Though not particularly exciting to watch, Blue Glacier is always flowing. The snow a glacier receives in the accumulation zone constantly adds mass at the head. Because a glacier is a single sheet of ice, gravity forces this weight to flow down slope until it melts from the relatively warmer conditions at a lower elevation. Ablation at the bottom also allows more ice to flow down to replace what melted. The more accumulation a glacier receives at its head, the faster it flows and the more melting occurs.

Blue Glacier is about 100 meters thick on average and moves quickly. In the accumulation zone and near the terminus the glacier moves about 20 meters a year. Within the icefall the speed is much greater and can reach 300 meters per year, or three centimeters in an hour at full throttle. This speed is unrelated to whether the glacier is advancing or retreating—even when the terminus appears to move back up the valley the ice still flows downstream to remain in equilibrium.

The general movement of ice down the valley can be divided into two types of flow: extending and compressing flow. Extending flow occurs at the top of an icefall when the ice pours down a steep slope without resistance. Compressing flow occurs at the base of the icefall where the speed of the ice decreases and the glacier flattens out. To stay in equilibrium, the ice from above must push the lower ice down the valley, compressing it like a bulldozer. In general, extending flow is found in the accumulation zone, compressing flow in the ablation zone.

On Blue Glacier the physical movement of flow occurs by internal deformation and basal slip. Imagine an aluminum pipe inserted into a hole drilled from the surface to the rock in the middle of the lower Blue. In only a year’s time, the pipe would be both curved and a few meters farther down the glacier. The transport of the pipe is caused by basal slip, the curve by internal flow. In fact, one can find the remains of such pipe experiments scattered around the lower glacier.

Basal slip, the simple process of ice sliding across rock, accounts for over half of the glacier’s movement. Due to the intense pressure at the base of the glacier, a layer of ice a few centimeters thick melts. To slide over the rough and uneven rock surface, this thin layer melts from the high pressure on the up-stream side of an obstacle and then refreezes when the pressure decreases on the down stream side. The rest of the glacier moves as a flexible solid atop this conveyor belt. Less intense pressure—not enough to cause the ice to melt—creates internal flow as ice deforms under its own weight due to gravity. Deformation is actually the net result of millions of tiny movements in the crystal structure of the ice and is most prolific in thick warm ice under high pressure.

If we were to follow a grain of firn along its travels down glacier, we would notice a distinct path. If our grain fell as snow just above the firn line, it would be in the area of fastest flow, it would stay near the surface, and it would quickly melt as it entered the ablation zone. On the other hand, if our grain landed at the top of the accumulation zone, in the cirque for example, it would be buried by subsequent snowfalls and eventually reach the base of the glacier. The firn grain would travel slowly along the bottom and emerge after a long and difficult trip at the terminus. On Blue Glacier the icefall complicates this flow path, but the principle remains.


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