Ice and a Slice of History
Middleton, Nick, Geographical
Nick Middleton explains how piles of rocks and upturned baskets of eggs can tell us where rivers of ice once carved their way through the landscape
Most people remember something about glaciation from their geography lessons at school. The study of glaciers, `glaciology', is packed with exotic sounding terms like firn, cirque and drumlin. The word `glacier' is used for any large mass of ice built up from snow that falls in an upland area and then flows downslope under the influence of gravity. The word itself is derived from the Latin for ice glacies, and its French derivative, glace. But a glacier isn't only made up of ice. It also contains air, water, and fragmented pieces of rock. The ice can be as large as a continent, such as the ice sheet over Antarctica. Or it may fill a small valley between two mountains, making a valley glacier.
Layer upon layer
It takes a long time for a glacier to form. Snow falling on top of more snow gradually accumulates over thousands of years and is compressed into ice. When falling through the air, fluffy snow flakes have an exquisite hexagonal shape, but on the ground they lose their edges and slowly become tiny grains, separated by air spaces. With each snowfall, underlying layers become more compressed. Their snow grains are loosely compacted and connected by air spaces. Ice in this form that has survived a season is known as firn. It looks like wet sugar but is hard enough to resist all but the most dedicated snow shovelling.
At depth, the ice crystals grow into the air spaces. Individual crystals can grow as long as a rugby ball. The ice only becomes true glacial ice when the air bubbles are so compressed that they become small and isolated. This complete transition occurs when the ice has reached a depth of about 100 metres below the surface. In Antarctica, it takes several thousand years for snow to turn to glacial ice.
The lifespan of any glacier depends on the balance between material being added to it as snow and material being lost. The loss of material from a glacier is known as `ablation'. In places where a glacier meets the ocean, huge slabs of ice at its snout are weakened and then broken off by the action of the rising and falling tides, creating icebergs. Icebergs float because glaciers are composed of flesh water which is less dense than the surrounding sea water.
Temperature and precipitation are the most important aspects of the weather that determine the health of a glacier. The temperature must be cold enough so that more ice accumulates than is lost, and the precipitation must be snow rather than rain. The basic requirement for any healthy glacier is that the accumulation of ice must be greater than, or as great as, ablation.
The eerie blue colour of glacial ice is a feature of the landscape in several parts of the world. In fact, glaciers are found on all the continents except Australia. They span the globe from high altitudes in equatorial regions to the polar ice caps. Huge ice sheets are found in the polar regions of the Arctic and Antarctic, where temperatures are well below freezing, even in the summer. …