What is a glacier?
- 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.
- 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
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.
- Glaciers cut distinctive U-shaped valleys with a flat floor and steep sides.
- The glacier widens, steepens, deepens and smoothes V-shaped river valleys.
u shaped v shaped
Arêtes and pyramidal peaks
- 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.
Moraines in more detail
- 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.
- Types of moraine
- 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.
- Ground moraines are disorganized piles of rocks of various shapes, sizes and of differing rock types.
Explained V and U shaped valleys
- Just like rivers, glaciers have tributaries. As the main glacier erodes deeper into the valley, the tributary is left higher up the steep sides of the glacier.
- U-shaped valleys ending with a waterfall at the cliff-face are called hanging valleys.
- When a river erodes the landscape, ridges of land form in its upper course which jut into the river.
- These are called interlocking spurs. A glacier cuts through these ridges leaving behind truncated spurs.
About Corries (explained)
- 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 back wall creating a crevasse or bergschrund.
- Plucked debris from the back wall causes further erosion through abrasion which deepens the corrie.
- Some of this debris is deposited at the edge of the corrie, building up the lip.
- 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.