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Studies conducted by glaciologists tell us that the Antarctic ice cap formed from layers of snow pressed together over millions of years. Freshly fallen snow is fluffy and full of air pockets. As the fresh snow layer gets thicker the complex shapes of the snowflakes are packed more closely into spheres. Scientists call this snow firn.
As the depth beneath the surface increases, the firn is compressed causing ice crystals to form. Well below the surface, from 230-330 feet (70-100 m), the pressure is so great that the pockets of air are closed off and the firn turns to bubbly ice. Deeper still the crystals merge together and absorb the air trapped in between forming pure ice.
Snow accumulation in the center of the ice sheet can be as low as 1-2 inches (2-5 cm) per year. That is less than the equivalent rainfall in the Sahara Desert! As you move out toward the coasts snow fall increases to about 10 inches (25 cm) per year, but it still may take thousands of years to form very deep ice.
As it forms, the weight of the ice cap causes the ice to spread outward and move toward the coasts, just like syrup flows over a stack of pancakes. Ice near the south pole moves about 33 feet (10 m) a year but near the coasts it can move as much as 660 feet (200 m) a year. In narrow valleys the ice moves even faster; in other areas the ice cap can break to form cracks, or crevasses, more than 100 feet (30 m) deep. These variations on the surface of the ice pose potentially dangerous challenges to the scientists working on at remote field locations drilling ice cores.
