Just as a river could be seen as a system of inputs, outputs, transfers and storage, so can a glacier. Whether a glacier grows or retreats is directly affected by the comparison between the inputs and outputs. This is described in more detail in the section on the Ice Budget.
The diagram below shows the different inputs, flows, stores and outputs of the glacial system:
Glaciers originate from heavy snowfalls over a prolonged period of time. The snow initially has many air or pore spaces between the flakes. Over time the weight of new snow above it compacts it all, squeezing the air out of the pore spaces, similar to what you might do when making a snowball. This compaction causes some of the snow to become freezing water that binds the compacted snow together even more, creating ice. As the sir is squeezed out of the ice it will turn a slight shade of blue.
Ablation - The melting of the ice, mainly during summer months, and usually at the snout end of the glacier.
Accumulation - The build up of the glacier due to snow being compacted into ice.
Calving - The splitting of the end of the glacier into smaller sections. These could become icebergs, if the glacier snout ended in the sea. .
Glaciation - The effect of large masses of ice on the landscape. Compressed snow accumulates to eventually form ice and create a glacier.
Ice Sheets - These are large masses of ice which cover an entire land surface. Antarctica is the best example as the ice sheet covers the entire continent.
Snout - the lower end of the glacier.
Valley Glaciers - The most common of the two types of glacier. These are confined by the valley sides that have already been carved out by a river. Valley glaciers can be found in all the main mountain ranges of the world, such as the Franz Josef Glacier in the Southern Alps of New Zealand, and the Rhone Glacier in Switzerland.
A glacier will grow and retreat depending upon how much precipitation is inputted into the glacier system. This growth can be seen on two time scales,either every year or over a much longer period.
In general glaciers around the world are retreating at the moment as the climate slowly warms up and they melt slightly every year. The evidence for this can be seen in some of the large Antarctic Ice Sheets either retreating or breaking off from the continent all together. Valley glaciers are also being measured to see how quickly they are retreating.
During a single year a valley glacier may well both grow and retreat, depending on the rate of accumulation compared to the rate of ablation. This is called the ice budget or glacial budget.
When the rate of accumulation is greater than the rate of ablation,the glacier will grow. This is called a positive regime.
When the rate of accumulation is less than the rate of ablation, the glacier will retreat. This is called a negative regime.