medium · GMAT Verbal

Passage: The dynamics of glacial movement are governed by two primary mechanisms: internal deformation and basal sliding. Internal deformation occurs when the weight of overlying ice causes individual ice crystals to shift and rotate, a process that happens throughout the glacier but is most pronounced at depth. Basal sliding, by contrast, involves the entire ice mass sliding over the underlying bedrock, often lubricated by a thin layer of meltwater. In temperate glaciers, where ice temperatures remain near the pressure-melting point, basal sliding can account for up to 90 percent of total movement. However, in polar glaciers, which are frozen to the bedrock, movement is restricted almost entirely to internal deformation. This distinction is critical for predicting glacial response to climate change. As meltwater from the surface reaches the glacial bed through conduits known as moulins, it can dramatically increase basal lubrication. Recent observations in Greenland show that even polar glaciers are experiencing increased basal sliding as surface meltwater penetrates the ice. This suggests that the historical categorization of polar glaciers as movement-restricted may be increasingly obsolete, potentially leading to faster ice sheet discharge into the ocean than previously projected by models focusing on internal deformation alone. The passage mentions 'moulins' primarily in order to:

1. Illustrate a specific geological feature that distinguishes temperate glaciers from polar glaciers.
2. Detail the primary method by which internal deformation is measured in modern glaciology.
3. Identify the primary source of the hydrostatic pressure that causes ice crystals to shift at depth.
4. Contrast the rate of ice discharge in Greenland with the rates observed in Antarctic polar glaciers.
5. Provide a mechanism that explains how surface phenomena can influence movement at the glacial base.

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