Water ice is a curious material: though apparently a solid, it can nevertheless flow, bend, and wrinkle. The mile-thick Antarctic ice cap is very gradually flowing out to the edges of the continent, through the mountains in glaciers, and when those glaciers leave their channels they spread out like hot fudge (only much slower).
Most of the time, though, what ice does is crack. In a glacier this makes a crevasse; on sea ice, it’s a short trip into some very deep, very cold water. When you’re travelling across the sea ice, it’s the cracks you need to worry about, and once you start looking for them they are everywhere. This would be enough to put off a cautious traveller but most of them are relatively harmless: the requisite sea ice training helps you determine just how risky your movements are.
First, you have to learn about how sea ice forms. You will not be surprised to learn that this happens when liquid water meets cold air.
The nice flat young sea ice is never allowed to let be, though – forces in the water underneath and air above, or even changes in temperature, put strain on it, and to release the tension it cracks.
Like a broken bone, or a fault line in rock, once ice has broken it is more likely to break again at the same place, so being aware of cracks will show you where you ought to be extra careful. Simple cracks are usually one-time releases of stress, and once refrozen rarely pose a risk, but you need to be wary of working cracks because they have shown they are unstable and their condition might change without notice.
Because they are much larger, working cracks tend to be more visible, sometimes even when the sea ice has a layer of snow on it. Pressure ridges show up very well and are a useful indicator that the ice has been moving around. Here is a very small pressure ridge just off McMurdo Station:
The snow blowing across the sea ice has been caught and drifted by the protrusions of the pressure ridge. When you have an recessed working crack, instead of a pressured one, the crack gathers the snow and shows up that way.
When you come across one of these, you have to find out what is going on under there before you know it’s safe to cross, so the following procedure is taken:
Here is the crack you saw earlier, with its snow-covered secrets revealed.
Each vehicle has different parameters for what a ‘safe’ crack is. Most tracked vehicles – snowmobiles, Hagglunds, etc – can safely cross a crack whose width is 1/3 the length of their track. So, if your snowmobile’s track is 90cm long, you can cross a 30cm crack even if it’s open water. (I would balk at doing it, myself, but it is theoretically safe.) You measure the maximum width of the crack to see if it’s within your vehicle’s safe crossing threshold. If it isn’t, it might still be safe to cross after all, but you will need to do some more measurements to find out.
Measuring sea ice thickness is great because you get to use a) a power auger and b) my favourite device in the Antarctic toolkit.
You drill either side of the original crack to make sure the ice – which is weakest here, remember – is within the specified necessary thickness to support your vehicle. You don’t have to memorise this number because your teacher has given you a laminated card with a table of all the vehicles and their requisite ice thicknesses. Your job is not to lose the card.
Once you’ve ascertained that the ice either side of the widest crack is safe, you find a point within the safe crack width for your vehicle and measure the ice there. So, using the example from before, if your snowmobile with 90cm tracks comes up to a 45cm crack, you cannot cross, so you find a point 30cm (the safe width) from your side of the crack and measure the thickness of the ice there.
If the ice at the safe width is thick enough for your vehicle, then that becomes the effective width of the crack, and you can cross over. The actual width is the objective total width of the crack, whereas the effective width is the width that matters to you practically.
The reason you have to drill again at (3) instead of just subtracting the change in surface elevation from the measurement you took at (2) is because of the irregularity of the underside of the ice. You don’t know if there will be a big lump of nice solid ice there, or if it will be a surprisingly thin patch, so you have to check. For this same reason, you can’t measure a crack in one place and cross in another, because conditions along the crack can change completely in just a few metres.
Part of the job of the Sea Ice Master* at McMurdo is establishing ‘roads’ for vehicles to take to regular destinations (e.g. Cape Evans, the seal camp at Turtle Rock, Penguin Ranch, etc). Usually the ice is thick enough that these roads can go more or less directly there, but this year the ice formed so late, and in some cases was so choppy, that the roads snaked and dog-legged all over the place to find safe crossings. On top of that, the early summer had been exceptionally warm, and there was a great deal of nervousness around just how long the sea ice would be traversible at all. Luckily there was a cold spell just before I arrived, which bought enough time for me to travel to Cape Evans. In my last few days at McMurdo the sea ice did finally get closed to all traffic and the flags marking the roads were brought in, so it was a close run thing.
*not the actual job title, but should be