Let's Talk About Crevasses—Deep Fractures in Antarctica's Ice
By Julian Spergel
As we prepare for our sixth flight of the season, I wanted to offer a glimpse of one of the types of glacial features that we are observing and studying as we map the Ross Ice Shelf: crevasses. The word sends shivers down the spine of anyone whose job involves working on a glacier. These cracks through the glacier can be hundreds of feet deep and hidden beneath a thin layer of snow. They are incredibly treacherous and have claimed the lives of many polar explorers and scientists. They also appear quite frequently in our sensor data as we fly our survey flights for Rosetta-Ice.
Crevasses are fractures in a glacier caused by the stresses of movement. They are like the cracks in the surface of clay as one pulls it apart past the limits of elasticity. They most often occur where the flow of a glacier increases, like in a steep deepening valley, and are thus oriented perpendicular to direction of flow. Crevasses that lie in a cross direction are called ‘transverse crevasses.’ There are also longitudinal crevasses that form parallel to ice flow and at an angle to the valley walls. These form as the glacier widens and the ice is pulled apart. These cracks in the surface of the ice shelf are an easily identifiable marker of areas of high stress within the ice flow. Mapping where crevasses appear is equivalent to mapping where the ice tensional stresses are highest.
On an otherwise featureless ice shelf, crevasses show where the different ice flows merge as they flow towards the open sea. Fahnestock et al. (2000) mapped crevasses, rifts, and glacial stretch marks they call “flow lines” on the Ross Ice Shelf in order to study the flow of ice from different glacial source areas to the Ross Sea and how these patterns may have changed in the past thousand years. From their study of the surface features, they were able to draw lines across the Ross Ice Shelf and identify whether a region of ice shelf was flowing in over the Trans-Antarctic Mountains from East Antarctica, or from the rapidly flowing ice streams from West Antarctica, named from southwest to northeast Ice Streams A, B, C, D, and E. A piece of ice takes about one thousand years to travel from the back of the Ross Ice Shelf to the front, and from its surrounding area we can tell where the piece of ice originated.
The Rosetta ice-penetrating radar shows us crevasses deep with in the ice. These cracks formed on the surface and were carried along and buried by centuries of snow and glacial flow. In the radar image, buried crevasses appear as thin arches. As the radar beam penetrates through the snow and ice like ripples in a pond, bouncing off of surfaces of changing density, the sharp corner of a crevasse scatters the ripples. When the radar image is processed from the echoes of the broadcasted signal, this sharp point of scattering becomes an arch descending down from an otherwise flat surface.
What do these buried crevasses tell us? Like digging to the bottom of a stack of papers on a desk, these ancient crevasses tell us of past glacial events. Their burial depth divided by the local snow accumulation rates gives an estimate for the period of stagnation required to stop flow, fill and bury the crevasses to the observed depth. They indicate past flow conditions, or even the locations of abandoned shear margins, where there used to be a boundary between ice streams moving at differing speeds.
Lastly, crevasses are interesting because sometimes fascinating things fall into them. A few studies have shown that wind-blown meteorites get caught in snow-filled crevasses. Knowing where to look for rare meteorites is a huge help to our friends in the astro-geology community. Crevasses are also the places where meltwater drains down to the base of the ice. This affects the slipperiness of the glacier’s bed, and can speed up its flow. Meltwater flowing through crevasses also widens the crack in a process called hydrofracturing. This can further weaken the structural stability around the crevasse, priming the area for a later break. While Rosetta-Ice is not specifically looking for extraterrestrial rocks or draining water, we are on the lookout in our radar data for anything that can tell us about the history or current flow conditions of the Ross Ice Shelf.
On another note, we’ve received questions from students at East Harlem School; here are answers to a few of them.
Is it possible for plants to grow in Antarctica?
Yes, a few. There’s a dozen native species that live on the Antarctic Peninsula, the thin peninsula of land that stretches north into relatively warmer parts of this continent. Everywhere else, only a few lichens (the crinkly stuff that grows on rocks and trees) survive.
How do you survive in the cold? What’s the hardest part about living/being in Antarctica?
With the right warm clothing and the right behavior, Antarctica’s conditions are very survivable. It’s very important to wear the right layers of clothes because you need to both stay warm, but also not let your sweat stay wet against your skin. I wear a thermal underwear layer that is warm and wicks sweat away. On top of that I’ll either wear a warm shirt or a thin sweater. On top of that, I wear a fleece or wool jacket, and then I wear my big red parka. Everyone has one and we call them our “Big Reds.” On my legs, I wear fleece pants and snowpants. I wear two layers of socks usually, and either my boots or the rubber boots that they gave us, called “Bunny Boots.” When I get too cold, I go inside, out of the wind, to warm up.
For me, the hardest part of living in Antarctica is the isolation. I personally use the internet a lot to connect to friends and family, but the combination of the slow internet connection and the time difference makes it difficult.
How has global warming affected how much ice will be there in 5 years?
We’re certain that the warming of ocean water is melting from underneath the floating ice shelves around Antarctica, and we predict that the warming atmosphere will lead to more melting and calving, but how much global warming-caused ice loss might there be within the next five years? There’s no way to know. What we still don’t know about how Antarctica’s climate works could fill a library. Weather over Antarctica is incredibly unpredictable, and we still cannot tell for sure how the multi-year climate cycles affect melting continent-wide. That question, how will global warming effect ice mass loss in Antarctica, is quite literally a multi-million dollar question. Thousands of scientists are studying every aspect of the Antarctic glacial system to get a sense of what is “natural” — what amounts of ice loss and gain are within the normal range of decades- or century-long cycles — and what can be interpreted as a result of human-caused climate change. Hopefully, Rosetta-Ice will yield a small piece of the puzzle.
For more information about Rosetta-Ice, check out our website and the archive of this blog. Have questions about Rosetta-Ice or about living and working in Antarctica? Feel free to email your question to firstname.lastname@example.org, and I will try to answer it in the next blog entry!
Julian Spergel is a graduate student at the Department of Earth and Environmental Science at Lamont-Doherty Earth Observatory and will be blogging from Antarctica. He works with Professor Jonathan Kingslake on analyzing spatial and temporal trends of supraglacial lakes on the Antarctic Ice Sheet using satellite imagery.