These glaciers grow to occupy entire valleys, before moving downslope due to gravity. As they move downslope, the glaciers join together to form ice sheets. These sheets can cover entire continents, and are also called continental glaciers.
Continental glacial sheets : Fig. 12.3 The extent of continental ice sheets
1. Fresh snow falls and accumulates on the ground. Newly fallen snow is soft and easy to compact as it is 90 per cent air.
90% air Snow
2. As more snow falls, the layers beneath it are compacted. Due to the pressure, the snow melts slightly before refreezing again. This changes the snow into rounder, small grains of ice called firn.
50% air
Granular ice
3. As more snow falls, the grains of ice are compacted further into a solid block of dense glacial ice. Glacial ice is also referred to as ‘blue ice’. It is blue as compaction has removed all oxygen from it.
20–30% air Firn
Glacial ice
: Fig. 12.4 The formation of glacial ice
12.2 Glacial Movement
As glaciers build up, they are pushed downslope by gravity and their own weight. This movement occurs in two ways:
1. Basal slide (also called basal slip) occurs in the same way as an ice sheet forms. Pressure from the weight of the ice melts a thin layer of water at the base of the glacier. This layer of water acts as a lubricant and allows the glacier to slide downslope. The gradient of the slope the glacier lies on reduces friction between ice and rock and affects the amount of movement that takes place. Most basal sliding takes place on steep slopes with a smooth profile. Basal sliding is most active during the summer months as higher temperatures allow more ice to melt.
224 EARTH
20% air as bubbles
EDUCATE YOURSELF The Last Ice Age
Snow – compaction – firn – compaction – blue ice
Glacial ice is formed in three stages, which can occur quickly or slowly depending on the rate of accumulation.
