Peering Beneath the Ice

Peering beneath the Ice

Anna Clinger—University of Michigan

Word was out. A large crevasse near camp had opened up and soon, a group of us would be heading out to explore it.  Armed with our harnesses and prussiks, we skied out to the site and began setting up anchors to belay down the openings. From a distance, it was hard to tell the difference between the crevasse opening and the rest of the snowpack. But this can be the scary thing about crevasses—you really have to keep an eye out since you might not realize just how close you might be….

   Ari tying her final safety knots with the crevasse opening several meters behind her. Photo by author.

Ari tying her final safety knots with the crevasse opening several meters behind her. Photo by author.

Much of our traverse has been spent zig-zagging around crevasses. Cautionary tales of slips and falls have been ever-present. During safety week at C-17, we learned to self-arrest and maneuver pulley systems. Knots upon knots were taught and tested. And then as we roped up to travel over Nugget Ridge and the Norris Icefall and during pretty much most of our treks in and out of camp, we kick and glide past these unassuming windows into a hidden world below. 

 Field of crevasses near our Camp 18 nunatak. Photo by author.

Field of crevasses near our Camp 18 nunatak. Photo by author.

While maneuvering the cracks has become a part of our daily life, it’s sometimes difficult to wrap our minds around the size and depth of a crevasse.  In lecture, we’ve talked about their formation in terms of glacier movement. The glacier is constantly moving, evolving, and deforming.  As the glacier flows down the valley slope due to its own weight, it travels over underlying rocks, between mountains, and around nunataks (exposed rock at the surface of the ice). The shape of the surrounding environment helps dictate the flow of the glacier and creates a path down the valley towards lower elevations. Each of these obstacles can provide resistance to glacial movement.

 

We can then characterize the movement by whether or not the flow is accelerating (extensional flow) or decelerating (compressional flow) which often depends on the thickness of the ice and the direction the rock under the glacier is sloping. During extensional flow, the bedrock slope underneath the glacier is getting steeper so the ice moves faster.  This process can cause the ice to break or fracture at the surface as the ice gets pulled apart. Inversely, if the bedrock slope is less steep down-glacier, compressional flow can occur, which essentially pushes the ice together and causes the surface ice to break apart. These fractures are the formations we’ve come to know as crevasses.

One by one, we took turns exploring down into the crevasse. I was one of the last to go and it was really strange to watch them slowly disappear into the snowpack for 15 to 30 minutes at a time but soon they’d lunge back over the snowpack lip and rejoin the group.

  Looking up, out of the crevasse. Photo by Lara Hughes

Looking up, out of the crevasse. Photo by Lara Hughes

Soon, my turn came around and, after a final double and triple check of my knots, I began to lower myself down into the hole. I didn’t realize how nervous I was until I noticed my arms shaking as I let more rope out.  I slowly made my way down and was soon hanging in mid-air, occasionally testing my weight on a sketchy snowbridge and scanning this new, unfamiliar world around me.   

 Drew all the way at the bottom of the crevasse. Photo by Lara Hughes

Drew all the way at the bottom of the crevasse. Photo by Lara Hughes

The most incredible thing was how illuminated the crevasse was. Light danced off the walls down onto the icy spires extending from below. The towers were twisted, interwoven, and warned me against testing my luck too far. As I tied my leg prussik, I attempted to balance between two drop-offs. Water was dripping everywhere. I zipped my rainjacket tightly and repositioned my rope which had become slightly forgotten in my state of awe. The crevasse extended much deeper than this year’s snowpits--I could see the layers of previous years’ snow, firn, and ice. They were lodged like history books hidden in the glacial walls. Books that help us better understand our icefield home which can seem so steady at the surface. But as our traverse continues, I’m learning more and more that we’re only just beginning to unfold its mysteries.

 Lara in a crevasse. Photo by Drew Higgins

Lara in a crevasse. Photo by Drew Higgins

And as I slowly began my ascent up, I couldn’t help but feel lucky to have this chance to explore the icy cavern but incredibly thankful to have helpful guidance (and a few strong pulls) from up above to carefully return back to solid ground.