Earth Science: Earth – Water – Glacier
Large areas of the Northern and Southern Hemispheres were once covered in mile-thick ice. Mighty moraines, carved-out valleys, and smooth-polished stones remain as reminders of the ice ages. Today the ice-cover of the polar areas and mountains is rapidly receding, and the full effects of global climate change are not yet certain.
Glacier Formation
Glaciers are masses of ice that flow slowly down a slope due to gravity. If excessive snow falls, the glacial advance is accelerated. If warm weather melts more ice than can newly form, the glacier retreats.
Mountain glaciers originate when snow and ice accumulate inside a cirque-a relatively small semicircular depression high in the mountains. In the so-called accumulation zone, above the snow line, fine snow crystals become densely packed, due to repeated melting and freezing, into coarsely grained firn ice. The constant pressure eventually results in air- and watertight glacial ice.
In the Alps this process only takes a few years, but it can take up to 200 years in the colder and drier Antarctic regions. The ice becomes a glacier only once the bottom layers begin to move downhill due to the weight of the ice. If the gradient is sufficiently high, the glacier crosses the firn line and the glacier’s snout pushes forward toward the valley.
Crevasses are formed when glaciers move faster than the flow rate of the ice can sustain. Transverse crevasses develop in areas of scarps and longitudinal crevasses develop in areas where glacial valleys turn wider. Radial crevasses form at the toe of a glacier, where they fan around the tip of the glacial snout.
Ice is lost through melting or evaporation in the so-called ablation zone below the snow line. It is continuously replenished by the glacial flow from the accumulation zone. The snout of the glacier moves forward when the ice flows at a higher rate than it can melt. The glacier retreats when the ice flows at a lower rate. The melted water of the ablation zone collects at the base of the glacier and exits in the form of milky white glacial creeks at the mouth of the glacial snout.
Rocks and debris that are transported by the glacier are deposited as a moraine. Solid material abraded from beneath the ice during glacial movements is carried along as a ground moraine. The material accumulates at the glacial snout and is pushed together to form an end moraine.
ICE AGES
Ice ages are geological time periods during which average temperatures were at least 8-9°F (4-5°C) lower worldwide than they are today, causing large-scale glaciations in mountain areas and toward the Poles.
The last major ice age period began 2.5 million years ago and ended about 11,000 years ago. During this period glaciation periods and warmer periods called in terglaciations alternated in a cycle.