Glaciers - The Work of Ice Glaciers 2% of the Earth's water is contained in glaciers. Of the nearly 15,000,000 square km of ice covered regions, about 14,500,000 square km are in Antarctica(12,600,000) and Greenland (1,800,000). Formation of Glaciers Requires a setting in which snow remains on the ground year round - SNOW LINE elevation above which snow remains year round Fresh snow is very porous - pore spaces are filled with air With increasing time and pressure snow flakes recrystallize and the porosity is reduced - firn After several years the lower part of the snow bank has been converted to firn With increasing time the porosity of the ice is reduced to ~0% - glacial ice When the ice reaches ~20 meters it begins to flow (plastic behavior) aided by gravity The upper portion flows more rapidly than the base (due to friction and freezing at the base) The upper portion may behave as a brittle substance and fractures (crevasses) are common Near the "head" of the glacier is the zone of accumulation Near the "foot" of the glacier is the zone of ablation (processes leading to melting) If accumulation is greater than ablation then the glacier advances If ablation is greater than accumulation then the glacier retreats Glaciers transport large volumes of rock material (poorly sorted) in front, on top and at the base of the ice. Glacier Types Ice Sheets - Antarctica and Greenland : some is more than 4000 meters thick Ice Shelves - ice sheets that extend over marine waters Ice Caps - small ice sheets Valley Glaciers Piedmont Glaciers Glacial Erosion Striated (groved) bedrock Glacial Polish Glacial Valleys - "U"-shaped Cirques - bowl-shaped areas Arette - glacial ridges - intersection of two cirques Horns - spires left behind - intersection of several cirques Depositional Features Till - unconsolidated glacial debria - poorly sorted Moraines - terminal, lateral and medial depending on where the material was transported. erratics - diamonds in Ohio Kames - Kettel Lakes Causes of Glaciation Plate Tectonics - lithosphere transported to a polar region Changes in temperature - prevent or encourage accumulation Variations in properties of the Earth's orbit.