Glaciers

Glatciation and glacial processes.

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Glacial processes

A glacier is a mass of ice that moves downhill very slowly. The formation of glaciers and the way they shape the landscape is called glaciation.

Glaciers used to cover large areas of the Earth and shaped the landscape around them.

Plucking is when rocks get frozen to the glacier and removed, leaving behind a jagged landscape.

Glaciers also erode by abrasion which is when the rocks and stones embedded in the glacier rub against the bedrock.

Melted water from glaciers can cause freeze-thaw weathering on joints and cracks in rock.

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Freeze-thaw Weathering, Plucking and Abrasion

Freeze-thaw describes the action of glacial meltwater on joints, cracks and hollows in rock. When the temperature reaches freezing point, the water inside cracks freezes, expands and causes the cracks to widen. When the temperature rises, the water thaws and contracts. This eventually causes rocks to break up. For freeze-thaw to take effect, the air temperature needs to fluctuate around freezing point.This produces angular rock fragments.

Plucking occurs when rocks and stones become frozen to the base or sides of the glacier and are plucked from the ground or rock face as the glacier moves. It leaves behind a jagged landscape.

 

Abrasion is when rocks and stones get embedded in the base and sides of the glacier and are then rubbed against the bedrock (bottom of glacier) and rock faces (sides of glacier) as the glacier moves. The landscape wears away as the glacier behaves like sandpaper. It leaves behind smooth polished surfaces which may have scratches in them called striations. Striations are carved out by angular debris embedded in the base of the glacier.

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Glacial erosion landforms

Glaciers widen, deepen and smooth V-shaped valleys into U-shaped valleys with flat bottoms. The U shaped valleys are called glacial troughs.

Glaciers have tributaries (small rivers that join the main river channel). As the main glacier erodes deeper into the valley, the tributary forms higher valleys. These are called hanging valleys and end in waterfalls.

When a river erodes the landscape, ridges of land form in its upper course which jut into the river. These are called interlocking spurs One of a series of spurs (ridges of land) jutting out from alternate sides of a river valley. A glacier cuts through these ridges leaving behind truncated spurs.

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Corries

Glaciers can also form corries in upland areas. Snowflakes collect in a hollow and compress becoming glacier ice. Erosion and weathering gradually make the hollow bigger.

Even though the ice is trapped in a hollow and unable to move down hill, gravity will still encourage it to move. This circular motion is known as rotational slip and can cause the ice to pull away from the backwall creating a crevasse or bergschrund. Plucked debris from the backwall causes further erosion through abrasion which deepens the corrie.

These processes create a characteristic rounded, armchair shaped hollow with a steep back wall.

When ice in a corrie melts, a circular lake is often formed at the bottom of the hollow. This is known as a tarn.

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Arêtes, Pyramidal Peaks and Ribbon Lakes

An arête is a knife-edge ridge. It is formed when two neighbouring corries run back to back. As each glacier erodes either side of the ridge, the edge becomes steeper and the ridge becomes narrower.

A pyramidal peak is formed where three or more corries and arêtes meet. The glaciers have carved away at the top of a mountain, creating a sharply pointed summit.

As a glacier flows over the land, it flows over hard rock and softer rock. Softer rock is less resistant, so a glacier will carve a deeper trough. When the glacier has retreated, (melted) water will collect in the deeper area and create a long, thin lake called a ribbon lake.

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Glacial deposition landforms

When glacial ice melts, it lays down rocks called boulder clay.

All glacial deposits are angular and mixed up (unsorted). The extreme of this can be seen in erratics. These are large rocks or boulders that are often found on their own, rather than in piles. They are unusual shapes, unusually large and of a rock type uncommon to the area they have been dumped.

Drumlins are elongated hills of glacial deposits. They can be one kilometre long and 500 metres wide, often occurring in groups. A drumlin swarm is a group of them. These would have been part of the debris that was carried along and then accumulated under the ancient glacier. The long axis of the drumlin indicates the direction in which the glacier was moving. The drumlin would have been deposited when the glacier became overloaded with sediment.

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Moraines

When glacial ice melts, different types of rock are laid down that have been carried along by the glacier. Piles of these deposits are called moraines.

  • Terminal moraines are found at the terminus or the furthest (end) point reached by a glacier.
  • Lateral moraines are found deposited along the sides of the glacier.
  • Medial moraines are found at the junction between two glaciers. In the middle. 
  • Ground moraines are disorganised piles of rocks of various shapes, sizes and of differing rock types.
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