Glacial Features Flashcards
What is plucking
Plucking occurs along the base and sides of glaciers. Glaciers freeze onto rock surfaces. When they begin moving downhill under their own weights and the force of gravity, they tear pieces of rock away with it
What is abrasion
The glacier moves downhill under its own weight and the force of gravity. As it does so the rocks frozen to the sides and base of the glacier scrape away at the land in a sandpapering motion
What is freeze-thaw
Water seeps into the cracks and fissures in a rock. The temperature drops and the water freezes and expands putting pressure on the rock. When the temperature increases again the water thaws and the rock contracts slightly. This process happens time and time again until the rock is too weak and beings breaking up
How do Corries form
- Snow gathers in north facing hollows, where there is lots of shade. It is compressed to form ice.
- Under its own weight and the force of gravity, the glacier begins moving downhill in a rotational fashion.
- The glacier freezes onto the back walls of the hollow and as it moves it plucks rock away. Rocks on the back wall are loosened by freeze-thaw. This steepend the back wall
- The rocks frozen to the glacier scraped away at the land in a sandpapering motion. This deepend the hollow
- The glacier lost momentum as it moved up and out of the hollow, causing it to leave behind a notch of rock at the entrance of the hollow. Called a rock lip
- Loose materials were bulldozed to the snout of the glacier
- This formed a terminal moraine at the entrance to the corrie
- At the end of the ice age glaciers melted
- The terminal moraine sealed in the melt water to form a tarn
How do aretes form
Aretes form when two corries erode back to back into the mountain. This narrows the area of land between them
How do pyramidal peaks form
- They form where three or more corrie glaciers erode back to back on a mountainside
- When the corrie glaciers erode their backwalls they create aretes, they also narrow the peak between each corrie
- This forms a mountain peak which is roughly pyramidal in shape
- Where glaciers are unable to sharpen the peak nivation will sharpen it. This will continue after the ice-age. It enhances the pyramidal shape
How do U-shaped Valleys form
- At the start of the ice-age precipitation fell as snow
- This collected and was compressed together to form firn. Further compression turned it into ice
- Under its own weight and the force of gravity the glacier began moving downhill through a pre-existing V-shaped valley
- Freeze-thaw weathering weakened the rocks in the area
- As it traveled the glacier froze onto the valleys sides. When it began moving again it plucked pieces of rock away with it.
- The rocks frozen to the sides of the glacier scraped away at the land in a sandpapering motion.
- This truncated the interlocking spurs, as well as steeping the valleys sides, widening the valley and straightening it out.
- Plucking and abrasion also occurred along the valley floor. Abrasion was greatest were the glacier was heaviest. This helped to deepen the valley.
How do Ribbon Lakes form
- In some areas along the floor of u-shaped valleys, the rock may be weaker
- At those points glaciers were able to erode more deeply. They were also able to erode more deeply where glaciers were thicker or heavier
- This created elongated hollows along the floor of the valley
- When the glaciers melted those hollows filled with the meltwater to form lakes.
- The meltwater was sealed in by the terminal moraine
- Glaciers plucked and abraded material such as sand, clays and boulders as they travelled. They then carried this material along with them as they travelled. Where it was bulldozed forward to the snout of the glacier and then left as a ridge across the entrance to a U-shaped valley, it formed a terminal moraine
How do truncated spurs form
- Before the ice-age started valleys were narrow river v-shaped valleys with rounded, gently sloping hills.
- The foot of hills formed interlocking spurs (areas of land which jutted out and rivers wound around them as they did not have the erosive power to remove them)
- During the ice-age glaciers eroded interlocking spurs
- The glacier would freeze onto the spurs and as it started moving it would pluck pieces of rock away
- Abrasion occured where rocks frozen to the base and sides of the glacier scraped away at the interlocking spur in a sandpaper motion
- This removed the end of the interlocking spurs truncating them
- This gave the sides of U-shaped valleys steep, flat sided appearances
