Glaciers and Ice

Glacial Systems An open system of flowing ice –Water input as snow –Transformed into ice –Ice flows under pressure –Water leaves by evaporation and melting

Glacial Ice Glaciers maintain a balance of ice –Zone of accumulation - net gain in ice –Zone of ablation - net loss of ice –Snow line - boundary between zones

Accumulation [i.e. snowline] Ablation

Types of Glaciers Glaciers divided into 2 subsystems: –Valley glaciers Ice streams flowing from mountain areas down valley –Continental Glaciers Large-scale Ice Sheets

Ice Flow Glaciers move by slow deformation of the ice due to increase in pressure from the overlying ice –Shifting and rotation of ice crystals –Pressure melting lubricates ice –Displacement along slip planes

Movement of is dependent on snow accumulation and slope –Accumulation is greatest at glacier head –Gravity pulls ice downward & outward –Flows parallel to surface through ablation zone

Movement is not uniform –Velocity increases from head to snow line –Velocity decreases after snow line Ice is brittle & ductile –Differential flow velocity creates tensional stress –Crevasses form as a result of differential flow

Glacial Equilibrium Water (ice) moves through the open glacial system –At EQ, input = output - mass balance –If input > output, glacier expands –If input < output, glacier shrinks

Erosion Powerful agents of erosion –Up to 0.35 mm/yr –Ice wedging tends to smooth surface Plucking pulls up loose fragments –Ground-up rock abrades surface

Valley/Alpine Glacier Systems Cirque glaciers –A protected bowl shaped depression –Bounded by steep cliffs, headwall –Open on the downslope side

Valley glaciers –Growing cirque spreads down a valley –Glaciers may fill long mountain valleys –Glacier pluck rock debris forming moraines

Glaciers produce unique geomorphic features –Arete- A sharp crested ridge bounded by 2 cirques –Horn - A sharp peak bounded by 3 or more cirques –Hanging valleys - characteristic U shape valley of a smaller glacier

arete horn

Continental Glacier Systems Most important glacial system –Modify pre-existing landforms –Cause crustal subsidence –Modify drainage systems –Important component of the hydrologic system & climate

Ice sheets –Continent-sized ice masses –Ice sheets include 95% of all glacier ice Antarctica & Greenland –Glacial ice accounts for 2.14% of all water on Earth

Ice caps –Covers large area of mountains or high altitude plains –Ice shows outward radial flow Ice shelves –Tabular ice bodies resting on the ocean floor

Geomorphic features indicate past glacial coverage –Till - debris that accumulates beneath and at glacier margins Drumlins - streamlined hills Ground moraine - deposited beneath ice –Eskers - melt water stream deposits