The underwater geometry of an ice island grounded off Prudhoe Bay, Alaska was mapped using a narrow beam sonar. Vertical section profiles of the submerged portion of the ice island were obtained by lowering a sonar transducer through the pack ice at eleven sites surrounding the ice island.
The ice island studied had previously been ballasted by freezing sea water on its upper surface in an attempt to ground it more firmly. This was part of a research effort to investigate the feasibility of using grounded ice islands as offshore platforms.
The results of the sonar investigation showed that the ice island was sharply undercut in some places. At a later date, the ice island broke into a few fragments and was partially abandoned.
Ice islands, as they are known in the Arctic, are tabular (flat) - icebergs (Figure 1); their origins are quite different from the more common pinnacled iceberg shown in most photographs and more commonly associated with Arctic waters. The ice islands calve (break off) from a few glaciers which float out over the water; the resultant buoyancy allows the pieces to be quite large before breaking off. In contrast, the pinnacled bergs (most of which come from western Greenland and float down Davis Strait and into the North Atlantic) break off from glaciers that end on land; the abrupt fall of the shore at thewater's edge and the height of the underlying land does not allow the tongue of the glacier to float. It breaks off into smaller vertically oriented pieces which usually fall a good way to float in equilibrium. Ice islands have been used for scientific stations; among these were an Arliss II, which broke up in early 1965 in the North Atlantic, and T-3, which is still occupied.
Occasionally these islands ground and then, because of resulting stresses break up. Recently, in 1968, an island estimated to be a few miles in extent, grounded off the north coast of Alaska, scattering fragments approximately on the 15 fathom depth line along a distant of over a hundred miles (Figure 2). Most of the pieces were about 120 feet thick, and averaged an acre in extent.
These islands, massive and strong as they are, can withstand the action of pack ice and resist its forces. Often impelled by subsurface currents (their 100–200 foot thickness is an order of magnitude greater than most pack ice) they can and do move at a different direction from pack ice, munching it up in a malevolent fashion (Figure 3). Pack ice appears to move as a result of forces generated by wind, surface currents, barometric pressure, shore contour and the rotation of the earth.
The massiveness and strength of these islands makes them ideal for stable platforms in the Arctic. Grounded, they are hard points, resistant to most pack ice action, and could be used for moorings, offshore platforms and the like.
The ice islands off Prudhoe Bay, Alaska were the subject of several scientific efforts.
