Ice has incredible power to shape the land and can completely transform the landscape underneath it. This is particularly true of valley glaciers and ice sheets, which transform the land underneath through a combination of weathering, erosion, transportation and deposition.
Glaciers are found in cold environments so the dominant form of weathering is MECHANICAL OR PHYSICAL. So, physical forces such as the wind, freeze thaw and frost shatter can have an impact.
Freeze thaw is the weathering of rock which occurs when the water, which has penetrated the joints and cracks, freezes. This process is, therefore, probably effective only in well-jointed rocks. Water expands by 9% when it freezes, and it has been suggested that this expansion causes the rock to shatter. This can create angular rock fragments known as scree. Freeze-thaw is most active where the temperature varies regularly around 0 °C. This can break down rock which can then be transported by the glacier.
The process of freeze thaw
There are 2 main processes of glacial erosion which occur because of the ice moving. A glacier is largely made up of ice but also has huge quantities of stones and boulders within it. These stones are likely to have come from freeze-thaw weathering or plucking. First, ice has the property of being able to freeze to rock. The glacier uses this to adhere to part of the ground. Then, as the glacier moves it forces the ice frozen to the ground to continue moving down slope, the rock may be pulled out of the ground and moved down slope as well. This is known as plucking and leaves an angular, jagged surface. Second, once glaciers have rock and sediment at their bases they can push this sediment against the ground and use it like sandpaper. This abrasion of rock against rock can scour the landscape and leave large gouges, small striations, or even a finely polished surface.
It is hard to understand that Ice moves given that it is a solid, but it can and does. Ice can move at extraordinary speeds, and glaciers in surge conditions are known to move at up to 30m a day. The Franz Joseph glacier in New Zealand has been known to surge in the past. Ice can move in many ways and this is determined by the glacial mass balance and the temperature and precipitation regime of the area the glacier is found within. Where accumulation and ablation are equal the glacier is said to be in a steady state (the front of the ice, the snout, doesn’t move).
This rarely happens, and 2 other states exist;
• When accumulation (gains via added snow) is greater than ablation (losses through melting and evaporation) leading to growth in ice mass and potential glacial advance down the valley.
• Ablation is greater than accumulation leading to a loss of ice mass and the potential retreat of the glacier up valley.
Over a year accumulation tends to be within the colder months and ablation within the warmer months.
Some of this movement is rotational, known as rotational slip. This is where gravity, the mass of the ice and the slope act to make the ice move down slope in a curved or rotational movement.
This all means that ice moves and so TRANSPORTS sediment within. At the front of the glacier, the snout, the ice bulldozes material forward. Soil, rocks and boulders are pushed forward by the huge mass of the ice descending the valley.
Due to the huge amounts of energy available to the glacier they can transport both tiny particles and huge boulders, and both of these processes result in glacial deposition.
Glaciers can carry ice and sediment far from where the ice falls into warmer regions lower down mountainous areas. This results into melting and where ever ice melts eroded and transported sediments known as moraines or tills can be deposited or dropped down. One type is ablation till, where the ice melts and dumps large amounts of unsorted materials. Deposition is also encouraged where ice becomes overloaded with sediment at its base, or as a glacier retreats.
Moraines above Lake Louise, Alberta, Canada
The ice that is melting also creates huge meltwater rivers that also carry away then deposit sediment far from the glacier. This sediment is known as outwash material and because it has been carried by water will appear more sorted and rounded than the glacial till deposits.
The glacial system
NEXT TOPIC - Glacial Erosion Landforms