Crevasse Orientation with Respect to Flow Velocity

Donald Jarrin

Colorado Mesa University

Here on the Juneau Icefield we have many hazards we must overcome on a day-to-day basis. These hazards range from hypothermia to snow blindness, but some of the biggest hazards we encounter are crevasses. These features are found across the glacier, and in this blog I will describe how ice flow velocity dictates crevasse orientation.

For the sketches here I will use the following color scheme: blue for the glacier, brown for the valley walls, yellow for glacier flow direction, green for lateral extension, red for zones of friction, and black for crevasses. It’s important to understand that there are arrows showing flow direction and speed, but there is an overall flow velocity throughout the glacier. The middle of the glacier flows more quickly than the sides of the glacier. There is friction from the valley walls which is causing the crescent shape in the flow velocity.  In all of figures (see Figure 1 below) we look down on the glacier from a bird’s-eye view.

 Figure 1

Figure 1

Tension

In general, when two things are pulled apart this causes extensional stress (tension). On glaciers extensional stress occurs when faster moving ice is followed by slower moving ice, as seen in Figure 2 below. Because the stress acting upon the ice is extensional and oriented down glacier, the crevasse forms perpendicular to the flow direction.

 Figure 2

Figure 2

Compression

 Figure 3

Figure 3

The opposite of extensional stress is compressional stress. Glaciers experience compressional stress when ice with a faster velocity is uphill of ice with a slower velocity. The compressional stress creates a lateral extensional stress as the ice spreads to accommodate the change in volume. As the ice flows outward laterally, crevasses form parallel to flow. This results in crevasses in the exact opposite orientation to the previous extension method. This process is shown in Figure 3 above.

Shear

 Figure 4

Figure 4

Shear stress is a little different from the two previous forms of stress. This is because shear stress is a product of both the ice flow and friction against the valley walls. As the ice grinds against the valley wall, the rest of the glacier moves at a more constant rate. This yields a crevasse orientation 45 degrees up glacier from the valley wall. Figure 4 above shows the flow direction of the ice at the shear zone, and a single resulting crevasse. Figure 5 below shows what the crevasses would look like as the glacier moves down valley and encounters friction on both sides of the glacier.  

 Figure 5

Figure 5

Terminus Crevasses

 Figure 6

Figure 6

The last type of crevasses are terminus crevasses. Once the glacier has made its journey down the valley is has the potential to form one last set of crevasses. If the glacier terminates in a broad flat area it can form a fan-shape toe (terminus). If this happens, the glacier begins to widen, thin, and create new crevasses that are sub-parallel to the original direction of flow- down the glacier. Figure 6 above shows that when the toe begins to expand the flow outward, lateral extension pulls the ice apart(shown in green). This is similar to the compression figure above but in this figure the ice is flowing down and outward to the lowest possible elevation.
    
Though crevasses are a major hazard to anyone who traverses any icefield, as well as being sobering reminders that glaciers are always in flux, they are also valuable teaching tools for looking at ice velocity and overall glacier movement.