Geology and Geography of Greenland
The Greenland Shield consists of the greater part of the exposed land surface of Greenland and is composed chiefly of granites and gneisses, like the Canadian Shield to the west. These Precambrian folded rocks have been strongly denuded, and at some points younger sedimentary rocks have been deposited on them, particularly in the north.
The high eastern and northeastern rim of the island is formed of rocks of varied age that have been metamorphosed, faulted, and folded, particularly in the Caledonian orogeny. During the Tertiary Period vulcanism, associated with the igneous activity of Iceland and northwest Scotland, led to outflows of lava in a belt that probably crosses the middle of the island. Dissected basalt landforms are found in the central west on Disko island and on the central east coast around Scoresby Sound.
The shape of the land beneath the ice cap has been likened to an elongated saucer with high rims that exceed 12,000 feet (3,700 meters) at Mt. Gunnbjörn. It is known that at several points the buried land surface in the center of the island is below sea level. At one time it was thought Greenland might be a group of islands like the archipelago north of Canada. However, it now appears that while deep fjord valleys penetrate beneath the ice cap, the landmass is in fact a single island. If the Greenland ice cap were to melt, the underlying crust would elevate as a plateau probably about 3,000 feet (900 meters) above sea level.
Greenland may be divided into two physiographic regions, the Greenland ice cap and the ice-free coastal areas. The ice cap covers more than 700,000 square miles (1.8 million sq km) and buries all the landforms except for a few nunataks (mountain peaks) that project through the ice near the margins. The ice cap generally does not reach the sea except in large outlet glaciers. An exception is in the northwest, in Melville Bay, where the ice sheet spills through the confining rim of mountains and numerous glaciers present an almost continuous ice face to the sea. The largest outlet glacier is the Humboldt Glacier in the northwest, which is about 60 miles (95 km) wide where it reaches the sea in Kane Basin.
The surface shape of the huge ice cap is semielliptical, rising inland from the coast, steeply at first and then more gently, toward an elongated dome that is asymmetrically situated on the eastern side at a height of about 10,000 feet (3,000 meters). Seismic measurements have shown that the greatest thickness of the ice cap is about 6,500 feet (2,000 meters) and that the average thickness is nearly 5,000 feet (1,500 meters).
Evidence from glacial striae, moraines, erratics, and other features indicates that the ice cap has advanced and retreated periodically and that at one time all Greenland, including the ice-free coast, was covered with ice. The interior ice cap, however, is not strictly a remnant of the far greater Pleistocene ice caps of the Northern Hemisphere; it is believed to have existed long before Pleistocene times. The effect of the existing ice cap on the climate is so strong, especially in keeping temperatures down, that it is doubtful that if the ice cap were to melt, the resulting milder climate would permit it to reform.
The ice-free coastal zones are generally mountainous and for the most part deeply dissected by fjords. Islands are numerous, the largest being Disko island off the west-central coast. North of Disko the coast, cut by fjords, changes gradually to one of islands and bays, until at Melville Bay in the northwest the ice cap reaches the sea. In the extreme north the mountains are deeply penetrated by fjords, but because of the low precipitation, glaciation is restricted and much of Peary Land is ice-free. On the east side of the island, the widest ice-free area is in the northern half, where the coast is dissected by large systems of fjords. The largest, Scoresby Sound in the middle of the east coast, is about 185 miles (300 km) long. Soundings show that these deep fjords continue as submarine valleys out onto the coastal shelf.
In southwest Greenland the inland ice cap is about 100 miles (160 km) from the coast, and the topography tends toward rounded summits and a relatively few small glaciers. Numerous long fjords intersect the area, as at Godthaab and Søndre Strømfjord. These long fjords have a climate that is less harsh than elsewhere, and they are the only areas in which there is any farming. The southwest is also the only part of the island where permafrost is absent.