A glacier is a mass of ice that moves very slowly downhill. They are found at high altitude across the globe, even on high mountains close to the equator, and at lower altitude in high latitudes close to the North and South Poles. The formation of glaciers and the process by which they shape the landscape around them is called glaciation.
Glaciation in the last ice age
18,000 years ago ice covered about 30 per cent of the land in the world. In Britain, ice covered land as far as the Bristol Channel. During the last ice age the temperature remained below 0°C which allowed the ice to remain on the land all year.
Map of Europe during the last ice age
The Khumbu Glacier, Nepal
Glaciers once covered large areas of the Earth and shaped the landscape around them. The legacy of ancient glaciers lives on – for example in areas such as the Lake District.
There are three processes by which glaciation affects the landscape – erosion,transportation, and deposition.
The predominant process is freeze-thaw weathering. 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.
Freeze-thaw weathering produces angular rock fragments.
Abrasion and plucking
Jakobshavn Glacier, Greenland
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 occurs when rocks and stones become embedded in the base and sides of the glacier. These are then rubbed against the bedrock (at the bottom of the glacier) and rock faces (at the sides of the glacier) as the glacier moves. This causes the wearing away of the landscape 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.