# Over longer timescales the size of glaciers is

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Gravity drives the movement of glaciers in short timescales. Over longer timescales, the size of glaciers isaltered by changes in climate. These masses of ice grow (advance) during colder periods and shrink (retreat) during warmer periods.Glacial MovementAreas of a glacier section are shown in the block diagram below. Characterize the style and velocity of icemovement at the indicated areas of this glacier. When answering this question, you should consider how pressure influences the brittle or plastic nature of ice, and how friction with bedrock affects ice movement.Hint 1. Glacial ice movement and depthIce in a glacier moves under the influence of gravity. Within a glacier, below a depth of approximately 50 m, pressure causes ice to undergo plastic flow. At the surface of the glacier, ice behaves as a brittle solid but rides “piggyback” style on top of the flowing ice beneath it.Hint 2. Bedrock and glacier velocityFriction with underlying bedrock or any rock contact can slow the velocity of flowing ice. Ice flows fastest where it is farthest from bedrock/rock.Glacial ice is not static. It changes by moving under the influence of gravity. Another way that glaciers can change is that their size can increase or decrease under the influence of climate. Changes to the volume of a glacier can occur at the same time that the glacial ice is moving due to gravity.Glacial BudgetsIn many circumstances, glacial ice moves downhill due to gravity, despite climate conditions. The size of a glacier is governed by the glacial budget that is calculated as:The difference between the zones of accumulation and wastage.The zone of accumulationis where the glacier “grows” because more snow and ice accumulate than melt each year. This typically occurs at the higher elevations of glaciers.The zone of wastageis where the glacier “shrinks” because annual ice loss exceeds accumulation.This typically occurs at the lower elevations of the glacier.The snowline is the boundary between the zones of accumulation and wastage. As climate cools over decades, centuries, or millennia, glaciers can become larger as the zone of accumulation grows relative to the zone of wastage. This process lowers the snowline. Over similar timescales of climatic warming, glaciers can decrease in size as the zone of wastage grows relative to the zone of accumulation. This process pushes the snowline higher.The snowline and zones of accumulation and wastage are labeled in the figure below. For each of the scenarios described, use the descriptions of a glacier to determine how the climate is changing.Hint 1. Balancing the glacial budget in a changing climateKeep in mind that change in a zone is a matter of which process is occurring more rapidly: ice accumulation or melting. For example, if more ice in the zone of wastage is melting each year than accumulating, the volume is decreasing.Hint 2. Snowline migration due to climate change The snowline is the boundary between the zone of accumulation and the zone of wastage, meaning that the volume of ice increases above the snowline and decreases below the snowline over annual timescales. In a cooling climate, the zone of accumulation will