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Icebergs form when glaciers sway in time to the rhythms of the sea

Among the factors that shape the growth of glaciers, scientists may also have to take account of what you could call the rhythms of the ocean waves. At least for glaciers with ice tongues extending offshore, the coupling between ocean swell and the natural oscillation modes of the ice seem to influence the calving of icebergs in the glacial cycle of growth and decay. Indeed, J. E. Glynn and G. Holdsworth of Canada's National Hydrological Research Institute at Ottawa think a glacial tongue will tune itself to the vibrational frequency of the waves just as a radio or TV set can be tuned to respond to broadcast signals.

The Canadian glaciologists are concerned especially with Antarctic glaciers which send ice tongues up to 100 kilometers out over the water from the coast or the socalled grounding line where they rest on the sea bed. Like the reed of an oboe or a ruler clamped on the edge of a desk, that ice tongue is fixed at one end and free to vibrate at the other. This, the glaciologists say, could explain why these tongues go through cycles of growth followed by breakup into large icebergs rather than calving off icebergs all the time. It could also explain why the iceberg break point does not usually come at the grounding line -- which seems to rule out tidal flexing as the cause of calving.

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The glaciologists have in mind a resonance effect, a phenomenon widespread in nature. When a forcing influence repeatedly acts on a system in tune with that system's natural oscillation frequency, it can produce a large response, as anyone who has pushed a swing in step with its natural rhythm knows. Not much happens if you push the swing with bad timing.

The same thing seems to occur with those ice tongues. Growing outward about 100 meters a year, the tongues change their resonant frequency as they grow. At some point, this frequency matches that of the ocean well. This is a critical point. The ice tongue has "tuned" itself to resonate with the swell. Large amplitude oscillations can build up, causing icebergs to begin breaking off.

McMurdo Sound in Antarctica may demonstrate this effect. The Erebus Glacial Tongue floating there broke up in 1911 and 1942. That ice tongue now has grown to a size where Glynn and Holdsworth estimate it has resonant modes with periods between 15 to 18 seconds. (A period is the time it takes for one complete oscillation.) Since the McMurdo swell has a period of about 16 seconds, the scientists believe the ice tongue is coming into resonance with the swell for at least some of the tongue's modes of oscillation so that a new iceberg epoch could start at any moment.

Glaciers are influenced by climate, by their underlying support, even by the cleanliness or dustiness of their surfaces. Now it seems that resonance -- the same basic physical process that lets you tune in the evening news -- also is involved.