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Gigantic tabular icebergs break off from the floating ice shelves which bound much of the Antarctic Ice Sheet. Smoothed on the surface by winds and by repeated melting and freezing at the base, these enormous, rectangular blocks of ice are the most sensational demonstration of how ice originally precipitated as snow on the deep interior of the Antarctic Ice Sheet is eventually returned to the sea.
This month, an iceberg roughly 150 km long and 35 km wide broke free of the Ronne Ice Shelf. The iceberg is composed of ice which fell on the Antarctic Plateau and drained through ice streams into the Ronne Ice Shelf. The ice remained intact until reaching the seaward edge of the ice shelf. There, shear cracks formed as the ice dragged along the western margin of Berkner Island. Eventually the shear cracks became long enough to slice off the huge block of ice.
Though unusual, antarctic icebergs of this size are not unknown. In October, 1987, a 154 km long 35 km wide iceberg broke free of the Ross Ice Shelf. Known as B-9, this iceberg formed when shear cracks formed along the margin of Roosevelt Island ruptured the ice shelf after at least 25 years of slow fracturing. A similarly sized chunk of the Filchner Ice Shelf calved in about 1986. Given that the ice speed near the margin of the Ronne Ice Shelf is about 900 m per year, we might expect that similar calving events might occur roughly every 30 years or so. Other places where we might expect new, large icebergs to form include the Princess Ragnhild coast where Radarsat imagery shows fractures nearly separating a large iceberg from the coastal ice shelf.
The mechanism that causes an iceberg to calve is poorly known. Several hypotheses ranging from strong storms to changing climate have been proposed but at this time, we cannot differentiate between these calving mechanisms. However, calve they must if the size of the Antarctic Ice Sheet is to remain nearly constant. Because the volume of the ice sheet is controlled largely by the amount of snow falling on the surface and the amount of ice lost by icebergs, an increase in the iceberg calving could signal ice sheet shrinkage. Similarly, a decrease in calving could signal ice sheet growth.
Radarsat gives an unparalleled look both at the processes leading up to iceberg formation and the iceberg themselves. This knowledge is important for eventually understanding how and why tabular bergs form and interpreting their signal about the changing environment of the Antarctic.
