Glaciers

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Glacials and Ice Ages

  • In the last 800,000 years, 10-15 glacials have occurred.
  • The average global temperature only needs to decrease about 2 for an ice age to occur.
  • The last ice age was the Pleistocene Epoch. It began about 2 million years ago and ended around 9600-9700BC. The maximum extent of glaciation occurred about 18,000 years ago.

Glacial period: A period of ice advance associated with falling temperatures.

Interglacial: A period of ice retreat associated with rising temperatures.

Ice Sheet: A large body of ice over 50,000km2.

Ice Cap: A smaller body of ice usually found in mountainous regions.

Glacier: A finger of ice usually extending downhill from an ice cap and occupying a valley. 

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Glacier Movements and its Causes

Accumulation: Inputs to the glacier budget, e.g. snowfall and avalanches.

Ablation: Outputs from the glacier budget, e.g. melting.

Snout: The front of a glacier.

Glacier Budget: The balance between the inputs and outputs of the glacier budget.

  • If the inputs to the glacier are greater than the outputs, then the glacier advances and vice versa. Ablation is greatest at the snout as the air temperature is highest there.
  • The weight of the ice increases the pressure at the base of the glacier. This increase the warmth at the base of the glacier, increasing melting rates, meaning that melt water becomes present. This provides lubrication, allowing the glacier to slide over the surface.
  • Gravity causes the movement of the glacier to be downhill. 

Basal Sliding: Meltwater acts as a lubricant, allowing glaciers to slide down surfaces.

Internal Deformation: The weight of the ice causes the ice crystals to change shape under pressure. The crystals act like plastic and ‘flow’.

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Glacier Processes

1.    Glaciers begin at the top of mountains, where it snows all year round and the temperatures are always below freezing.

2.    The glacier is so heavy that it begins to move slowly down the mountain.

3.    The glacial ice builds up over many years until the ice is so thick that the bottom layer melts a little and mixes with the dirt and soil. This forms a slick layer which makes it easy for the glacier to move as gravity pulls it downwards.

4.    As the glacier travels down the mountain, new snow falls to replace the glacier at the top of the mountain.

5.    As the glacier moves, it scrapes along the sides and bottom of the valley, picking up boulders and sediments and transporting them down the mountain.

6.    Valley glaciers cause a lot of damage to the land they pass over, making many new landforms.

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Freeze-thaw Weathering

1.    Meltwater runs into the cracks in a solid cliff.

2.    The Meltwater freezes, expands and forces the cracks to widen.

3.    This ice melts again and loose blocks fall as scree. Water fills the new crack.

4.    A pile of scree forms below the cliff as the process repeats continuously.

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Causes of Glacial Retreat & Advance

Sun spots: The output of the sun slightly varies over decades due to sunspots. High levels of sunspot activity reduce annual ablation rates.

Volcanic Eruptions: Volcanoes can eject km3s of dust into the atmosphere. This acts as an insulator of the earth from the sun’s rays, causing temperatures to fall and short periods of glacial advance.

Global Warming: The increasing usage of fossil fuels has lead to a measurable increase in global temperatures. This has caused an increase in ablation rates.

Ice Ages: Over long time periods, the changes in the earth’s orbit are thought to contribute to the causes of ice ages. The orbital changes cause a series of climatic responses and this can result in most of the world becoming covered in ice. These cycles last for 10,000s of years or more.

Plate Tectonics: When the Indo-Australian plate converged with the Eurasian plate, the Himalayan Mountains were formed. This caused an upset in planetary wind systems, precipitation and temperature patterns.

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Glacial Erosion: Abrasion

Abrasion: The sandpaper effect caused by the weight of the ice scouring the valley floor and sides using the angular rock material trapped underneath.

Striations: Deep scratches/grooves caused by abrasion.

Plucking: Individual rocks are removed from the valley floor and sides as water freezes them to the glacier.

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Formation of Corries

Corrie: A hollow in a mountain formed by stones being dragged over the surface by a moving glacier.

