The ox is slow, but the earth is patient


First, about how glaciers turn into ocean water.

Consider this experiment. Take a large open-top drum of water and poke a hole near the bottom. Measure the rate at which water comes out of the hole. As the amount of water in the drum goes down, the rate of flow out of the hole will normally decrease because the amount of water pressure behind the hole decreases. Now, have a look at a traditional hourglass, where sand runs from an upper chamber which slowly empties into a lower chamber which slowly fills. If you measure the rate of sand flow through the connecting hole, does it decrease in flow rate because there is, over time, less sand in the upper chamber? I’ll save you the trouble of carrying out the experiment. No, it does not. This is because the movement of sand from the upper to lower parts of an hourglass is an entirely different kind of phenomenon than the flow of water out of the drum. The former is a matter of granular material dynamics, the latter of fluid dynamics.

Jeremy Bassis and Suzanne Jacobs have recently published a study that looks at glacial ice as a granular material, modeling the ice as clumped together ice boulders that interact with each other either by sticking together or, over time, coming apart at fracture lines. This is important because, according to Bassis, about half of the water that continental glaciers provide to the ocean comes in the form of ice melting (with the water running off) but the other half consists of large chunks (icebergs) that come off in a manner that has been very hard to model.

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