 A glacier is a thick mass of ice, composed of compacted and recrystallized snow that forms over thousands of years.  Glacier only flow or move over land  The movement is caused by the weight or by gravity  Icebergs and sea ice are not glaciers.  Glaciers cover nearly 15 million square km or about 10% of the Earth’s surface  They account for 75% of all freshwater.

 Glaciers can be seen today on all continents except Australia  Glaciers form at high altitude or high latitude where snowfall in winter does not entirely melt in summer.  There are tow types of glaciers;  Valley glaciers and  Continental glaciers

 Valley glaciers, also called alpine glaciers are confined to mountain valleys at high altitudes with temperatures low enough to accumulate snow.  Valley glaciers vary in length, width, and depth  They move in response to gravity; they flow down hill.  They have smaller tributary glaciers, like a river has tributaries.

 These glaciers are not confined by topography and cover large landmasses. Also called ice sheets.  There are only two continental glaciers on Earth today;  Antarctic ice sheet and  Greenland ice sheet

 These glaciers flow outward from the center where ice thickness is greatest.  The thickest part of a glacier can be up to 3km in the center and thins towards the margins of the glacier

 Glacier can form if an area receives more snowfall in winter than melts in summer, resulting in a net accumulation of snow.  Snow turns into glacial ice through a process of compaction  Firn are snow granules fused together.

 The movement of a glacier is termed flow.  When the ice reaches a critical thickness, about 50m, the pressure is enough to allow for plastic flow, having the ice move like toothpaste.

 Above 50m depth the glacier does not move plastically.  The top portion is forces to move, such that it is brittle, that the ice fractures. This is known as the fracture or brittle zone.  Crevasses are formed due to this action.

Travel on a Glacier

The other mechanism of flow is termed basal slip. This takes place at the bottom of the glacier, where the ice slides over a thin layer of water.

 Expansion and decrease in size of a glacier is in response to accumulation of snow and ice and melting, wastage.  It is described in terms of a glacial budget.  The upper part of the glacier is the accumulation zone  The lower section is the wastage zone

 Glacial ice is lost through melting, sublimation, and/or calving.  The line that separates the zone of accumulation from the zone of wastage is termed the snowline or equilibrium line.

 When the equilibrium line moves down a glacier, terminus or toe is said to advance. A positive budget.  When the equilibrium line moves up the glacier it is said to be retreating or a negative budget.