Anatomy of a Glacier
Glaciers form where more snow falls than melts. A glacier's accumulation area, located at higher elevations, accrues a wealth of snow and ice. The ablation area, located at lower elevations, loses ice through melting (downwasting) or calving. A glacier's terminus or face advances when more snow and ice amass than melt, and it retreats when melt exceeds accumulation. When melt equals accumulation, a glacier achieves equilibrium and its face remains stationary. Whether the glacier's face is advancing or retreating, glacial ice persistently glides down-valley.
Coerced by gravity, ice pursues the path of least resistance. Ice depth and bedrock angle influence the rate of glacial flow. Glaciers contain two zones of ice flow. The zone of plastic flow, ice closest to the bedrock, experiences extreme pressure from the weight of the ice above and conforms to the anomalies in the bedrock. The zone of brittle flow, the upper 150 feet of glacial ice, lacks this pressure and reacts in-elastically to the bedrock features, forming elongated cracks called crevasses which fluctuate with the glacier's flow. Tubular chutes or moulins drain surface meltwater, and formidable spires of ice called seracs reach skyward. Ice plummets over particularly steep terrain creating ice falls. One theory suggests that differences in seasonal flow rates over an icefall create the convex bands called ogives at the base of the falls, which undulate down glacier. The erosive power of glacial flow changes the landscape and scrapes much of the soil and rock from the valley walls that channel its irrepressible flow.
The landscape around a glacier clearly illustrates the effects of Pleistocene and Holocene glaciation. Ice excavates the bedrock, forming bowl-shaped cirques, pyramidal horns, and a series of jagged spires called arête ridges that separate glacial valleys. As glaciers carve U-shaped valleys, rocks plucked from the bedrock and frozen in the ice etch grooves and striations in the bedrock. Rocks scoured from surrounding valley walls create dark debris lines called lateral or medial moraines along the edges and down the center of glaciers. Pulverized rock called rock flour, ground by the glacier to a fine powder, escapes with glacial meltwater producing the murky color of glacially fed rivers and lakes. Glacial recession unmasks trimlines, slightly sloping changes in vegetation or weathered bedrock on the valley walls that indicate a glacier's height at its glacial maximum. Meltwater transports glacially eroded material to the outwash plain, an alluvial plain at the edge of retreating glaciers. Icebergs break away or calve from the faces of glaciers ending in lakes or the ocean.
Cracked pieces of rock, plucked or torn from the bedrock, are carried with other debris in and on the glacier. This debris scrapes the valley walls and floors, leaving grooves and striations. Rock debris is crushed and ground into fine grains, called rock flour.
Did You Know?
Interglacial stumps can range from 250 to 10,000 years old. Some of these stumps are remnants of forests that predate the Little Ice Age and can help researchers understand the climate history of Glacier Bay.