Glacier is a body of ice and firn composed of recrystallized snow and frozen melt water, which lies wholly or mostly on land and shows evidence of present or former motion. Glaciers originate in areas where snow accumulation exceeds loss. Glaciers flow downward and outward toward areas where the losses exceed any mass gain.
Sea ice is not considered as glacier because it does not lie on land and does not move by itself. Permanent snowfields are not glaciers because they lack motion.
Most glaciers have accumulation zone, equilibrium line and ablation zone. Zone of accumulation is an area of positive gain of snow/ice mass. Vectors of particle motions are directed downward into the ice mass. Zone of ablation is an area of loss. The ice of lower glacier area is lost by melting, sublimation, erosion, calving into water (icebergs), and breaking away of body of dead ice. Equilibrium line is the boundary of net (zero) annual accumulation.
When Accumulation >Ablation a glacier will advance. When Accumulation < Ablation a glacier will retreat. When Accumulation = Ablation a glacier is in a steady-state.
Glacial Classifications:
1. Dynamic
2. Morphologic
3. Thermal
Dynamic classification of glaciers:
1. Active, the glacier is in motion (advancing or retreating).
2. Stagnant, the glacier is not in motion, it is dead.
Thermal classification of glaciers:
1. Temperate (=Wetbased) glaciers (Where ice is at, or near, pressure-melting temperature. + The transition from firn to ice is relatively fast. + Ice temperature decreases downward due to the effect of increasing pressure on the melting temperature).
Zero degrees Celsius and below: ice may coexist with water at the base of a glacier due to the high-pressure environment and as result to give: 1) ice deformation and 2) glacial movement.
2. Polar (cold-based) glaciers (Ice is entirely below the pressure-melting point. + The glaciers are frozen to the base. + Melting does not occur and the conversion of firn takes much longer than in temperate glaciers. + Ablation is only by calving, wind erosion, or sublimation. + Movement??).
3. Sub-polar glaciers (Frozen to base, as in Polar, but melting occurs in the summer months, i.e. intermediate (seasonally temperate/polar)).
Morphologic classification of glaciers:
1. Ice sheets; Ice caps (i.e. continental glaciers) that flow from a central topographic high. They are large enough not to be confined by topography. Modern ice sheets are Greenland and Antarctic glaciers, modern ice caps are e.g. Baffin Island, ice caps on Severnaya Zemlya archipelago etc.
2. Alpine (mountain) glaciers, which are bound by topographic features such as mountains or valleys. a. Valley glaciers b. Cirque glaciers
Cirque (or kar) glaciers occupy amphitheater-like hollow in alpine areas near the firn line (above). They are more frequent on northward or eastward facing slope in the Northern Hemisphere, where solar energy is least. In areas of high precipitation cirque glaciers may flow down into valley and join to valley glaciers.
Valley glaciers flow well beyond their cirques in long, winding valleys.
3. Piedmont glaciers. They form when glaciers flow out mountain valley onto foothill plain as a broad, radially flowing lobe. Piedmont glaciers may also extend into standing bodies of water.
