Continental margin

(geology) Those provinces between the shoreline and the deep-sea bottom; generally consists of the continental borderland, shelf, slope, and rise.

The submerged portions of the continental masses on crustal plates, including the continental shelf, the continental slope, and the continental rise. All continental masses have some continental margin, but there is great variety in the size, shape, and geology depending upon the tectonic setting.

The most common settings are the trailing-edge margin and the leading-edge margin. The former is associated with tectonic stability, as exemplified by the Atlantic side of the North American landmass (see illustration). Here the margin is wide and geologically relatively uncomplicated, with thick sequences of coastal plain to shallow marine strata dipping slightly toward the ocean basin. By contrast, the leading-edge margin (for example, the Pacific side of the United States) is narrow, rugged, and geologically complicated. The global distribution of these widespread continental margin types is controlled by the plate tectonic setting in which the landmass resides. Some major landmasses, such as Australia, are surrounded by wide margins, but most, such as North and South America, have some of both types. See also Plate tectonics.

Continental margin off the northeastern United States.

Any consideration of the continental margin must include a general understanding of global seal-level history over the past few million years. As glaciers expanded greatly just over 2 million years ago, sea level was lowered more than 300 ft (100 m). The cyclic growth and decay of glaciers during this period caused the shoreline to move from near its present position to near the edge of the present continental shelf on multiple occasions. These sea-level changes had a profound effect on the entire continental margin, particularly the shelf and the rise. During times of glacial advance, the coast was near the shelf edge, causing large volumes of river-borne sediment to flow down the continental slope and pile up on the rise; deltas were poorly developed for lack of place for sediment to accumulate. During times of high sea-level stand similar to the present time, little sediment crossed the shelf and large volumes of riverine sediment accumulated in large fluvial deltas. See also Delta.

The continental shelf is simply an extension of the adjacent landmass. It is characterized by a gentle slope and little relief except for shelf valleys (see illustration), which are old rivers that were active during times of low sea level. The outer limit of the shelf shows a distinct change in gradient to the much steeper slope.

The continental slope and rise of the outer continental margin includes the relatively steep slope and the rise that accumulates at the base of the slope. This continental material has the same general composition as the landmass.

The leading-edge continental margin that is commonly associated with a crustal plate boundary displays a very different geology, geomorphology, and bathymetry than the outer continental margin. In this type there is no distinct shelf, slope, and rise. Like the trailing-edge margin, the leading-edge margin exhibits the same characteristics as the adjacent landmass, in this case a structurally complex geology with numerous fault basins and high relief. The borderland is narrow and overall steep. Its geomorphology consists of numerous local basins that receive sediment through numerous submarine canyons. The canyons commonly extend nearly to the beach; there is no shelf as such. See also Submarine canyon.

The continental margin contains a vast amount and array of natural resources, most of which are being harvested. The primary fishing grounds around the globe are in shelf waters. The Grand Banks off northeastern North America and the North Sea adjacent to Europe are among the most heavily fished. There are also many mineral resources that are taken from shelf sediments, including heavy minerals that are sources of titanium, phosphate, and even placer gold. Important commodities such as sand, gravel, and shell are also taken in large quantities from the inner shelf. Salt domes that underlie the shelf, especially in the northern Gulf of Mexico, provide salt and sulfur.

Probably the most important resource obtained from the continental margin is petroleum, in the form of both oil and gas. Production is extremely high is some places, ranging from the deltas at the coast across the entire shelf and onto the outer margin, and reserves are high. See also Marine geology; Oil and gas, offshore.

Shelf circulation is the pattern of flow over continental shelves. An important part of this pattern is any exchange of water with the deep ocean across the shelf-break and with estuaries or marginal seas at the coast. The circulation transports and distributes materials dissolved or suspended in the water, such as nutrients for marine life, fresh-water and fine sediments originating in rivers, and domestic and industrial waste. Water movements over continental shelves include tidal motions, wind-driven currents, and long-term mean circulation. The inflow of fresh water from land also contributes to shelf circulation, because such water would tend to spread out on the surface on account of its low density. Rapid nearshore mixing reduces the density contrast, and the Earth's rotation deflects the offshore flow into a shore-parallel direction, leaving the coast to the right. A compensating shoreward flow at depth is deflected in the opposite direction, adding to the complexity of shelf circulation. See also Nearshore processes; Ocean circulation.