About 75% of the fresh water on the planet is in the form of ice.
During the Pleistocene: 2 000 000 years BP this was greater.
Alpine glaciers have lost 33% of their mass in the last 100 years.
Think of ice in the freezer. Glacier ice is not as pure and is not the result of water freezing. It is formed by snow being compacted: placed under pressure. This removes the air and increases the density.
Much of the snow's thickness is caused by pockets of air. Density of newly fallen snow is 0.06-0.08 i.e: over 90% air. Natural compression will reduce this to 0.2 within a couple of days. When it reaches a density of 0.5, which may take several years, it reaches an intermediate stage called firn or neve. Densities of 0.8 and above will take centuries to reach.
TYPES OF GLACIER
Different scales of ice form.
ICE SHEET: Continental in scale and may be over 3km thick e.g. Antarctica (& N. Europe during Pleistocene)
ICE CAP (or JOKULL in Iceland): May sit across a mountain or mountain range.
GLACIER: Most common. A flowing stream of ice, often restricted by surrounding relief. May still be of considerable size e.g Beardsmore in Antarctica extends over 200km and is up to 20km wide.
There are several types of glacier:
VALLEY GLACIER: often follow existing river valleys, originating in ice-collection centres in the highlands (corries)
CORRIE GLACIER: fill their distinctive hollows, but don't venture far from them
PIEDMONT GLACIER: may make up the bottom stretches of a valley glacier: the ice is broader and shallower as it spreads out onto the lowland
OUTLET: outlets for ice from ice sheets e.g. the Beardsmore