Regardless of a glacier's advance or retreat, glacial ice persistently glides down valley. Although the Juneau Icefield is at least 3,000 years old, the ice remains young because its steady flow perpetually renews itself through snowfall at upper elevations. Glacial ice at the terminus of the Mendenhall Glacier flows only 80 to 120 years on its twelve-mile trek to Mendenhall Lake. 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 - it flows almost like a huge stream of honey. The zone of brittle flow , the upper 150 feet of glacial ice, lacks this pressure and reacts inelastically 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 icefalls, which undulate down glacier.