What is continental shelf and its Origin? What is the formation or structure of continental shelves?

Continental shelves are a component of the continent even though they are submerged. The margin of the continental shelf, not the coast, is where a continent’s true boundary lies.

What is continental shelf?

The undersea margin of a continent is referred to as a continental shelf. The seven major land divisions on Earth are referred to as continents. A continental shelf stretches from a continent’s shoreline to a point known as the shelf break, where it stops. In what is known as the continental slope, the shelf drops from the break to the deep ocean floor.

A broad, generally shallow undersea terrace of continental crust that forms the edge of a continental landmass is known as a continental shelf. Continental shelves feature a gently sloping terrain known as ridge and swale, and its geology is frequently comparable to that of the neighboring exposed piece of the continent. Oceans cover around 8% of the earth’s surface, which is made up of continental shelves.

Continental shelves are a component of the continent even though they are submerged. The margin of the continental shelf, not the coast, is where a continent’s true boundary lies. Continental shelves come in various widths. The continental shelf, for example, stretches less than a kilometer over areas of California, the United States (.62 miles). However, the shelf stretches 1,290 kilometers along Siberia’s northern coast (800 miles). A continental shelf’s average breadth is 65 kilometers (40 miles).

The majority of continental shelves are vast, gently sloping plains with shallow sea. The continental shelves have an average water depth of 60 meters (200 feet). The shallow waters allow sunlight to penetrate, allowing a variety of organisms to thrive, from minuscule shrimp to enormous seaweed known as kelp. Currents in the ocean and river runoff provide nutrients to creatures that dwell on continental shelves.

Continental shelf provide ideal feeding grounds for sea animals because to plants and algae. The shelves cover less than ten percent of the ocean’s entire surface area. Despite this, the bright waters are home to all of the ocean’s plants and a variety of algae.

Deep gorges and canals cut across the continental shelf in several places. Submarine canyons receive very little light and are often the least-explored parts of continents. Submarine canyons are frequently produced around river mouths. Strong river currents erode rocks above ground and cut deeply into the soft material of the continental shelf. The Congo Canyon is 800 kilometers (497 miles) long and 1,200 meters (3,900 feet) deep, stretching from the Congo River’s mouth. Africa is home to the Congo Canyon.

Origin of continental shelf

Continental shelves, according to American oceanographer Donald J.P. Swift, are “palimpsests,” parchment writing tablets on which stories are written after all previous writing has been erased. Each fresh rise or decrease in sea level “writes” a new story of sedimentation on the shelf, after the preceding episode has been wiped by the rise or fall that came before it, but with remnants of the previous deposition environment or last erosional event remaining. The wave is the “eraser,” a high-energy force that erodes and reworks everything it passes over, winnowing out the finer-than-sand particles and leaving the coarser stuff behind.

The continental shelf southwest of Cape San Blas on Florida’s panhandle is made up of a convoluted array of channels (eroded and later filled), old deltaic deposits, ancient erosional surfaces, and winnowed sand bodies, as seen by an interpreted seismic line.

The impact of the foregoing processes on a given margin is determined by its tectonic setting and the size of the rivers that flow into it. The difference between high and low sea-level stands on continental shelves backed by high mountain ranges, such as the Pacific coasts of North and South America, may be difficult to detect, being one of degree perhaps only noticeable by marginally increased sedimentation rates during lowstands, or intervals of decreased sea level.

Continental shelves on tectonically active borders are similar to lowstands on trailing-edge, or passive, margins in many ways at present sea levels.

Rivers are revived when sea level is dropped on a trailing-edge shelf with no neighboring high mountains, such as the Atlantic coast of North America. In other words, their base level drops, and they begin to erode their beds, transporting material from the continent across the now-exposed former continental shelf and depositing it along the new shore. The coast is on the continental slope when the sea level falls below the shelf break. Small and medium-sized river mouths drown and estuaries form as sea level rises again on tectonically stable or sinking shelves, trapping silt and starving the shelves.

In these circumstances, the erosion of the shoreline as the surf zone moves landward with rising sea levels is the primary source of sediment for the shelf. Fine-grained debris is sifted out and either returned to the estuaries or moved across the shelf in stages by advective processes to deeper water beyond. As a result, continental shelf surfaces on trailing-edge edges where no significant rivers flow are veneered with a sand layer over a complex of older deposits, some of which peep through the surface as outcrops—relics of a previous story inscribed on the palimpsest. Vast rivers, such as the Mississippi, that drain a large, high continent can keep up with increasing sea levels and carry enough sediment to keep the continent afloat.

Broad rivers that drain a large, high continent, such as the Mississippi, can keep up with increasing sea levels and carry enough material to prevent an estuary from emerging, and even fill their whole shelf area at a high stillstand like the current one.

For many years after World War II, when many of the world’s continental shelves were first described in detail, it was assumed that sand deposits on continental shelves were “relict,” deposits stranded by a higher sea level from the higher-energy regime of the surf zone that passed over them perhaps a few thousand years ago. Since the mid-1970s, geophysical studies of the shelf area have revealed the presence of a variety of sand waves and ripple marks in seafloor sediments, indicating that submerged continental shelf sediments are continually being reworked and eroded.

