Ogives:Glacial Masterpieces
Joel Wilner, Middlebury College
If you’ve made it to this website, chances are you’ve seen a picture of the dramatic Gilkey Glacier and its ogives – curving bands of ice on a glacier that alternate from dark to light. Also known as band ogives or Forbes bands, ogives (pronounced oh-jives) are among the planet’s most extraordinary natural phenomena in terms of both aesthetic quality and scientific intrigue.
On yesterday’s long traverse to Camp 18 from Camp 10, my trail party was stuck in a whiteout for most of the trek. Today, though, the skies cleared and we were afforded an astonishing panoramic view of the Gilkey Trench, the Vaughan Lewis Icefall, and the Gilkey Glacier’s famous ogives. The sudden sight of the ogives is admittedly overwhelming, particularly after a “ping-pong ball day” in a whiteout.
A close-up view of the arced bands of ogives below Camp 18. Photo by author
Scientists have proposed several different ideas about how ogives form. However, there are certain things about ogives that we know for sure: 1) all ogives form at the foot of icefalls (icefalls are jumbled, chaotic regions of a glacier in which ice moves much faster than elsewhere in the glacier, usually because of a steep slope), although not all icefalls create ogives; 2) the ogive bands begin as a series of bumps on the surface, sometimes five meters tall initially, but eventually flatten out; and 3) each pair of dark and light ice bands in an ogive system usually corresponds to one year of a glacier’s movement. An early idea proposed that the ogives are formed by pressure waves, just like pushing a spoon through a bowlful of thick honey. This idea theorized that as the icefall slides faster each summer than it does in winter, the speed increase causes the icefall to compress the ice below, gradually forming annual waves in the ice.
The Vaughan Lewis Icefall (bottom left) and the ogives (right) it produces. Photo by author.
However, some scientists have seen holes in that theory, arguing that compression force alone cannot fully explain the bands, so a second theory was studied and proposed. This second theory contends that since ice speed is far greater in an icefall than elsewhere in the glacier, ice stretches as it enters the icefall, similar to water stretching as it flows over a cliff into a waterfall. As a result, the surface area of any ice that enters the icefall increases. This means that in summer as ice from the icefall melts, much more ice melts at a time than anywhere else on the glacier. More melt means that more debris and dust that was stuck inside of the ice is revealed, creating the dark troughs of the ogives. Ice that spends winter in the icefall is able to accumulate more snow and reveals less debris, emerging from the icefall as the light crests of the ogives. Glaciers flow faster in the middle, so the bands are shaped into arcs.
So, why do some icefalls produce ogives and others don’t? Scientists speculate that it again has to do with stretching at the beginning of the icefall. In order for the right amount of ice to be stretched, it needs to travel through the zone of rapid stretching in a short amount of time – six months or less. If it takes much longer for ice to move past the onset of the icefall, the stretching will be off and the icefall won’t generate ogives. Many different factors can affect this, including steepness and climate conditions.
I feel incredibly fortunate to be able to observe these rare, unique natural wonders firsthand. Sometimes, we forget that nature isn’t all just random disorder. From the shattered shards and chaos of icefalls emerge these works of art, with remarkable regularity and precision. Nature is indeed an artist, and in her chaotic ways she paints masterpieces.
The Vaughan Lewis Icefall. Photo by author