Differential Erosion on Ice

March 20, 2020

Sawyer Glacier bergie bit_smMarch 2020 Viewer's Choice

Photographer: James J. Van Gundy 
Summary Author: James J. Van Gundy 

This approximately 30 foot (9 m) diameter chunk of ice was calved from the active front of South Sawyer Glacier, a tidewater glacier that sits at the head of the Tracy Arm fjord in southeastern Alaska. A house-sized chunk of calved ice such as this is often referred to as a bergie bit in distinction to smaller, piano-sized chunks called growlers and even smaller sized fragments that are referred to in the aggregate as brash ice.

This particular piece of ice has been rotated approximately 90 degrees from right to left from its original orientation on the face of the glacier. Ice from mountain glaciers is typically stratified with layers of dense clear blue ice alternating with thin layers of sediment and layers of white ice filled with air bubbles. These different layers absorb, transmit, and scatter sunlight to different degrees. For example, the blue-colored ice absorbs nearly all of the long wavelengths of visible light (red, orange, yellow).

Ridges and valleys have been eroded into the surface of this ice mass from differential melting by sunlight. Some layers absorb more solar energy than others and therefore melt faster. The resulting surface looks rather like a satellite view of the ridge and valley section of the central Appalachian Mountains, a landscape dominated by elongate ridges of resistant rocks and parallel valleys eroded into softer rocks. Although the two landscapes are very different in terms of spatial scale, time scale, materials involved, and erosive agents, the basic process of different rates of erosion on materials of differing erodibility remains the same. Photo taken on August 16, 2018.