As scientific knowledge of the physical processes that affect continental shelves grew, it was discovered that currents created by large winter storms, monsoons, hurricanes, and typhoons are reworking the bottom by winnowing out fine-grained materials and transporting them back into estuaries or beyond the shelf break, where they are lost from the system.

In short, just as with continental margins in general, the type of sediment that covers the surface of a continental shelf is determined by the interaction of the tectonic setting, the size of the rivers that empty into it (size based on how much sediment they carry), and the wave energy that affects it. Carbonate sediments may cover shelves such as those in western Florida that have been cut off from clastic input (that is, sediments made primarily of quartz and clay minerals produced from continental erosion). The carbonate shelf, known as a bank, is cut off from a continental source in some circumstances, such as the Bahamas islands.

Quartz sands dominate continental shelves with rivers that bring sediments from continents to the shelf and beyond only at low sea levels, as well as those that drain hilly areas on high-energy coasts. Furthermore, shelves with rivers that drain huge continental areas and transport enough material to keep up with sea-level rise or dominate ambient wave-energy conditions will build muddy sediment deposits across their surfaces.

Since the 1970s, a growing number of researchers have attempted to explain the origins of continental shelves and their associated structures using plate tectonics.

The Pacific Ocean shelves, for example, originated as the leading edges of continental margins on lithospheric plates that terminated either at fracture zones (places where two plates move past each other) or at subduction zones, according to this idea (sites where one of the colliding plates plunges into the underlying partially molten asthenosphere). Such shelves are usually steep, distorted, and coated with a thin layer of erosional debris. The continental shelves of the Atlantic, on the other hand, display little or no tectonic deformation and are covered in a thick layer of sedimentary material. They’re supposed to be the trailing edges of the massive plates that split apart and withdrew to form the Atlantic basin many millions of years ago.

Large volumes of sand, silts, and mud from the continents settled and piled along the seaward side of the plates as they gradually contracted and sank.

Formation or structure of a Continental Shelf

A continental shelf extends 100–200 meters (330–660 feet) from the coast to depths of 100–200 meters (330–660 feet). It has a gentle inclination to the sea, with an average slope of about 0.1°. It almost always comes to an abrupt end at its seaward edge, known as the shelf break. Below this, at a depth of 4,000 to 5,000 meters, the continental slope, a significantly steeper zone, usually combines with the continental rise, a part of the ocean bottom (13,000 to 16,500 feet). A few continental margins, such as those off France’s Mediterranean coast and Porcupine Bank off Ireland’s western coast, lack a sharply marked break in slope and instead preserve a typically convex form to the seafloor.

Continental shelves have an average breadth of 65 kilometers (40 miles). Almost everywhere, the shelves are little more than a continuation of the continental landmass beneath the ocean’s edge. As a result, they’re narrow, rugged, and steep off mountainous shores, but broad and level offshore from plains. The shelf along the United States’ mountainous western coast, for example, is narrow, measuring only 32 km (20 miles) wide, whereas the shelf along the country’s eastern coast measures more than 120 km (75 miles). Off the coasts of northern Australia and Argentina, there are exceptionally broad shelves. From the coast of Siberia to the Arctic Ocean, the world’s biggest continental shelf stretches 1,500 kilometers (930 miles).

A layer of sand, silts, and silty muds commonly covers continental shelves. Small hills and ridges alternate with shallow depressions and valleylike troughs on their surfaces, giving them some respite. Submarine canyons with steep walls cut deeply through both the shelf and the slope below in a few cases.

Organic and inorganic materials accumulated over millions of years to produce continental shelves. As rivers carried sediment—bits of rock, soil, and gravel—to the margins of continents and into the ocean, inorganic material accumulated. These sediments collected in layers at the margins of continents over time. Organic matter, such as plant and animal remains, accumulated as well.

A lot of the continental shelf used to be dry land. Much of the Earth’s water was frozen into massive volumes of ice called glaciers 18,000 years ago, at the peak of the most recent ice age. As the water level lowered, continental shelves became visible. Sea levels were possibly 100 meters (330 feet) lower during this glacial period, according to scientists.

During the Ice Age, the continental shelves between North America and Asia were most likely exposed. According to some scientists, the shelves served as a “land bridge” connecting the two continents. This land bridge, now known as the Bering Strait, may have allowed people to move from Siberia to what is now Alaska, becoming the first humans in North America.

Biologists have also discovered the remains of land-based flora and animals on now-underwater shelves. For example, off the coast of the northeastern United States, scientists uncovered 11,000-year-old mastodon teeth and spruce pollen. Scientists can prove that the mastodon and pollen survived during the latest ice age using scientific instruments. Glaciers migrated over the shelves when they were above water, changing their surfaces.

[We took help from Britannica online dictionary, National Geographic and other websites.]

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