The shelf break generally follows the ancient shorelines that existed at the peak of the continental glaciation periods of the ice age when sea level was as much at feet meters lower that present sea level. The continental shelf around Florida shown in red gradually transitions to the continental slope yellow and green. Florida displays features of a typical "passive continental margin" having wide coastal plains, wide continental shelves, and gentle slopes extending into deep water.
Continental Slope A continental slope is the slope between the outer edge of the continental shelf and the deep ocean floor. The continental slope is cut by submarine canyons in many locations. The continental slope marks the seaward edge of the continental shelf. Continental slopes typically follow the boundary between continental crust and oceanic crust. Continental slope range in steepness from 1 to 25 degrees, average is 4 degrees.
The dominant process influencing slopes are sediment deposition and erosion by turbidity currents discussed below. The continental slope off the coast of Virginia is cut by numerous submarine canyons that drain sediments to the continental rise at the base of the slope. Continental Rise A continental rise is a wide, gentle incline from a deep ocean plain abyssal plain to a continental slope. A continental rise consists mainly of silts, mud, and sand, deposited by turbidity flows, and can extend for several hundreds of miles away from continental margins.
Although it usually has a smooth surface, it is sometimes crosscut by submarine canyons extending seaward of continental slope regions.
The continental rise is generally absent in regions where deep-sea trenches exist where subduction zones are active. Continental rises feature deep-sea fans.
In appearance they are much like alluvial fans on land found along the fronts of mountain ranges. Deep-sea fans are accumulations of sediment deposited by turbidity currents called turbidites at the foot of the continental slope.
Turbidites are underwater landslide deposits. Over time they build up the large deep-sea fans that coalesce to form the continental rise along some continental margins. Bathymetry of the Monterey Bay offshore region highlights the character of the continental slope and rise.
Submarine Canyons Submarine canyons are similar to river gorges carved in mountainous regions on land, however they tend to be both much larger and deeper. Turbidity currents transport sediment into deep ocean basins via submarine canyons. Hudson Canyon offshore of New York City. Turbidity Currents and Development of Submarine Canyons and Fans A turbidity flows is a turbid, dense current of sediments in suspension moving along downslope and along the bottom of a ocean or lake.
In the ocean, turbidity currents can be massive episodic events. They typically form and flow down through a submarine canyon carved by previous turbidity flows and accumulate near the base of the continental slope on deep-sea fans. Turbidity flows produces deposits showing graded bedding Figure Slowing turbid currents drop their coarser fractions first gravel and sand and the finer silt and clay fractions settle out last.
A deep-sea fan is a fan- or delta-shaped sedimentary deposit found along the base of the continental slopes, commonly at the mouth of submarine canyons. Deep sea fans form from sediments carried by turbidity flows density currents that pour into the deep ocean basin from the continental shelf and slope regions and then gradually settle to form graded beds of sediment on the sea floor.
Deep-sea fans can extend for many tens to hundreds of miles away from the base of the continental slope and an coalesce into a broad, gently sloping region called a continental rise.
Graywacke is a fine-to-coarse-grained sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base commonly called a "dirty sandstone or mudstone" because of its mixed size fractions. Graywacke is the general term applied to sediments deposited by turbidity flows, and they commonly show graded bedding. Graywacke is common in the Coast Ranges of California and other active continental margin regions.
It is exposed on land where tectonic forces push up rocks that originally formed in the deep ocean examples in Figures to Conglomerate typically occurs in thicker beds and were originally deposited as gravel and mud on ancient submarine fans closer to the mouths of submarine canyons or in channels carved into the seabed. Turbidity flows are essentially underwater landslides or density-driven currents. Sediments laden with sediment are heavier than clear seawater.
Seas stacks composed of submarine channel deposits mostly conglomerate exposed at Gazos Creek State Beach, California. Passive continental margins occur where the transition between oceanic and continental crust which is not an active plate boundary.
Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin.
The Atlantic Coast of the United States is characterized by wide beaches, barrier islands, broad coastal plains see features discussed below. Emergent and Submergent Coasts In some regions around the world, tectonic forces are pushing rocks up along coastal regions, mostly in regions associated with active continental margins.
There areas are called emergent coasts and display features including sea cliffs and marine terraces see below. Where sea level is rising faster than land is rising, or where coastal areas are sinking, it is called a submergent coast. Submergent coasts are associated with passive continental margins with wide coastal plains and continental shelves. Estuaries are associated with submergent coastlines formed when sea level rises and floods existing river valleys.
Active margins can have both emergent and submergent coastlines in close proximity to each other. Active and passive margins of North America. Passive margin: North Carolina's Outer Banks region showing coastal plain, rivers, tidal estuaries, lagoon, barrier islands, and shallow Atlantic continental shelf. Active margin: San Francisco Bay and Monterey Bay region has actively rising coastal range mountains and sinking coastal basins.
Comparison of active and passive continental margins. Passive margins are on the trailing edge of a moving continental landmass. Deep-Ocean Basins Deep-ocean basins cover the greatest portion of the Earth's surface.
The Amazon River, whose source is in the Andes Mountains the active margin drains east across the interior of South America to the coast, where it enters the Atlantic Ocean and deposits the tremendous volume of sedimentary materials it eroded from the continent.
The North American plate also serves to illustrate this difference. The west coast is the active margin, and is the location of earthquakes, volcanoes, and mountains. The Eastern Seaboard is a passive margin, as is the Gulf Coast.
Active continental margins are also associated with subduction zones, often include a deep offshore trench. The Pacific Coast is an active margin that is characterized by narrow beach, steep cliffs, rugged coastlines with headlands and sea stacks see features discussed below.
Passive continental margins occur where the transition between oceanic and continental crust which is not an active plate boundary. The continental shelf ends at the shelf break , which is the point where the angle of the seafloor begins to get steeper. The shelf break averages about m deep. After the shelf break, the seafloor takes on a steeper angle about 4 o as it descends to the deep ocean.
This steeper portion of the margin is the continental slope , and it extends from the shelf break down to m. In some parts of the ocean, large submarine canyons have been carved into the continental slope; for example, Monterey Canyon in Monterey Bay, California, is a submarine canyon similar in size to the Grand Canyon!
These canyons may be carved out by turbidity currents , which are essentially landslides of sediment, rocks, and other debris down the face of the slope. At the bottom of the slope is the continental rise.
This area represents where the continental crust meets the oceanic crust, as the slope begins to level off to become the deep ocean floor. The rise consists of a thick layer of accumulated sediment coming from the continent, so it is difficult to tell where the slope ends and the rise begins.
0コメント