The coast rests on the landward limit of the continental margin, typically
described as a three-component system composed of the shelf, slope, and rise.
The continental shelf is a gently dipping shallow platform or terrace (ave. slope 1:100) that extends from the shore to an abrupt break in slope known as the shelf edge. The water depth over the shelf is generally less than 200 feet. Beyond the shelf edge lies the more steeply dipping (ave. slope 1:40) continental slope. Both the shelf and slope are underlain by thin continental crust. As the continental slope encounters the relatively flat abyssal ocean floor the gradient decreases. The resultant apron of accumulated sediment resting at the foot of the slope and lying on oceanic crust is the continental rise. Although the rise is considered a component of the continental margin it actually lies on and is part of the ocean floor.
The continental shelf is cut by deep submarine canyons that mimic those carved by rivers. The excavation of the canyons has been attributed to turbidity currents and continental rivers flowing toward a lower (approx. 300 ft.) ice-age sea level. In regions, such as the Pacific west coast, where the continent shelf is narrow these canyons are sediment sinks that funnel sediment away from the coastal zone.
The shape of the continental margin reflects its tectonic evolution and subsequent geologic history. For example a passive continental margins, such as the eastern margins of North and South America, do not lie along active plate boundaries. Passive margins are typically depositional and more irregular characterized by promontories and salient. Most passive margins were formed by rifting accompanying the formation of ocean basins. Active continental margins, such as the the west coasts of North and South America, lie along a plate boundaries. Active margins lack broad shelves, are backed by mountainous regions, and typically straighter than a passive margin.
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Passive and active continental margins of the Eastern Pacific and Atlantic oceans. NOAA image obtained and modified from <http://wrgis.wr.usgs.gov/docs/parks/province/atlantpl.html> |
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