What does the word flow bring to mind? Maybe a river? Water coming out of a sink? What about pouring syrup out of a jar? If it didn’t already, ice sheets should also come to mind. Ice isn’t stationary, it slides! Here’s maps of surface ice velocity for both Greenland and Antarctica.
Take a minute to really look at these maps. The first time I saw maps of velocity for Greenland and Antarctica I was surprised by what I saw. You might guess that the velocity would be fastest near the ice margins (the edge of the ice where it meets the ocean) and progressively slower towards the middle of the ice sheet. This is sort of the case, but there is a lot of variability. Do you notice the very fast flowing areas that resemble streams within the ice, almost like veins? Fittingly, glaciologists call these “ice streams”. Some of these streams start very far in the interior (near the middle of the ice sheet). While it’s all ice, these streaming areas move orders of magnitude faster than ice adjacent to the streams. Remember, you can see these streams when looking at a velocity map, however if you just look at a picture of an ice sheet from space (see below), you can’t really identify where the ice is moving fast and slow.
What dictates exactly where and how these ice streams develop is not entirely known. Some of it has to do with topography of the bed below the ice and some has to do with how slippery the bed is. What’s important, is that these ice streams are like a conveyor belt transporting ice to the ocean where it eventually melts. Warming the ice sheet only causes the ice to flow faster, contributing to sea level rise.
If you’re curious to look more closely at ice flow, take a look at this video showing just how dynamic the Antarctic ice sheet is.
Here’s a thought to leave you with. What causes the ice to flow this way? Why doesn’t ice sit complacently at rest, like a pile of sand or rocks? On the other hand, why does an ice sheet last at all? Why doesn’t the ice immediately flow off the land like water would? I’ll give you a hint, this has to do with viscosity. Let’s pick this idea back up next time.