"When an icefall falls to pieces, it's with big chunks of ice," said the University of Nottingham's Barbara Turnbull, a fluid dymanicist with a special research interest in ice and snow. "So it's more like a rockfall."
But even without that lubrication, the house-sized blocks of ice in the Khumbu Icefall are primed to fall because the glacier itself is buckling and turning over steep, uneven ground. This creates constant stress in the ice, which means avalanches have a hair trigger.
Primed as it is to fail, there are at least a couple of things that set an avalanche in motion in an icefall, Turnbull explained. One trigger, also seen in snow avalanches, is people. As small a thing as a person tramping on the surface of such an unstable area can be enough to compress and shift some layers of snow or ice, causing a small amount of ice to melt, and the avalanche begins, Turnbull said.
Another very well known trigger is the sun. When it rises in the morning and shines on the glacier, the sunlight starts melting some of the ice. That meltwater percolates down and weakens the glacier, making it even more unstable. This is also a common trigger for snow avalanches, Turnbull said.