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14.2: Glacier Movement

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    As the ice accumulates, it begins to flow downward under its own weight. In an early study of glacier movement conducted in 1948 on the Jungfraufirn Glacier in the Alps, glaciologists installed hollow vertical rods in the ice and measured the changes in movement over two years. The study showed that the ice at the surface was fairly rigid and the ice within the glacier was actually flowing downhill. Ice actually melts under pressure (one of the unique properties of water) so ice at the base of a typical glacier is actually melting. About half of the overall glacial movement was from sliding on a film of meltwater along the bedrock surface and half from internal flow [3]. These studies show that the ice near the surface (about the upper 165 feet [50 meters] depending on location, temperature, and flow rate) is rigid and brittle [4]. This upper zone is the brittle zone, the portion of the ice in which ice breaks when it moves. Large cracks along the top of a glacier are called crevasses. These crevasses can be covered and hidden by a snow bridge and thus are a hazard for glacier travelers.

    Deep cracks in the surface of glacial ice
    A person crosses over a large, deep crack in a glacier.
    Figure \(\PageIndex{1}\): Left: Glacial crevasses. Right: Crevasse on the Easton Glacier in the North Cascades.

    Below the brittle zone, there is so much weight of the overlying ice (typically exceeding 100 kilopascals-approximately 100,000 times atmospheric pressure) that the ice no longer breaks when force is applied to it but rather it bends or flows. This is the plastic zone and, within this zone, the ice flows. The plastic zone represents the great majority of the ice of a glacier and often contains a fair amount of sediment of various sizes from boulders to silt and clay which act as grinding agents. The bottom of the glacier slides and grinds across the bedrock surface and creates a zone of significant erosion.

    Cross-section of a glacier shows upper part of the glacier moving en masse and breaking in a brittle fashion while the lower part flows ductiley.
    Figure \(\PageIndex{2}\): Cross-section of an alpine glacier showing stress (red numbers) increases with depth under the ice. The ice will deform and flow where the stress is greater than 100 kilopascals, and the relative extent of that deformation is depicted by the red arrows. Down slope movement is shown with blue arrows. The upper ice above the red dashed line does not flow but is pushed along with the lower ice. The rate of ice movement is slow near the bottom, and fastest in the middle, with the top ice being carried along on the ice below. (Source: Steve Earle)

    This page titled 14.2: Glacier Movement is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher (OpenGeology) via source content that was edited to the style and standards of the LibreTexts platform.