1.    Corries start as sheltered hollows near the top of mountains. Snow collects in these hollows and over time this turns to firn, then glacial ice. This is called a corrie glacier.

2.    As the glacier moves, the processes of plucking and abrasion start to erode the hollow. Abrasion mainly occurs at the bottom of the hollow, making it deeper. Plucking and freeze-thaw weathering occur at the back wall of the corrie, making it steeper.

3.    The corrie glacier moves downhill due to gravity and basal sliding. The movement of the glacier is known as rotational slip as it doesn’t move directly down-slope, it curves.

4.    As the glacier flows out of its hollow and into the main valley glacier it erodes less at the front edge, forming a rock lip.

5.    Sometimes when the ice melts a small lake forms at the bottom of the corrie. This is known as a tarn.

Rotational Slip: The slippage of ice down a curved surface. Tarn: A small mountain lake formed at the bottom of a corrie. 

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Arêtes

An arête is another feature of upland glacial erosion.

Arête: A sharp ridge of rock that’s found between two main areas of glacial erosion, e.g. between two corries.

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Formation of Pyramidal Peaks

1.    Snow accumulates in the hollows in mountainsides, becoming compressed into ice.

2.    This forms corries, and between these corries, arêtes.

3.    Frost shattering attacks exposed areas above the ice, making the top of the mountain jagged and pointy.

4.    The pyramid shape is due to the formation of the corries on each side of the mountain.

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Identifying Glacial Features on an OS Map

  •  As corries are rounded hollows, they look like a horseshoe shape of contour lines on the map. Remember tarns can be found within them. Corries can also be known as coire, cwm, cirque or cove.
  • Glaciers form U shaped valleys, whereas river valleys are V shaped.
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Formation of Glacial Troughs

A glacial trough is a form of lowland glacial erosion.

 Glacial Trough:  A valley with steep sides and a flat base, once occupied by a glacier.

1.    On the glacier’s journey downhill it takes the easiest path, often passing through a V shaped river valley.

2.    The moving glacier causes abrasion, wearing away the base and sides of a valley and carving it into a U shaped trough.

3.    As it erodes the old valley it cuts off the interlocking spurs creating truncated spurs.

4.    Due to differential erosion, the tributary valley is left hanging above the main valley (this is called a hanging valley).

5.    Eventually, the glacier melts and a misfit stream or ribbon lake occupies the valley. The hanging valley creates a waterfall.

Ribbon Lake: A long narrow lake on the floor of a glacial valley which is often blocked by a terminal moraine.

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Glacial Deposition

Glacial till: Can also be known as boulder clay as it is made up of clay, medium sized boulders or both. Glacial till is a mixture of sizes as plucking and freeze-thaw weathering break off big angular rocks, whereas abrasion erodes smaller pieces of rock. The rocks are unsorted as they were held firmly in the ice when being transported, then suddenly dropped.

Moraine: An accumulation of unsorted debris formed by a glacier. This can occur in a formerly or currently glaciated area. Moraines can range in size from silt-like glacial flour to large boulders.

Ground Moraine:  Material dragged underneath the glacier, usually picked up by abrasion, is left behind when the ice melts.

Lateral Moraine: Forms at the edges of the glacier and is mainly made up of scree that has fallen off the sides of valleys due to freeze-thaw weathering. When the glacier melts it forms a slight ridge on the side of the valley.

Medial Moraine: When a tributary glacier joins a main glacier, their two lateral moraines merge to form a single one. When the glacier melts, a line of sediment along the base of the glacier remains.

Terminal Moraine: Huge amounts of material build up at the snout of a glacier, forming a high ridge (often 10s of metres high across the valley). It represents the furthest extent of the glacier’s advance.

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Drumlins

  • Made up of moronic material.
  • Occur in clusters on the floor of glacial troughs.
  • Moulded and shaped by moving ice rather than being dumped.

Drumlin: An elongated mound of glacial deposits.

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