What holds glaciers up?

If the glaciers and ice sheets that rest on top of Earth's continents could slide away into the ocean at once, they would cause sea level to rise by several hundred feet. The terminations of ice ages have witnessed sea level changes of similar magnitudes in the past, but most current rates of flow are reassuringly glacial. Important exceptions occur on the margins of Greenland and Antarctica, where ice streams and outlet glaciers slide along at breakneck speeds of up to several miles per year past less dynamic ice ridges that stick to more normal velocities that are 100 times slower. This curious behavior is of more than simple academic interest. The most recent IPCC report emphasizes that projections for future sea level rise have no representation of ice stream behavior beyond the naive expectation that they will simply continue to behave in the same way as is seen today, no matter how the conditions that surround them change.

The ice streams sit on top of porous sediments that contain water at pressures that are almost high enough to support the entire glacier weight – almost, but thankfully not quite. A small, but important part of the load is supported instead by intermolecular forces that act between the ice and the sediment grains themselves so that friction can help to resist even more rapid sliding. In a recent paper published in the Journal of Geophysical Research – Earth Surface (Paper), Alan Rempel outlines a mathematical model that accounts for the microphysics of these ice–particle interactions.

The theory demonstrates how the same forces that cause needles of ice to grow and lift up bark mulch and small clumps of dirt at the Earth's surface (see photo above and to the right) are instrumental in determining the fraction of ice stream weight that keeps them from flotation and firmly grounded on a layer of till instead. Further work is underway to examine how variations in the overall level of till support produce thick fringes of partially frozen till (see graph to the left) that might mark the boundaries between ice streaming regions and the more stagnant ridges alongside.