June 25, 2011

Accretionary Wedge #35: No Jive, it's Ogive

Gilkey glacier ogives, rimmed by medial moraines
(58° 49.280'N 134° 21.481'W)
Evelyn over at Georneys is hosting AW #35, and the bloggers of the geoblogosphere are submitting their favorite geology words. Coming up with a favorite geoscience word was tough. I thought of going for the comically crude, but none would have been my favorite. So I went from experience and what pops into my head first. On the rare occasion I've ventured across the top of a glacier, and when doing so my companions and I always played a game of 'spot the feature'. You had to get right the specific type of crevasse or moraine, and there were no points for pointing out firn or glacial ice. My greatest success at spotting a feature was straight off the chopper at the Gilkey trench in the Juneau Icefield. The Gilkey glacier had these strange alternating bands of light and dark crescents pointing westwards towards Berner's Bay. "Ogives!"

So what are these patterns on the surface of valley glaciers, and how do they form? Ogives are curved bands across the surface of a glacier, with convexity facing downhill. The bands are characterized by alternating dark and light groupings. The darker bands are devoid of ice-bubbles, are formed from melting & refreezing of ice in the summertime, and contain sediment accumulated at icefalls where open crevasses become a pit of deposition. The lighter bands are filled with snow & air bubbles from the non-summer months when precipitation is greatest, and a fresh snowpack acts as a layer of protection against weathering. Thus ogives are a seasonally created phenomenon. The crescent shape is due to velocity/friction differences between the lateral edges of a glacier where velocity is low & friction is high, and the center of a glacier where velocity is high & friction is low.

glacial flow lines relative to surrounding bedrock
Due to their darker color, the summer bands have greater conductivity to solar radiation, and thus are topographically lower due to increased melting. My experience traversing the Gilkey glacier was that the trough created is noticeable but pretty minimal, on the order of a 8-10 foot amplitude between a dark bands trough and a light bands crest. Interestingly, the combined width of one light + one dark band corresponds to the distance a glacier traveled in a year, thus it is a proxy element of glacial motion that can give a decent measurement of an advancing glaciers speed.
During summer, the glacier's surface melts and crevasses collect windblown particles, creating the dark band
During winter, the surface is covered with snow, protecting it from weathering and creating the light band
There you have it. Ogives! A wonderful pattern seen in some of natures freezers. The Vaughn icefall in the Juneau Icefield is as close to an idealized conveyor belt of the banded pattern you can get, but there are others. Soon I plan to visit Mt. Rainier, whose alpine glaciers are purported to have some of their own ogives.
A valley glacier replete with ogive banding, stemming from near Mont Blanc in the Graian Alps
(Credit goes to Sue Ferguson for the title of this post, an homage to her excellent guide book "Glaciers of North America: A field guide")

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