Into the ice
We learn a lot about the ice by sampling, drilling ice cores and reading radar profiles. That is not always enough. Sometimes we just have to take a walk inside it.
Location: Fimbulisen, 70º 40’ S, 5º 9’ E, 70 metres a.s.l.
Weather: All clear, -16 C, wind 8 kts
Troll Station has its “port” on the Fimbulisen ice shelf 300 km north of the station itself, and the transport route between the station and the ship unloading site crosses the grounding line. This is where the inland ice sheet meets the sea and transitions to a floating ice sheet. The area is heavily crevassed, and over the years a lot of work has been put in to create a safe crossing. A route through the area was found by an initial helicopter survey some 20 years ago, recent crevasse radar surveys have been carried out to check the size and development of crevasses, and snow bridges have been constructed and maintained. But the ice moves seaward by some 50 metres per year, and the area is constantly changing.
Since we do have a crevasse detecting radar along (see the diary for December 31), three of us stopped for a couple of days in this “hinging zone” while the rest of the team continued down to the ice edge to meet the ship. The route across the grounding line was manually surveyed and marked before the summer season, but given the opportunity we wanted to do a radar verification of the crevasse pattern and the snow bridges. Thus we spent those two days in the area, driving the crevasse radar up and down along and around the track.
It has been a snowy summer, and currently the hinging zone has a fairly thick cover of fresh snow. Nevertheless, locating the crevasses with the radar is straightforward. We could easily check that the crevasses are indeed where we expect them to be and nowhere else. Usually it is also fairly easy to see the width of the crevasses on the radar screen. But the tricky part is to see the structure and thickness of the snow bridges. Are they thick and solid enough to carry a heavy vehicle and container sledges across the gap?
Mostly they are, by a wide margin, but there are always reasons for doubt. The crevasses have an abundance of natural features inside – irregular ceilings and walls, remains of old, collapsed snow bridges, huge snow mushrooms blooming right off the interior walls, stalactites suspended from the ceiling, gaps and abysses of all shapes. Some of the constructed snow bridges have been undermined by snow collapsing deep down, and some of them have substantial gaps forming between the filled-in snow and the crevasse walls, as the ice is always on the move.All of this tends to confuse the radar image and makes it difficult to interpret what we are seeing. There is only one way around the problem: We have to open the crevasses to see their inside form, gauge their dimensions and thus verify what we see on the radar.
The effort is time-consuming, but has its rewards. Descending into the eerie, blue stillness of a huge crevasse is an awe-inspiring experience. The air is completely still, all sounds are muffled, and nothing can be heard from the surface. The diversity of forms, a variety of surfaces of shapes, labyrinths of ice and snow, inch-size ice crystals, gaps and cracks leading down and away into darkness – it is a scenery that never ceases to fascinate and spellbind. But above all: Crevasse anatomy is sheer fun!
Kirsty going for a walk in the crevasse we have come to know as “H8”. Photo: Stein Tronstad/NPI