How long is the continental slope

According to paragraph 4 a of article 76 For the purposes of this Convention, the coastal State shall establish the outer edge of the continental margin wherever the margin extends beyond nautical miles from the baselines from which the breadth of the territorial sea is measured, by either: i a line delineated in accordance with paragraph 7 by reference to the outermost fixed points at which the thickness of sedimentary rocks is at least 1 per cent of the shortest distance from such point to the foot of the continental slope; or ii a line delineated in accordance with paragraph 7 by reference to fixed points not more than 60 nautical miles from the foot of the continental slope.

Paragraph 4 b of article 76 gives two methods for determining the location of the foot of the continental slope: In the absence of evidence to the contrary, the foot of the continental slope shall be delineated as the point of maximum change in the gradient at its base.

A schematic diagram of a margin that is clearly divided into shelf, slope, rise and abyssal plain Along morphologically complex margins, establishing the extent of natural prolongation of the land mass requires consideration of crustal structure, sediment deposition patterns, plate tectonic history, and other aspects of the evolution of the continental margin.

Analysis of geophysical and geological data may, in some areas, help identify the region of the base of the continental slope where the maximum change of sea floor gradient rule can be applied, or show that the outer edge of the continental margin determined by this rule is not a reliable location of the extent of natural prolongation of the land mass.

Finding the foot of the continental slope There are two steps to finding the foot of the continental slope 1. The complexity of the margin and availability of data will dictate the choice of evidence to use. In areas where the morphology of the continental margin is clearly divided into slope, rise and abyssal plain, and there is no contradicting geological and geophysical evidence, identification of the region of the base of the continental slope and location of the point of maximum change in gradient within this region are based on morphological evidence and are relatively straightforward.

For many reasons, along some margins the outer edge of the continental margin may not be reliably identified solely on the basis of morphological data. The location of the foot of the continental slope within that region can be determined by the maximum change in sea floor gradient rule, or, if this is inappropriate, by locating the inner edge of the continent-ocean transition. Maximum change in the gradient at the base of the continental slope In areas where the morphology of the continental margin can be clearly subdivided into shelf, slope, rise and abyssal plain, the region of the base of the continental slope is where the lower continental slope meets the rise, or where it meets the abyssal plain in cases where a rise is absent.

The morphology of the seabed where the continental slope merges with the rise may be an abrupt boundary where gently-dipping rise sediments onlap a smooth, relatively steep slope, or it may be a complex transition where local relief on the ocean floor meets an irregular lower slope.

Regional gradients can be used to narrow the search for the region of the base of the continental slope. Regional gradients less than 1 degree are generally considered representative of the rise and abyssal plain, and regional gradients greater than 2 degrees are generally considered representative of the slope. There are many exceptions to these values, however, and other evidence, such as the erosion effects of deep sea currents, underwater slides, margin collapse, local volcanic activity and deep-sea canyons, must be considered before finally establishing the region of the base of the continental slope.

The width of the region of the base of the continental slope is typically 4 — 10 kilometres, but can vary according to the complexity of the margin. A schematic showing identification of the region of the base of the continental slope from the margin morphology. Having established the region of the base of the continental slope, the point of maximum change in gradient is determined by computing the second derivative of the bathymetry within that region.

The foot of the continental slope has the maximum value of the second derivative of the bathymetry. A schematic showing the determination of the foot of the continental slope using the maximum change in gradient rule. Evidence to the contrary and the Continent-Ocean Transition COT Analysis of geophysical and geological data can help identify the region of the base of the slope where the maximum change of sea floor gradient rule can be applied. Convergent plate boundaries For active convergent plate boundaries, the Commission Guidelines 6.

The use of these locations is applicable where deep ocean sea floor is being subducted, but is not relevant where the continental landmass occurs on both sides and the plate boundary does not disrupt continental prolongation.

In those cases the outer edge of the continental margin lies at the outboard edge of the continental blocks. Because continental landmass is on both sides of the plate boundary the fault does not disrupt continental prolongation. Geological and geophysical supporting evidence In morphologically complex margins, where there are several points with maximum change in sea floor gradient, the region of the base of the continental slope might be identified by determining the location of the COT from geological and geophysical data.

Even though in many instances the extent of COT may be difficult to identify it is clear from paragraphs 1 and 3 of article 76 that the edge of the continental margin lies within the COT, so the inner and outer edges of the COT can be used to constrain the region of the base of the continental slope.

If a point of maximum change in sea floor gradient can be identified in this region, then it establishes the location of the foot of the continental slope. A schematic showing the use of geological and geophysical data as supporting evidence to distinguish the region of the base of the continental slope and use of the maximum change in gradient rule to identify the foot of the continental slope.

