Ocean Coasts and Continental Margins
Antony R. Orme
As befits a continent that extends from the tropics to the High Arctic, the coasts of North America experience a wide range of physical and biological processes, which are reflected in a variety of environments. Furthermore, except along the Pacific rim, the mainland shore is often fronted by broad continental shelves and backed by coastal plains, notably around the Gulf of Mexico, the unglaciated Atlantic coast, Hudson Bay, and portions of the Arctic coast. Because these shelves and plains have been influenced by eustatic and isostatic sea-level changes during late Cenozoic time, they are integral components of the coastal zone. Along such coasts, quite subtle fluctuations in sea level may have major impacts on coastal processes, such as sedimentation, biotic migration, and wetland evolution. Such is not the case along the mountainous Pacific coast where the effects of sea-level fluctuations are limited to a narrow rim, glaciated in the north, that descends steeply onto narrow shelves or fault-controlled borderlands. Nor is it true of Greenland, eastern Baffin Island, and Labrador where recent glaciation and crustal adjustments have left a legacy of rugged coastal terrain. Such contrasts between steep rugged coasts and gently shelving coastal zones reflect the continent's tectonic and geomorphic evolution, but details within this framework are due mostly to processes at work during and since the large major eustatic rise of sea level that culminated less than 5000 years ago.
Unlike Africa or Australia where long stretches of coast are unbroken by sizable inlets, the North American coast is augmented by many gulfs, estuaries, and islands, sufficiently numerous to render coastline measurements rather meaningless. However, a useful perspective is gained by noting that the continental mainland is bordered by about 4000 km of temperate Atlantic coast from Newfoundland to the Florida Strait, by 4500 km of subtropical coast around the Gulf of Mexico, by 7000 km of temperate Pacific coast from Baja California to the Alaska Peninsula, and by long cold coasts on the Bering Sea, Arctic Ocean, Hudson Bay, and Labrador Sea.
Beyond the mainland shore, continental shelves add 6.74 × 106 km2, or 28%, to the continent's land area of 24.06 × 106 km2 (table 1.1), a large addition with major physical and biological implications during eustatic sea-level changes. Off the northern coast, a vast shelf supports the Arctic Archipelago, including such large islands as Baffin (476 × 103 km2), Ellesmere (213 × 103 km2), and Victoria (212 × 103 km2). Beyond these lies Greenland (2176 × 103, km2), the world's largest island below continental size, tenuously attached to the continent's tectonic framework. Farther south, off Newfoundland (111 × 103 km2), the continental shelf covers 345 × 103 km2. The shelves underlying 600 × 103 km2 or 38% of the Gulf of Mexico, widest in the carbonate platforms off Florida and Yucatan, and 1120 × 103