling oxygen. Snow that has survived at least one summer is known as firn - and is an inbetween phase of snow and ice

Formation of glacial ice (step 4)

4. This process continues, with further compaction from subsequent years of snowfall expelling more oxygen. Eventually, it becomes glacial ice - with a higher density than snow (0.91g/cm3). This process is known as diagensis.

• This usually takes anywhere from 30/40 to 1000 years to take place

• True glacial ice is found at a depth of about 100m, and is characterised by a blue colour

Diagenisis

The process by which loose snow is compacted and transformed into firn, and eventually into solid glacial ice caused by immense pressure & de oxidation of snow.

Factors that influence the movement of glaciers:

• Gravity - the fundamental cause of movement in an ice mass

• Gradient - the steeper the gradient, the faster it will move if other factors are excluded

• The thickness of the ice, as this influences basal temperature and PMP

• Internal temperatures of ice

• Glacial budget - a positive one (net accumulation causes the glacier to advance)

How does the thickness of ice influence basal temperatures & glacial movement?

• The greater the thickness, the higher the pressure

• The higher the pressure is, PMP can be lowered, and so lowering the MP of the ice

• This means that meltwater can form at the base even if temp. are lower than 0 degrees

• More meltwater allows for more basal sliding - glacial movement.

Two types of glacial movement:

1. Basal sliding (temperate glaciers)

2. Internal deformation (polar glaciers)

What is basal sliding (simplified)

Due to PMP, ice at the base of the glacier in temperate glaciers is above melting point - and so a thin layer of meltwater lubricates the valley floor, allowing for easy glacial movement.

Slippage

When ice slides over the valley floor, meltwater reduces friction. Additionally, the friction creates from the ice/debris coming into contact with the valley floor (bedrock) is transferred into thermal energy - heating ice and creating more meltwater

What is Regelation slip/creep?

When ice deforms under immense pressure, due to obstructions on the valley floor (EG protruding dyke, roche moutonne) and allows ice to spread over the object before refreezing

Example of a warm based glacier

Franz Joseph, New Zealand. It moves approximately 300m per year. On average, basal sliding accounts for 60% of movement in a glacier

Example of a polar glacier

Merserve glacier, Antarctica. Moves 3 to 4m per year, and its movement is 100% internal deformation.

What is intergranular flow (internal deformation)

Individual ice crystals reorientate themselves and begin to move in relation to eachother, slowly moving the glacier along.

Why do polar glaciers use internal deformation?

Due to basal temperatures being below pressure melting point - no meltwater at the base to lubricate the glacier.

What is laminar flow? (internal deformation)

Layers within the ice moving over each other, creating a slow and smooth flow of movement

How do crevasses form?

When ice moves over a steep slope, it is unable to deform fast enough and so it fractures, leaving behind crevasses

What is extending flow?

When the valley slope gets steeper, the glacier accelerates as it moves downhill - however, it is unable to deform fast enough - and so the ice fractures, leaving crevasses and sometimes seracs (ice blocks or step faults). It is common in the upper ablation zone

What is compressing flow?

When a valley flattens out after a steep section/the gradient is reduced, compressing flow occurs as the ice thickens and the following ice pushes over the slower moving, leading ice.

How do glacial landscapes develop?

Due to a variety of interconnected geomorphic proccesses

Physical weathering

The in situ breakdown of rock, without any chemical change.

Freeze - thaw weathering

• a type of physical weathering

Water enters cracks/joints in rock - expands by 10% as it freezes. This exerts pressure onto the rock and causes it to split into pieces/break off (more effictive/common in areas which have fluctuations below and above 0.

What are the other two types of physical weathering?

Frost shattering and pressure release

What does freeze-thaw weathering produce?

scree

Chemical weathering?

The breakdown and decay of rocks due to chemical reactions, altering the mineral composition of the rock.

Example of chemical weathering?

Oxidation

Biological weathering

The breakdown of rocks by living organism either through physical growth or chemical proccesses

Erosion

The wearing away of material by (glacial)/mass movement

Mass movement

Occurs when the forces acting on the slope material (Mainly RF of gravity) exceed the forces keeping material on the slope (predominantly friction).

The two types of mass movement

1. Rock fall (when the angle of a slope exceeds 40 degrees) - when rocks become detached, often from weathering and fall under gravity. Sometimes material accumulates into a scree slope

2. Slides. Common in weaker rock, as the valley sides are steepened or undercut by erosion. Rotational slides are known as slumps.

Plucking

Meltwater seeps into joints of the rock (on the valley floor or sides).

It freezes, and becomes attached to the glacier.

As the glacier advances, it pulls pieces of rock away. It is more effective at the base of the glacier, as the weight of the ice may produce meltwater. When rock is highly jointed, it will also be more effective.

Abrasion

As glacier moves over surface, sediment embedded within wears away at the surface rocks. Coarse material will scrape & scratch rock, smoother material will polish it. The glacial debris is worn away by this, forming a fine rock flour - giving glacial streams a milky white appearance

Rates of abrasion are influenced by:

• Presence of glacial debris

• Size/shape of debris (angular is most effective)

• Hardness of bedrock & debris

• Ice thickness

Where does the sediment that ice carries originate?

