"he sees a glacier in action from below"). Here he saw that the glacier has holes filled with water, these pockets can contain up to about twenty litres of water. The commentator puts forth the hypothesis that these pockets are generated by the pressure of the ice mass above and that these holes make the glacier less rigid and hence move faster.
The beautiful thing about glaciers is that they are rivers of ice, really behaving like a liquid. The bottom meter (or so) is even filled with sediments (sand, gravel, stones) like they are picked up by the flow and then whirled into the ice. (We even have a word for these processes: glaciofluvial.)
A typical glacier will move several meters (up to about 30 meters) a day, with the bottom parts moving slower due to the friction with the bed. However, the enormous pressure in the bottom parts of the glacier, make the ice more fluid than that on the top layers. During the flow the top layers of the glacier will often crack, because this part (the 'fracture zone') is so rigid.
Now, I was wondering. May it not be possible that the pockets of water in the lower parts of the glacier are a result of a solid phase water being forced into the liquid phase by the energy the glacier has to absorb on its way?
I tried to find an answer on this wonderfully informative site: Martin Chaplin's Water structure and science (here: phase diagrams). however, I could not find the answer there easily. Probably, because most of the information on the site is too specialised for me (or maybe even just too difficult and complicated).
By the way, it seems that the water in these pockets can't be colder than -20°C; looking at the phase diagram from professor Chaplin's site.
What a wonderful place we live in!
Dear mr. Boers,
ReplyDeleteI remembered your blog from my times at the JvO and when I stumbled upon this problem I figured I might be able to offer some insight. Considering your last post is almost four years ago, I imagine you're not an active blogger anymore, but since I think it's an interesting question so here's a reply anyway.
Intuitively, I would say that this problem has to do with the fact that the density of ice is lower (!) than that of liquid water. This is because of the hexagonal crystalline structure the water molecules form. This configuration is generally energetically favored (because of hydrogen bonds) in atmospheric conditions. (More on this can be easily found on Wikipedia and the like.)
Now consider the existence of air bubbles in ice. How exactly these things come to be is a whole question in itself, but maybe more intuitive. For example if the top layer of some body of liquid water is frozen, air bubbles will form against the surface. While they move along the surface the encounter other bubbles and together form a bigger bubble, this bubble encounters another bubble and so on. Meanwhile the water around it freezes and we are left with an air pocket.
So here's what I think. The air inside these air pockets exert pressure on the surrounding ice and vice versa. However, these two will not be in equillibrium, i.e. the air pushes harder on the ice than it can push back. To achieve this equillibrium the water will want to go into the higher density phase, to lower the pressure. Ergo, the surrounding ice will melt. This process will continue till equillibrium is achieved and we're left with a water pocket. However, I might be oversimplifying thins, since II haven't done the calculation explicitly, but I imagine it can't be too involved, so you might want to give it a try.
Regards,
Danny Hetharia