Though it is the shortest line to the summit, the 300 meter-high icefall pouring onto the lower Blue Glacier is avoided by even the most seasoned mountaineers. An icefall’s movement is not as rapid as the name implies—it is not an avalanche—though by extending flow it does move about 15 times more quickly than the rest of the glacier.

The icefall on Blue Glacier results from the steep gradient of the slope and the confining rock features. A sheet of ice cannot deform quickly enough to smoothly cover such an extreme elevation change and so breaks into crevasses and seracs. It is these towering and unstable blocks of ice that convince climbers to find an alternative route; often large explosive sounds can be heard as house-size chunks of ice break off and collapse. The Khumbu Icefall on Mount Everest is a classic zone of treachery that climbers are forced to traverse.

Blue Glacier has two accumulation zones, the cirque and the snowdome. Each of these feeds the lower glacier and extends all the way to the terminus as a distinct stream. Before forming the lower reaches of the glacier, the two ice streams merge as they pass through the icefall. Consequently this area plays a significant role in shaping the structure of the rest of the glacier.

The ice below a fall is under high compressive force. As firn consolidates to ice, the resulting crystal size is determined by factors such as temperature and pressure. Glacier ice on the Blue is classified into three distinct types based on the size of the crystals and the presence of bubbles. Coarse-clear ice is usually bright blue and has large crystals. Coarse-bubbly ice is white and the most common as it makes up 90 to 95% of the lower glacier. Fine ice has small crystals and can be found along the margins and down the center. These ice types can be seen as variations in color in the photograph at right.

In the icefall the chaos of snow, ice blocks, and rock dust are consolidated and recrystallized. During this process the three ice types form distinct layers known as foliation bands. (The ability of ice to undergo deformation and recrystallization causes geologists to consider ice a metamorphic rock because they both behave similarly.) The predominant foliation bands on Blue Glacier are transverse and in the shape of nested spoons cupping up glacier. The two ice streams each have a separate set. Large-scale repetitive foliation patterns that form topographical swells are known as ogives and can be faintly seen below the snowdome.

In other parts of the glacier the foliation patterns are aligned down glacier, or longitudinally. Where the two accumulation zone ice streams are united at the base of a rocky rib in the icefall, intense horizontal compression between the two flows forms what is called a longitudinal septum (See the terms page). About 80 meters wide and extending vertically throughout the ice, this band of longitudinal foliation forms a foliation divide or center spine in the lower glacier. The predominantly fine ice found here is slightly dirtied as it contacts the rock in the fall, leaving a faint dark line marking the septum all the way to the terminus. Longitudinal foliation also occurs along the glacier margin from the intense pressure difference between stationary valley walls and flowing ice.



intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
pdf version | glacier glossary | bibliography | about blue ice

Benjamin Drummond 2002