5.3 Active and Passive Continental Margins


5.3 Active and Passive Continental Margins

Two main types of continental margins have been identified, passive and active. Passive margins are found along most of the coastal areas that surround the Atlantic and Indian oceans, including the east coasts of North and South America as well as the coastal areas of Europe and Africa. Passive margins are not situated along an active plate boundary and therefore experience very little volcanism and few earthquakes. In these passive areas, weathered materials eroded from the adjacent land mass accumulate to form a thick, broad wedge of relatively undisturbed sediments.

By contrast, active continental margins occur where oceanic lithosphere is being subducted beneath the edge of a continent. The result is a relatively narrow margin, consisting of highly deformed sediments that were scraped from the descending lithospheric slab. Active continental margins are common around the Pacific Rim, where they parallel deep ocean trenches.




Passive Margins

First, we will discuss passive margins in more detail. Later we will give details on active margins.

The features comprising a passive continental margin include the continental shelf, the continental slope, and the continental rise.

First, the continental shelf. This is a gently sloping, submerged surface extending from the shoreline toward the deep ocean basin. Because it is underlain by continental crust, it is clearly a flooded extension of the continents. The continental shelf varies greatly in width. Although almost non-existent along some continents, the shelf extends seaward more than 900 miles along others. On average, the continental shelf is about 50 miles wide and 425 ft deep at its seaward edge. The average inclination of the continental shelf is only about one-tenth of 1 degree, a drop of only about 10 feet per mile. The slope is so slight that it would appear to an observer to be a horizontal surface.

Although continental shelves represent only 7.5 percent of the total ocean area, they have economic and political significance because they contain important mineral deposits, including large reservoirs of oil and natural gas, as well as huge sand and gravel deposits. The waters of the continental shelf also contain many important fishing grounds, which are significant sources of food.

Even though the continental shelf is relatively featureless, some areas are covered by extensive glacial deposits and thus are quite rugged. In addition, some continental shelves are dissected by large valleys running from the coastline into deeper waters. Many of these shelf valleys are the seaward extensions of river valleys on the adjacent land mass. Such valleys appear to have been excavated during the Pleistocene epoch, or Ice Age. During this time great quantities of water were stored in vast ice sheets on the continents. This caused sea level to drop by 300 feet and more, exposing large areas of the continental shelves. Because of this drop in sea level, rivers extended their courses and land dwelling plants and animals inhabited the newly exposed portions of the continents. Dredging off the coast of North America has retrieved the ancient remains of numerous land dwellers, including mammoths, mastodons, and horses, adding to the evidence that portions of the continental shelves were once above sea level.

Most passive continental shelves, such as those along the east coast of the United States, consist of shallow water deposits that can reach several miles in thickness. Such deposits have led researchers to conclude that these thick accumulations of sediment are produced along a gradually subsiding continental margin.

Next – the continental slope. Marking the seaward edge of the continental shelf is the continental slope, a relatively steep structure, as compared with the shelves, that marks the boundary between the continental crust and oceanic crust. Although the inclination of the continental slope varies greatly from place to place, it averages about five degrees and in some places may exceed 25 degrees. Further, the continental slope is a relatively narrow feature, averaging only about 12 miles in width.

The continental rise. In regions where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise, where the slope drops to about a third of the degree or about 30 feet per mile . Whereas the width of the continental slope averages about 12 miles, the continental rise may extend for hundreds of miles into the deep ocean basin.

Continental rise consists of a thick accumulation of sediment that was moved down slope from the continental shelf to the deep ocean floor. The sediments are delivered to the base of the continental slope by turbidity currents that periodically flow down submarine canyons. When these muddy currents merge from the mouth of a canyon onto the relatively flat ocean floor they deposit sediment that forms a deep sea fan. As fans from adjacent submarine canyons grow, they merge laterally with one another to produce a continuous covering of sediment at the base of the continental slope, forming the continental rise.

Active Continental Margins

So now we will move on to some details about active continental margins. Along some coasts the continental slope descends abruptly into a deep ocean trench. In this situation, the landward wall of the trench and the continental slope are essentially the same feature. In such locations, the continental shelf is very narrow, if it exists at all.

Active continental margins are located primarily around the Pacific Ocean in areas where oceanic lithosphere is being subducted beneath the leading edge of a continent. Here sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the overriding continent. This chaotic accumulation of deformed sediment and scraps of oceanic crust is called an accretionary wedge. Prolonged plate subduction, along with the accretion of sediments on the landward side of the trench can produce a large accumulation of sediments along a continental margin. A large accretionary wedge, for example, is found along the northern coast of Japan’s Honshu Island.

Some subduction zones have little or no accumulation of sediments, indicating that ocean sediments are being carried into the mantle with the subducting plate. These tend to be regions where old oceanic lithosphere is subducting nearly vertically into the mantle. In these locations the continental margin is very narrow, as the trench may lie a mere 30 miles offshore.

active continental margin,active continental margin vs passive continental margin,active continental margin features,passive continental margin,passive continental margin vs active continental margin,passive continental rifting,continental margins,continental slope shelf and rise,continental slope video,continental slope and rise,continental shelf drop off,continental shelf,offshore fishing depth,how deep is the continental shelf