Chapter 18: Glaciers and Glaciation

Glaciers form through the progressive transformation of snow into firn and finally interlocking crystalline ice, accumulating on land in thick masses that persist year-round. The chapter distinguishes between valley glaciers confined to mountain regions and extensive ice sheets covering Greenland and Antarctica, along with related forms such as ice caps, piedmont glaciers, and outlet glaciers. Glacial ice dynamics involve plastic flow within the ice mass and basal sliding facilitated by meltwater, with the upper brittle layer generating crevasses where ice velocity changes. The glacier mass balance, determined by the interplay between accumulation from snowfall and ablation through melting, sublimation, and calving, controls whether glacier margins advance or retreat, though ice always flows downslope regardless of margin position. Glacial erosion operates through two primary mechanisms: plucking, which removes fractured bedrock blocks, and abrasion, which smooths surfaces and produces fine sediment that creates characteristic turquoise-colored glacial lakes. Alpine glaciation generates distinctive landforms including U-shaped valleys, truncated spurs, hanging valleys, cirques and tarns, arêtes, and horn-shaped peaks, while continental ice sheets create subdued streamlined features such as roches moutonnées. Glacial deposits, collectively termed drift, consist of till deposited directly by ice and stratified drift sorted by meltwater, producing landforms like moraines, drumlins, outwash plains, kames, and eskers. Beyond local topography, Quaternary glaciations profoundly altered Earth's surface through isostatic crustal adjustments, dramatic sea level fluctuations exceeding one hundred meters, and disruption of drainage patterns that created modern lake systems. The chapter concludes by synthesizing glacial theory and ice age mechanisms, including plate tectonic positioning of continents, orbital variations described by Milankovitch theory involving eccentricity and axial tilt, and climate feedbacks involving atmospheric greenhouse gases and surface albedo changes that amplify temperature fluctuations over geological timescales.