Greenland's massive ice sheet covers 80% of the island and sits in a topographic basin surrounded by highlands. Over 1500 miles long (north to south) and nearly 700 miles across at its widest point, the ice sheet has an average thickness of 1 mile; its maximum thickness is almost 2 miles at a point where the ice surface is more than 10,000 feet above sea level.
Greenland's ice sheet began to form by the Miocene Period, about 15 million years ago; since that time, its thickness and surface area has waxed and waned in concert with Earth's climate. Composed of compressed snow that, until the end of the Pleistocene, had been accumulating for about 100,000 years, the weight of the ice sheet has depressed the rock floor of its basin, which now sits below sea level at its center. Along its rim, the ice sheet breaks through the highland barrier, forming stark canyons and feeding outlet glaciers which calve icebergs. Meltwater also pours out through these canyons while, far from the edge, massive pools of meltwater develop atop and beneath the ice sheet; some geologists believe that melting on the underside of the ice sheet is due to heat radiating from Earth's mantle, conducted through Greenland's relatively thin crust.
Now in full retreat due to global warming, hydrologists and glaciologists are working to determine both the process and rate of melting and what effects that will have on sea levels, ocean currents and weather patterns across the globe. It has been calculated that there is enough water within Greenland's Ice Sheet (8% of the freshwater on Earth) to raise sea level almost 24 feet. Should that occur, a process that, depending on the rate of global warming, may take 2000 years, Greenland would become an oblong archipelago, formed by the mountains that now surround the ice sheet; eventually, the basin floor would rebound upward, reconnecting the islands. Of course, the major question today is how rapidly and to what degree that scenario will unfold.