• Rockfall (weathered debris falls onto glacier)

• Avalanches

• Aeolian deposits (fine material carried by wind)

• Volcanic eruptions (source of ash and dust)

• Plucking

• Abrasion

Supraglacial debris:

Material carried on the surface of the glacier

Englacial debris

debris within the ice - often used to be supraglacial, and sunk further in.

Subglacial debris

Debris embedded in the base of the glacier, either derived from abrasion or travelled down the glacier

Till

Unsorted, unstratified, angular sediment deposited directly by the ice

Outwash

Material deposited by meltwater (also known as glacial-fluvo material)

What percent of the earths surface do glacial deposits cover?

8%

Lodgment till

deposited as the glacier advances

Ablation till

Deposited as the glacier retreats (loses mass).

Ablation till landforms:

• Moraines

• Erratics

• Till sheets

Lodgement till landforms

• Drumlins.

What is a corrie

An armchair shaped rock hollow in a glacial landscape. They have a steep backwall, tarn and a corrie lip (signifying the point of terminal movement)

Corrie formation:

• Development begins by nivation in rock hollows

• Over time, hollows enlarge and collect more snow & diagenesis occurs, and is compressed into glacial ice.

• Freeze thaw also occurs at this point as water enters joints in the backwall

• Ice acquires rotational movement under its own weight

• Rotational movement causes plucking of the back wall, steepening it

• Rotational movement also pulls the ice away from the backwall, opening a bergschund crevasse

• Debris derived from plucking and other weathering (scree) falls into the crevasse.

• This abrades the hollow, deepening it. Once the hollow has deepened, the thinner ice at the front is unable to erode so rapidly and a higher lip is left.

• The lip may also consist of moraine deposited by the ice as it moves out the corrie

• After ice has melted, the Corrie becomes filled with water forming a corrie lake or tarn

What is an arête?

A narrow, steep-sided ridge found between two corries. Described as ‘knife-edged’. Formed from headward erosion and retreat of two back to back corries.

What are pyramidal peaks?

When three or more corries develop around around a hill or mountain top & their back walls retreat, the remaining mass will be steepened to form a pyramidal peak.

How do U-shaped valleys form?

As glaciers flow down pre-existing valleys, they erode the sides and floor of the valley as the mass of ice has more erosive power than the river that previously ran through & so can make new landforms. There are variations in the long profiles of them, as where there is compression they are over-deepend to form rock steps and basins (more apparent where there is alternating bands of hard and soft rock)

What are hanging valleys?

Smaller, higher up valleys visible in glacial troughs. They have their own glacier - which is smaller, and doesnt reach the main U-shaped valley.

What is a ribbon lake?

Vertical erosion leaves a depression refilled with water as the glacier retreats - showing the path of glacial movement

How do roche moutonnee form?

As a glacier moves downslope, it passes over protruding areas of rock. The up side (stoss) of these ‘dykes’ is abraded by glacial debris, and smoothed over. This results in a gentle slope, with striations. Temperature decreases as pressure decreases over the down (lee) side, and meltwater freezes to it - plucking away and making it rough.

Example of a roche moutonne

Maes Caradoc in the nant ffrancon valley (5m tall)

What are striations

Lines or grooves on the surface of a rock formed by abrasion, as the glacier moves over it and debris carves them out.

Features of a U-shaped valley:

• misfit streams

• hanging valleys

• truncated spurs

• ribbon lakes

What are moraines?

Ridges of unsorted, unstratified angular till forming a mound

What are the three types of moraine?

• Terminal

• Lateral

• Reccesional

What is a terminal moraine?

A ridge of till extending across a glacial trough. They mark the position of maximum glacial advance. They tend to be crescent shaped, and steeper on the up side.

What is a lateral moraine?

A ridge of till running along the edge of a glacial valley. Material accumulates on top of the glacier after being weathered from the sides. As the glacier melts, material sinks through the ice and is deposited. W

What are reccesional moraines?

They run transversly across glacial troughs, and are paralel to eachother. They are found further up in the valley than a terminal moraine. They form during a pause in glacial retreat, and so aren’t usually very big (don’t exceed 100m)

What is an erratic?

An individual rock, deposited by the glacier different in composition from surrounding bedrock.

How are fluvo-glacial landforms developed?

By meltwater from glaciers

Outwash

Sediment deposited by glacio-fluvial streams. Outwash is:

• Smooth and rounded

• Sorted

• Smaller than till

• Stratified

What are outwash plains?

Flat expanses of deposits from meltwater. the channels that develop in outwash plains are known as braided streams

What are geomorphic procceses?

The physical and chemical interactions between the Earth’s surface and the natural forces acting upon it to produce landforms. 

What are eskers?

Sinuous (lots of curves) ridges of sand and gravel, parallel to ice movement. Sediment is carried in englacial or subglacial streams

What are kames?

Rounded hills of deposited material from either sediment deposited in a proglacial lake, or from sediment deposited into pools with stagnant ice.