The region of the Western Lau Terrace, north of New Zealand is an example of the use of geological and geophysical evidence to identify the region of the base of the continental slope. This part of the New Zealand continental margin is characterised by terraces and sediment-filled basins formed by tilted basement blocks that step down to the deep ocean floor of the South Fiji Basin.

On these profiles the foot of the continental slope is the point with the maximum change in sea floor gradient within the transition zone. Western Lau Terrace - Geological and geophysical supporting evidence.

Basement fault blocks are shown by red dashed lines Evidence to the contrary If the maximum change in sea floor gradient does not reliably locate the foot of the continental slope, then that position can be located on the basis of geological and geophysical evidence. As mentioned above, on some margins foot of the continental slope positions located at the inner edge of the continent-ocean transition zone may be used to define the outer edge of the continental margin according to the formulae of article 76 paragraph 4 a CLCS Guidelines 6.

In order to use geological and geophysical evidence to locate the foot of the continental slope, the data must clearly demonstrate that the natural prolongation of the landmass extends to that point Geophysical evidence, such as seismic reflection data, gravity and magnetic data, and geological evidence from dredges or drill-holes, might establish the crustal structure, plate tectonic and geological history, and sediment depositional patterns of a region.

Any of this information could help define the natural prolongation of the landmass and the outer edge of the continental margin. A schematic diagram of a rifted continental margin where geological and geophysical evidence might establish the COT. The CLCS Guidelines state that the landward limit of the transition zone may be used as the foot of the continental slope from which the outer limit of the continental shelf can be established by the formulae of article 76 paragraph 4 a Hikurangi Plateau — evidence to the contrary The Hikurangi Plateau, east of New Zealand, is a large igneous province that sutured to the New Zealand continent in the Cretaceous and now forms the base of much of the continent.

It is an example of a continental margin where geological and geophysical evidence may be used to establish the natural prolongation of the landmass and the foot of the continental slope.

Powered by. A continental shelf is the edge of a continent that lies under the ocean. Continents are the seven main divisions of land on Earth. A continental shelf extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the shelf descends toward the deep ocean floor in what is called the continental slope.

Even though they are underwater, continental shelves are part of the continent. The actual boundary of a continent is not its coastline, but the edge of the continental shelf.

The widths of the continental shelves vary. Along parts of the U. But along the northern coast of Siberia, the shelf extends about 1, kilometers miles.

The average width of a continental shelf is 65 kilometers 40 miles. Most continental shelves are broad, gently sloping plain s covered by relatively shallow water. Water depth over the continental shelves averages about 60 meters feet. Sunlight penetrate s the shallow waters, and many kinds of organisms flourish—from microscopic shrimp to giant seaweed called kelp. Ocean currents and runoff from rivers bring nutrient s to organisms that live on continental shelves.

Plants and algae make continental shelves rich feeding grounds for sea creatures. The shelves make up less than 10 percent of the total area of the oceans.

In some places, deep canyon s and channel s cut through the continental shelves. Little light penetrates these submarine canyon s, and they are sometimes the least-explored areas of continents. Often, submarine canyons are formed near the mouth s of rivers.

Strong river currents cut deeply into the soft material of the continental shelf, just like they erode rocks above ground. The Congo Canyon, extending from the mouth of the Congo River, is kilometers miles long and 1, meters 3, feet deep. The Congo Canyon is part of Africa. Formation of a Continental Shelf Over many millions of years, organic and inorganic materials formed continental shelves.

Inorganic material built up as rivers carried sediment —bits of rock, soil, and gravel—to the edges of the continents and into the ocean. These sediments gradually accumulate d in layers at the edges of continents. Organic material, such as the remains of plants and animals, also accumulated. Many continental shelves were once dry land. The sea level dropped, exposing continental shelves. During this glacial period , scientists say that sea levels were perhaps meters feet lower than they are today.

People may have used this land bridge—now the Bering Strait—to migrate from Siberia to what is now Alaska, becoming the first human beings in North America.

Biologists have also found the remains of land-based plants and animals on shelves that are now underwater. For example, scientists have discovered 11,year-old mastodon teeth and spruce pollen off the coast of the northeastern United States. Scientific instruments can show that the mastodon and pollen lived during the time of the last ice age.

When the shelves were above water, glaciers moved over them and changed their surfaces. As huge alpine glacier s moved quickly downhill, they gouged deep, narrow valleys. Now, the valleys are filled with seawater. These narrow, flooded valleys that descend into the continental shelf are known as fjord s.

Oil on the Shelf A lot of fuel we use is collected from beneath the continental shelves. For example, 30 percent of all the oil and 20 percent of the natural gas produced in the U.