takes its gross shape from the properties of the ice shelf or ice tongue from which it derived and the deformations that liberated it as a distinct ice mass from those terranes. The floating ice sheets come under the influence of waves, tides, and the internal flow that results from thinning as land-based ice spreads out across the sea. Infinitesimal cracks preserved in the ice fabric propagate, and large inherited fissures persist. New stresses, particularly near the exposed terminus, develop into new fractures. Seawater etches along crystal boundaries, and brine infiltration along cracks and within the firn lowers the strength of the ice dramatically. The processes involved may be complex and competitive, but the composite mechanical effects of wind, tide, and wave cause exposed ice to bend and fracture, fatigue and fail. The calved ice mass becomes an iceberg.
The iceberg accelerates the motions, and the shaping, of its ice terrane. The ice mass is subjected to a new range of external stresses from wind and wave, as well as new internal stresses resulting from its recent liberation from a confining ice field. Its inherited structures are quickly reworked. Previously, the ice mass slowly accumulated or, at the least, it was conserved. Changes in form resulted from the rearrangement of ice. Now the ice is shed. Processes other than those characteristic of confining ice begin to operate, and the berg breaks up and melts. The entire mass, not only its internal fabric, is restructured. The shapes become more visible and vastly more exotic, the rate of change accelerates, and the ice reverts to water. The most interesting period in the ice shaping is also the most transient. In Antarctica, ice begets ice. The ice—its composition, shape, movements—exists because of its informing ice field. When the ice field breaks up, the ice mass loses its identity as ice.
The process of disintegration is both mechanical and thermal. Each aids the other. Melting begins while the ice mass remains within an ice shelf or glacial tongue. Some melting scours the exposed ice front, some scours the top, and some scours the floating bottom. Along the bottom there will be both melting and freezing. But if the freezing predominates before the ice mass calves, it will soon be superseded by melting once the berg takes to sea. Top melting will result in the percolation of meltwater through the porous firn. Side melting, especially active when accompanied by waves, carves out the flanks, and the berg acquires a scalloped texture. Small convective cells eat away at the sides; the released fresh water and air bubbles form a turbulent boundary layer along both ice and seawater and rise through a series of thermal terraces. These carved subsurface facets expose yet more ice surfaces and encourage still faster melting. If sufficient meltwaters accumulate on the top, they may flow down the sides to give the berg a fluted appearance. But the roughened bottom, gouged by large crevasses, disintegrates most rapidly. The fissures widen and rise; the berg thins, making it more susceptible to wave-induced flexing; and the berg itself calves.
Of the two processes, melting ultimately triumphs over mechanical disintegration. Melting prepares the ice mass for ruptures, large and small, and unlike breakage it reduces the volume of the ice. Melting is the final solution: ice is no longer reformed into ice but transformed into water, a change of state that will remove it from the ice field completely. Yet the mechanical processes assist melting by increasing the proportion of the total ice that is exposed to thermal activity. Some ice spalls off the sides. Some is mechanically eroded by waves—melting into exposed pinnacles that quickly rot and into terraces that, as overhanging cliffs, soon fail and drop. The thinning of the berg encourages rupture by allowing the ice mass to flex amidst long ocean swells. Some disintegration follows from simple collision, especially where a grounded berg is struck by a free-floating one. Grounded bergs, in fact, are a prominent source of brash ice. Differential heating—the sharp contrast between cold ice and warmer sea—can lead to thermal spalling, with chunks and slabs of ice breaking free like exfoliating granite and sandstone. The permeability boundary between firn and glacial ice, a zone of potential penetration by brine, may lead to the large-scale slumping of firn, a process of mass wasting.
The marks of the strain that produced calving will persist for a time, although fissures will slowly heal shut, and some of the fractures may become zones of weakness for further mechanical disintegration as the berg experiences a new set of stresses. The intensity of this activity will vary with the size of the berg, and it will be reflected in the berg’s shape. Gigantic bergs—with dimensions measured by tens of kilometers—will not undergo much internal change. Only the edges of the berg will be affected. Smaller bergs, with higher proportions of newly exposed edges, will show proportionately greater change. No longer subjected to a high confining pressure and no longer protected by an enveloping shield of ice, the sides of the ice mass will ablate rapidly.
The berg’s motions, too, are a curious amalgamation of its history and its present state. All the movements that have characterized the ice mass on its journey are present. Past motions are preserved in the internal ice fabric. Current motions are revealed by the gross movement of the berg, as many former stresses vanish and new stresses appear. Its free-floating motions give the berg many of its distinctive characteristics. Unlike other ice masses, the berg will not merely flow internally, within the confines of the rigid ice field, but will respond more or less freely to its new environment of fluids, the sea and the air. The berg will drift in ocean currents and wind fields. It will bob, rock, and spin. It will tilt or even overturn as erosion modifies its density profile. From the simplest of motions, that which governs the settling and compression of snow, the iceberg has acquired an almost limitless mobility. The price paid for this mobility is disintegration. Time itself accelerates; events crowd one upon the other; the more rapidly the berg moves, the more swiftly it decays. The ice began in a nearly timeless state, 15 prolonged over centuries, even millennia, because the extreme cold of the source region slowed movement to a vanishing point. But as the ice acquires composition, shape, movement, variety on its journey outward, it correspondingly wastes away. The smaller the berg, the greater its mobility and the faster its disintegration.
Very large tabular bergs are the least mobile and show the greatest persistence. They cling to the shore, grounded or entrapped in pack ice. The heavy concentration of large bergs near the coast contributes to the preservation of a wide belt of fast ice and shore ice even during the summer. Only when they break up into smaller units and proceed through the pack do bergs respond freely to wind and wave. In broad terms, their drift is set by ocean currents. Because of its large draught, its keel, the berg behaves as a current integrator, sailing roughly eastward with the nearshore flow of the Antarctic circumpolar current and ultimately breaking free of shore influences to enter the broad west wind drift out to the Antarctic convergence. There are considerable variations, however.
The act of breaking loose from the coast is typically not a single event. Several years may pass before the berg actually becomes a free-floating vessel. Frequently, the berg grounds as it attempts to sail from the continental shelf or to pass around peninsulas of ice or land, or as it encounters eddies within the current that trap it temporarily in a frozen gyre of bergs and pack ice. From its first calving the berg may be recaptured several times by the ice field—to be refrozen, reincorporated, then liberated again. The vast majority of large bergs hug the coast, and most of the bergs that survive many years do so because they move ponderously out from the ice shore. The average longevity of an Antarctic iceberg is four to six years. The colossal Trolltunga berg, however, was tracked for over eleven years, slowly creeping along the frozen shoreline of Queen Maud Land before it was absorbed within the Weddell gyre, where it spent more than two years before being catapulted into the South Atlantic. Once it enters more or less open ocean, a berg advances an average of 8–13 kilometers a day.
Other influences shape the actual drift track of the berg. The Coriolis force, pronounced at high latitudes, gives a small northward component to the berg’s circumpolar drift. Tides work to lift grounded bergs and to move others, while winds shape considerably the local pattern of drift. How influential winds become depends on the strength of the wind, the size of the berg, and the proportion of sail to keel. High winds (over 50 kilometers per hour), small bergs, and large sails make for responsive ice. Even large bergs will be sensitive to very high winds and the waves they generate. But the storm cells that orbit the continent follow the general trend of the Antarctic circumpolar current. While, in the short term, wind and current may compete, in the larger scale of
