The Ice. Stephen J. Pyne

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      Antarctica, showing major physiographic provinces. Redrawn, original courtesy National Science Foundation.

      The Ice is, in turn, a constituent of an ice regime broadcast throughout the solar system. Pluto is an entire ice planet; the satellites of the outer planets are ice moons. On Mars and Earth, there are polar ice sheets; on some asteroids and amid the rings of Saturn, ice debris; and in the form of comets, interplanetary icebergs from the Oort cloud. The Earth’s cryosphere joins it to other worlds and other times—to the outer solar system and to vanished geologic pasts. It is a white warp in space and time. That the Earth’s ice consists not of ammonia or carbon dioxide or methane but of water, that it is crystalline rather than amorphous, and that the planet’s temperature range falls within the triple point of water account for the Earth’s uniqueness, and dynamism, as a member of the ice cosmos.

      The berg contains a record of all this. Its travels have a mythic quality, a retrograde journey out of an underworld. It is a voyage that joins microcosm to macrocosm, that builds from a single substance—ice crystals—a vast, almost unbounded continent. Yet a descent to this underworld—from the ice-induced fog that shrouds the continent to the unblinking emptiness that commands its center—does not lead to more splendid scenes, as a trip through the gorge of the Grand Canyon does, or to richer displays of life, as a voyage to the interior of the Amazon does, or to more opulent civilizations, living or dead, as the excavation of Egypt’s Valley of the Kings or the cities of Troy does, or to greater knowledge, as the ultimately moral journeys of Odysseus, Aeneas, Dante, even Marlowe do. It leads only to more ice. Almost everything is there because almost nothing is there.

      Antarctica is the Earth’s great sink, not only for water and heat but for information. Between core and margin there exist powerful gradients of energy and information. These gradients measure the alienness of The Ice as a geographic and cultural entity. The Ice is profoundly passive: it does not give, it takes. The Ice is a study in reductionism. Toward the interior everything is simplifed. The Ice absorbs and, an imperfect mirror, its ineffable whiteness reflects back what remains. Toward the perimeter, ice becomes more complex, its shapes multiply, and its motions accelerate. The ephemeral sample, the berg, is more interesting than the invariant whole, the plateau. The extraordinary isolation of Antarctica is not merely geophysical but metaphysical. Cultural understanding and assimilation demand more than the power to overcome the energy gradient that surrounds The Ice: they demand the capacity and desire to overcome the information gradient. Of all the ice masses in Greater Antarctica the berg is the most varied, the most informative, and the most accessible. The assimilation of The Ice begins with the assimilation of the iceberg.

      The great berg hesitates.

      A cloud passes before the sun. The berg glows blue amid a tar-black sea. Then almost imperceptibly it retreats, drawn back into the fog and pack ice, back into the first of the great veils of The Ice.

       Glaciology of the Berg

      The berg synopsizes the natural history of Antarctic ice. The whole ice field derives from the recrystallization and rearrangement of a single substance, water, in a single state, the solid, under the influence of a single force, gravity. Its geology contains virtually an entire rock cycle based on a solitary mineral, ice. Its larger geography is organized into roughly concentric ice terranes: the berg, the pack, the shelf, the glacier, the sheet, and the source domes. Within its ice the berg contains a frozen record of these terranes and a history of movement through them. Thus the berg is by far the most complex ice mass, and its dazzling whiteness masks a dense fabric of acquired ices and shapes—of old materials, now reworked into new stuff; of old shapes, now subjected to new internal stresses and external sculpturings; of old motions, now propelled into new movements; of inherited appearances, now brilliantly illuminated by the contrasts of the berg’s new surroundings. Like the world-encompassing summas of medieval philosophy, in which a single principle of logic could endlessly ramify a limited body of texts, the berg evolves out of ice a grand synthesis of structure upon structure.

      The process begins with the simple fall of ice crystals. In the interior of the continent, this involves little more than the settling of ice dust consisting of tiny prisms. Outward, along the coast, aggregates of prisms form snowflakes. The piling of crystal upon crystal leads to a process of ice lithification and metamorphosis. A stratigraphy of snow develops. Loose debris is transformed into structured layers. As snow builds up, compression and sintering round off individual crystals and reorganize the strata into a texture of toughened ice grains saturated with irregularly shaped bubbles of trapped air. Under further heat and pressure, the ice recrystallizes into ice slates, ice schists, and ice gneisses. Some air is squeezed out, other bubbles reshape into tiny globes, and swaths of bubbles form flow bands of white ice. Relentlessly, the density increases and firn becomes glacial ice—blue, hard, translucent. The speed of this metamorphosis varies with temperature and the thickness of the superimposed snow, both of which increase toward the perimeter of the ice field. With a density about one-third that of average rock and about 90 percent that of seawater, glacial ice floats. The heavy snowfall along the coast increases the proportion of snow and firn to glacial ice, with the result that Antarctic bergs can be relatively buoyant.

      But there is more to the berg than its source ice. Chemically, Antarctic ice is the purest form of water on the planet. Yet from the beginning the ice fabric incorporates air, and glacial ice will typically show bands of blue ice and milky white bubbles. Other snows and other impurities will be acquired by the ice during its journey to the sea. Additional terrestrial dusts, nitrates, radioactive fallout, and extraterrestrial particulates all become embedded into the ice, often as nuclei for precipitants. Coastal glaciers may acquire rocky debris from adjacent mountains and eroding beds, СКАЧАТЬ