A glacier forms when winter snowfall in an area exceeds summer melt and therefore accumulates year after year. Snow is compacted and converted to glacial ice, and when the ice is about 40 m thick, pressure causes it to flow.

Glacier Types 1. Valley (Alpine) Found in mountainous areas Smaller than ice sheets The length is greater than its width Only cover a small region Transform V-shaped valleys into U - shaped valleys

Alpine Glaciers

Glacier Types 2. Ice sheets (Continental glacier) Large scale – cover 10% of Earth’s land Found in polar regions Greenland – 1.7 million km 2 Antarctica – 13.9 million km 2

Continental Glaciers

So where does glacial ice come from? Ice that makes up glaciers originally fell on its surface as snow. To become ice, this snow underwent changes that caused it to become more compact and dense. Glacial ice has a density of about 850 kilograms per cubic meter. After the snow falls, the crystals can be reduced by the effects of melting and sublimation. Scientists call this process ablation. For most glaciers, ablation is a takes place in the summer months. The snow also undergoes compaction through melting and refreezing. At first, these processes cause the original snowflakes to be transformed into small round crystals. This partly melted, compressed snow is called névé. Névé has a density exceeding 500 kilograms per cubic meter. If the névé survives the ablation that occurs during the summer months it is called firn. When this process happens year after year, a number of layers of firn can accumulate. Accumulation then causes a further increase in density, changing the firn into glacier ice, as the lower layers of firn are compressed by the weight of the layers above. On average, the transformation of névé into glacial ice may take 25 to 100 years.

Fig. 14-2a, p. 323 The conversion of freshly fallen snow to firn and snow ice. Firn is a partially- compacted type of snow that has been left over from past seasons and has been recrystallised. It is ice that is at an intermediate stage between snow and glacial ice. Firn has the appearance of wet sugar, but has a hardness that makes it extremely resistant to shovelling.

Fig. 14-2b, p. 323 A sample of Firn in the Alps

Glacier’s Budget In - Zone of accumulation  Snow accumulates and forms ice  Outer limit of accumulation is called the snowline Out – Zone of wastage ablation – general term for loss of ice or snow from a glacier 1. Sublimation ( Process where ice changes into water vapour without first becoming liquid) 2. Melting 3. Evaporation 4. Calving - icebergs

Glacier as a system

The glacier’s budget The behavior of a glacier depends on its budget, which is the relationship between accumulation and wastage. If a glacier possesses a balanced budget, its terminus (end) remains stationary; a positive or negative budget results in advance or retreat of the terminus, respectively.

How a glacier changes in response to inputs and wastage. Inputs=outputs=glacier remains stationary Inputs > outputs = glacier advances Inputs< outputs = glacier retreats.