Why do glaciers move?
That’s the basic premise of my PhD research. The answer can be simple enough to include in a book for 6 year olds: ‘downhill under the force of gravity’. There isn’t much wrong with that answer, but obviously it’s pretty simplistic and would leave me 74,994 words short of a thesis…Glacial flow gets more complicated when we start to look at the exact mechanisms by which they move. Basically we can separate these into three: the ice itself flowing, the ice sliding over the ground beneath it, and the sediments beneath the ice flowing due the weight of the ice (subglacial deformation). Unfortunately it tends to be pretty tricky to observe exactly what’s going on at the base of a glacier, so our understanding of the last two mechanisms is lacking in quite a few areas.
That’s where the Glacsweb group here at Southampton comes in. Glacsweb is an interdisciplinary group involving glaciologists from the Geography department and web-scientists from ECS. The techie side of the group make wireless probes which measure a variety of different variables such as shear stress, temperature, orientation, water pressure. These probes are then popped them into the sediments under a glacier, so under about 80 m of ice, and we analyse the data that gets sent back from them. Its a pretty neat bit of design.
I think to the layman it seems bizarre that it is so difficult to get this kind of data, but not only are the stresses under a glacier huge, its also very difficult to communicate wirelessly from the base to the surface because the large quantities of water in the glacier. This means a lot of the time when the probe tries to send data to the base station that sits on the glacier surface it can’t make a connection. Because battery life is at a premium (there’s no way to get at the probes once they’re under the glacier) they can only try and communicate every now and then, so they have to be clever enough to work out when to save data that hasn’t sent, and what to sample from its instrumentation to make sure there isn’t an overload in the amount of data that needs sending. Basically the techie side of things is very complicated and I’m very thankful to the guys slaving away in ECS making sure the probes work! There have been a few problems in the past too, like the entire project falling into a lake because the glacier melted faster than expected…The current field site at Skalafellsjökull, Iceland, is nice and high up the glacier so hopefully that won’t happen again.
Coming back to exactly why glaciers move, aside from the weight of the glacier, water is the key variable. Although the glacier is obviously a frozen mass of ice, there’s lots of water about from the surface and the base melting. As it builds up in the sediments under the glacier, increasing porewater pressure, it effectively weakens the sediments to the point when they can be deformed by the weight of the glacier.
Although the raw probe data is really useful for helping us understand what happens in the subglacial environment as porewater pressure varies, I’m also doing some geotechnical lab work to further understand it. This involves replicating the processes occuring under the glacier in a ‘back pressure shear box’ in Durham University’s landslide lab. Shear boxes allow you to measure the force required to deform a material, and the cool thing about this ‘back pressure’ version is that you can also vary the porewater pressure. So the plan is to use the data from the probes on shear stress and porewater pressure variations under the glacier and replicate these conditions in the shear box. As far as I know its one of only two instruments like this, and certainly will be the first time the instrument has been used to study glaciology. Basing the experiments on actual data is also pretty groundbreaking (if sensible) as previously similar geotechnical experimentation has had to use theoretical values. One minor catch is that to do the testing I need to bring back three 30cm cubed samples of intact sediment from the glacier margin in Iceland. I’m under absolutely no illusions that this will be easy! Any tips from people that have taken similar size samples would be appreciated.
So all in all I’m getting to do some pretty fun and cutting edge research. The flip side to that is that cutting edge stuff has a tendency to go wrong. I’ve already had one experience of that as the probes that are going under the ice this summer were actually supposed to have gone under the ice last summer. Fingers crossed it all works fine this year…