A glacier is a moving river of ice on land. These large masses of ice require specific climatic and geographic conditions to form and advance: mainly, high snow fall and cool summer temperatures, so that snow is able to accumulate in the winter, without being lost to melt in the summer. Today, most glaciers are found at the poles (continental glaciers), but can still be found at higher altitudes in alpine climates on every continent (alpine glaciers).
Glaciers are formed as snow falls and becomes compacted and recrystallised under the pressure of its own weight. Initially this process turns the snow into firn, which is a more granular, partially compacted snow (Figure 1.).
After approximately 45m depth, the firn is further compacted and changes into glacial ice. The crystals are now packed much tighter together and the ice is impermeable to air and water (Figure 1.).
Glaciers are formed as snow falls and becomes compacted and recrystallised under the pressure of its own weight. Initially this process turns the snow into firn, which is a more granular, partially compacted snow (Figure 1.).
After approximately 45m depth, the firn is further compacted and changes into glacial ice. The crystals are now packed much tighter together and the ice is impermeable to air and water (Figure 1.).
How do glaciers move?
Glaciers are open systems with inputs, processes, stores and outputs.
To be classed as a glacier the mass of ice must be capable of motion. The sheer weight of the layer of ice and the gravity acting upon it, causes glaciers to flow very slowly downhill -over mountain valleys, across upland plains or stream into the ocean.
To be classed as a glacier the mass of ice must be capable of motion. The sheer weight of the layer of ice and the gravity acting upon it, causes glaciers to flow very slowly downhill -over mountain valleys, across upland plains or stream into the ocean.
The upper part of the glacier is where input material - snow fall - exceeds the output - meltwater. Therefore more mass is gained than lost over a period of a year. This is called the ACCUMULATION zone.
The lower part of the glacier is where the output - melt water - exceeds the inputs - snow fall. Therefore mass is lost rather than gained over the period of a year. This is called the ABLATION zone.
Between the two zones is the line of EQUILIBRIUM. This separates net loss from net gain and represents the line of snow upon the glacier.
The difference between the total accumulation and the total ablation during the period of a year is known as the NET BALANCE.
In temperate glaciers, there will be a negative net balance in the summer warm temperatures, due to ablation exceeding accumulation and then therefore the reverse in winter. If these seasonal budgets cancel each other out then the glacier will remain stationary.
The glacier can begin to move down the valley and cover an increasing area of land, as the supply of snow exceeds the loss of glacial ice. This is known as GLACIAL ADVANCE.
When there is larger amount of glacial ice lost than gained, the glacier will lose volume and the snout will move its position up the valley. This is known as GLACIAL RETREAT.
The lower part of the glacier is where the output - melt water - exceeds the inputs - snow fall. Therefore mass is lost rather than gained over the period of a year. This is called the ABLATION zone.
Between the two zones is the line of EQUILIBRIUM. This separates net loss from net gain and represents the line of snow upon the glacier.
The difference between the total accumulation and the total ablation during the period of a year is known as the NET BALANCE.
In temperate glaciers, there will be a negative net balance in the summer warm temperatures, due to ablation exceeding accumulation and then therefore the reverse in winter. If these seasonal budgets cancel each other out then the glacier will remain stationary.
The glacier can begin to move down the valley and cover an increasing area of land, as the supply of snow exceeds the loss of glacial ice. This is known as GLACIAL ADVANCE.
When there is larger amount of glacial ice lost than gained, the glacier will lose volume and the snout will move its position up the valley. This is known as GLACIAL RETREAT.