The Brunt Ice Shelf, sastrugi and crevasses

Halley is by far the strangest place where I’ve lived so far. Having an address in Antarctica is already weird enough but living in an ice shelf, floating over the ocean takes the thing to a different level.

That’s right, Antarctica this, Antarctica that and the truth is that we are about to hang around for more than a year, hoovering a few dozen of meters above the antarctic ocean.

What’s an ice shelf after all? Well, in a very simplistic way, an ice shelf is nothing more than a glaciar spewing out into an ocean.
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A glaciar is just a “river of ice”. It is an immense mass of ice that flows towards a certain direction due to gravity, just like a normal river, but since it is just too cold for it, the water is all frozen. But it flows anyway, only very, very slowly.

When this glaciar, again, just like a normal river, gets to the ocean, then you get an ice shelf forming because, since ice is lighter (less dense) than liquid water, the ice spreads over the ocean, just like a pair of ice cubes in a glass of ice tea.
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But this shelf is a tad different from other shelfs in the business. In our case, the Brunt Ice Shelf is formed by chunks of continental glaciar that “fall” into the ocean bellow. It was one of these chucks that sunk a certain intercontinental ship and provided a reason for drowning Leonardo DiCaprio so that Kate Winslet could keep the floating piece of wood all to herself.
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When the glacier reaches the edge of the continent, just like a strangely slow waterfall, it pushed itself over the edge until a heavy piece breaks off and falls into the ocean. When that happens and the temperatures are not that extreme, the chuck of ice floats slowly towards North, scaring people on ships and providing a crazy background for selfies on ships. But if that break off happens too low South wise, the ocean freezes again in hours or days and keeps this and other icebergs firmly stuck in place.
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But the continent never ceases on dumping these icebergs into the ocean. As such these ones are being pushed North, little by little, either by pressure of the ice behind, either by the natural flow of the ocean and ice that form in its surface. We then have these icebergs going North, very slowly and all stuck together by the sea ice. During that trip, and since we are in a very cold place, every once in a while it snows or there are wind bouts that carry snow from the continent or other places on top of our icebergs. Given enough time and this snow is able to fill every space between the icebergs and when this ice jumble gets to were Halley is, the shelf appears all uniform, pretty and ready to take with eight modules with 200 tones each on top of it.

This apparent uniformity its only valid when someone gets to Halley by air. In reality, the surface is actually ridden with small bumps. These small mounds of snow created by the wind are called “sastrugi”.SastrugiThese are a sort of small snow dunes. Since the wind around here is mostly from East to West, these things stay aligned in that direction. They don’t bother us that much during Summer. Its not that cold and the snow is soft enough to destroy one of these just by stepping on it. But in Winter things change in that regard. Though the snow is usually harder by then, the wind is still strong enough to rip the lighter layer of surface snow and create this constructions still. But it only takes a colder day after a blow to get completely surrounded of these booby traps. With the cold this things freeze in a night and become hard enough to make us fall face down on the ground while trying to step on it. Besides they make ski-doo traveling quite an adventure, specially if we are out and about in areas that didn’t see much action lately.PhotoAlbum-003Then it depends: if we travel along the East-West line, we are going in the same direction as the sastrugi. Our comfort depends in our ability in avoiding the bigger ones. But when we get along the North-South line, then we are going straight through these hard snow lines with very powerful and fast machines. The end result is normally a very sore set of butts and backs.Modules twilight - mkrzysztofowicz-20160424-_MK10920-_MK10924-ND750
But the Brunt Ice Shelf still has a surprise for us. Has it happens, during the filling up of those inter iceberg spaces with snow, sometimes the job ends up sub par and there are holes and caves hidden in the inside of the ice – these ones we call “crevasses”. There’s plenty of room for it also. This ice shelf gets to be hundreds of meters thick were it gets born, that is, near the continent, but it “stretches” as it moves North. By the time that we get to its edge, its thickness is around 40 meters tops. Yet, the further away from the continent we are, the more dangerous this crevasses become since they had more time to hid themselves under the snow. A crevasse is a hole in the snow surface that can either go all the way to the bottom of it, that is, all the way to the ocean or can be just a half a dozen meter deep hole full of soft snow and distracted penguins dead bodies. But the problem is that some of these crevasses are covered, sometimes with just a thin snow bridge, just to disguise the danger.
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And that’s why in Halley we have a Field Guide as part of the Wintering team and we cannot even leave the station’s perimeter without him, at least. The Field Guide is normally someone with plenty of mountaineering experience. BAS has the habit of hiring experienced mountain guides for this type of contracts. They are usually the people in the team more used to snow and all the dangers that lurk under it and with enough survival training to get along fine in remote and cold areas… like Antarctica. Since you can also find these crevasses in the Alps, Andes or any other mountainous range with plenty of snow, these people know what to look for when they end up in the middle of nowhere with a distracted scientist, loaded with GPSs and other expensive scientific gadgets.

In Halley, in the last days of Winter, when we are expecting the ship at any day, the Field Guides (sometimes more than one) go out and survey the route that the machines are supposed to use (bulldozers and tractors with 150 tones worth of cargo and sledges behind), looking for any crevasses that may have surfaced during Winter. The rule is to use the same route every year if possible but the shelf doesn’t rest, always expanding and twisting all over the place, which can create or move a crevasse, smaller that it may be, to that route or very close to it.

My first Summer was an example of it. During a routine maintenance visit to one of my GPS stations from the Lifetime of Halley project, we discovered a big crevasse just about 150 meters from the route that goes from N9 (the chosen docking point from the Ernest Shackleton for the last couple of years) to Halley.DSC01523In this case it was an authentic ice cave covered by a snow ceiling. Part of this ceiling collapsed, probably during a storm, and revealed the crevasse before anyone went over it unknowingly. From what we were able to see, the cave was not that deep but it was still a considerable fall in any case. The biggest danger would be for a heavy machine with a bored driver that went over it.DSC01504Just from the top of my head, I would say that it was able to withstand one ski-doo going over it fast. Two maybe. But I seriously doubt that a 50 ton tractor could survive the transverse.

In this situation, after carefully investigating the surrounding area properly and after we got sick of taking pictures and videos, the Field Guide signaled the area with red flags for danger. Though the crevasse is easily visible even from afar, in a day with poor contrast and with a tired and bored driver, it isn’t that hard to provoke an encounter with the bottom of the ice shelf. In this particular case, since the crevasse was quite near to the normal route, besides warning everyone back at the station of it, the Guide changed the route’s flag line, inserting a bend around the flagged area just for safety.
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Last Winter there was a fatality on the other side of the continent. An atmospheric scientist working from the North American station of McMurdo, with plenty of experience I might add, fell into a crevasse while investigating an area ridden with them. We also have something similar here. The Hinge Zone, which is a strip of several kilometers where the continent meets the ice shelf, is filled with crevasses and all sorts of ice holes, which at this point are nothing more than a bunch of icebergs before getting a few good meters of fresh snow on top.

And this is what makes a year in Antarctica so special. For one, I don’t have to worry about crossing the street, tropical diseases or pianos and anvils suspended from dubious ropes. But on the other hand I had to look closely where I set my feet down.

 

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