Glaciation Flashcards
What is freeze thaw, plucking and abrasion?
The base of glacier ice melts because of pressure and friction. This allows water to freeze into cracks in the rocks and when the glacier moves it pulls out chunks to leave a jagged surface.
This provides material for abrasion and this process is called plucking. Rocks which get stuck in the ice grind away the bedrock under the glacier and this is known as abrasion.
Freeze thaw is a process where water in cracks in the rock freezes and expands, forcing open the gap. When the ice melts more water can get into the crack and freeze again.
After many cycles of freezing and thawing lumps of rock are broken off the surface - this is called freeze thaw. These small pieces of rock are called scree and often build up forming scree slopes on mountainsides.
Explain how a corrie is formed
A corrie is an armchair shaped hollow high on a mountain with steep back and side walls. Snow gathers in mountain hollows, especially north facing hollows, where there is more shade. This snow builds up and compacts to ice. The action of gravity means the ice moves downhill.
As it travels, ice sticks to the back walls and plucks rocks from the surface. Rocks on the back walls are also loosened by freeze-thaw action.
Ice moving with loose rock acts like sandpaper and deepens the hollow by abrasion. Most erosion occurs where the weight of the ice is heaviest. Stones frozen in the base of the ice grind or abrade the corrie base, deepening it.
Ice in a corrie has a rotational movement which means that the front of the corrie is less eroded and a lip forms. The glacier retreats and melts, often leaving a tarn/corrie loch in the base of the corrie.
Explain how aretes and pyramidal peaks are formed
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. Glaciers erode backwards towards each other, carving out the rocks by plucking and abrasion. Freeze thaw weathers the top of the mountain, creating a sharply pointed summit.
Explain how U-shaped valleys are formed
Glaciers cut distinctive U-shaped valleys, or troughs, with a flat floor and steep sides. The glacier uses the processes of plucking and abrasion to widen, steepen, deepen and smooth ‘V’-shaped river valleys into a ‘U’ shape.
The interlocking spurs in the narrow V-shaped river valley are cut-off by the ice, creating truncated spurs. After glaciation, a misfit stream/river or ribbon lake can sometimes occupy the floor of the U-shaped valley.
Explain how hanging valleys are formed
a hanging valley is formed when there is a tributary glacier to the main glacier that joins on but isnt as deep or wide. when the glaciers melt the smaller u-shaped valley is left joined onto the main u shaped valley. there are often waterfalls.
Explain how a ribbon loch is formed
A ribbon lake is a large, narrow lake occupying a U-shaped valley. It forms in a hollow where a glacier has more deeply eroded less resistant rock or it may fill up a valley behind a wall of moraine across the valley.
