The bottom of the World Ocean is divided by depth into the following components: continental shallows (shelf), continental (coastal) slope, bed, deep-sea (abyssal) basins (trenches) (Fig. 2).
Mainland Shoal- the coastal part of the seas and oceans, lying between the coast and the continental slope. This former coastal plain is expressed in the topography of the ocean floor as a shallow, slightly hilly plain. Its formation is mainly associated with the subsidence of individual land areas. This is confirmed by the presence within the continental shallows of underwater valleys, coastal terraces, fossil ice, permafrost, remains of terrestrial organisms, etc. Continental shallows are usually distinguished by a slight bottom slope, which is practically horizontal. On average, they decrease from 0 to 200 m, but within their limits there can be depths of over 500 m. The relief of the continental shallows is closely related to the relief of the adjacent land. On mountainous coasts, as a rule, the continental shelf is narrow, and on flat coasts it is wide. The continental shelf reaches its greatest width off the coast of North America - 1400 km, in the Barents and South China Seas - 1200-1300 km. Typically, the shelf is covered with clastic rocks brought by rivers from land or formed during the destruction of coastlines.
Rice. 2. Relief forms of the ocean floor
Continental slope - the inclined surface of the bottom of the seas and oceans, connecting the outer edge of the continental shallows with the ocean bed, extending to a depth of 2-3 thousand m. It has fairly large angles of inclination (on average 4-7°). The average width of the continental slope is 65 km. Off the coast of coral and volcanic islands, these angles reach 20-40°, and on the coral islands there are angles of greater magnitude, almost vertical slopes - cliffs. Steep continental slopes lead to the fact that in areas of maximum bottom inclination, masses of loose sediments slide to the depths under the influence of gravity. In these areas, a bare slope or muddy bottom may be found.
The relief of the continental slope is complex. Often the bottom of the continental slope is indented by narrow deep gorges-canyons. They are often found near steep rocky shores. But there are no canyons on continental slopes with a gentle bottom slope, as well as where there are stumps of islands or underwater reefs on the outer side of the continental shallows. The tops of many canyons are adjacent to the mouths of existing or ancient rivers. Therefore, canyons are considered as an underwater continuation of flooded river beds.
Another characteristic element of the relief of the continental slope is underwater terraces. These are the underwater terraces of the Sea of Japan, located at depths from 700 to 1200 m.
ocean bed- the main space of the bottom of the World Ocean with prevailing depths of more than 3000 m, extending from the underwater edge of the continent into the depths of the ocean. The area of the ocean floor is about 255 million km 2, i.e., more than 50% of the bottom of the World Ocean. The stock has slight angles of inclination, on average they are 20-40°.
The relief of the ocean floor is no less complex than the relief of the land. The most important elements of its relief are abyssal plains, oceanic basins, deep-sea ridges, mid-ocean ridges, hills and submarine plateaus.
In the central parts of the oceans are located mid-ocean ridges, rising to a height of 1-2 km and forming a continuous ring of uplifts in the Southern Hemisphere at 40-60° S. w. From it to the north extend three ridges extending meridianally in each ocean: the Mid-Atlantic, Mid-Indian and East Pacific. The total length of the mid-ocean ridges is more than 60 thousand km.
Between the mid-ocean ridges there are deep-sea (abyssal) plains.
Abyssal plains- flat surfaces of the bottom of the World Ocean, which lie at depths of 2.5-5.5 km. It is the abyssal plains that occupy approximately 40% of the area of the ocean floor. Some of them are flat, others are undulating with a height range of up to 1000 m. One plain is separated from the other by ridges.
Some of the single mountains located on the abyssal plains protrude above the surface of the water in the form of islands. Most of these mountains are extinct or active volcanoes.
Chains of volcanic islands above a subduction zone, occurring where one oceanic plate subducts beneath another, are called island arcs.
In shallow waters in tropical seas (mainly in the Pacific and Indian oceans), coral reefs are formed - calcareous geological structures formed by colonial coral polyps and some types of algae that can extract lime from sea water.
About 2% of the ocean floor is occupied deep-sea (over 6000m) depressions - trenches. They are located where oceanic crust subducts beneath continents. These are the deepest parts of the oceans. Over 22 deep-sea depressions are known, of which 17 are located in the Pacific Ocean.
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The topography of the bottom of the World Ocean is no less complex than on land, but its detailed study is due to the fact that it is buried under the water column. The irregularities can only be discerned by exploring the depths of the ocean.
Methods for studying the relief of the bottom of the World Ocean
Currently, there are several ways to determine the topography of the bottom of the World Ocean:
- using an echo sounder- a special device that sends sound signals to the bottom; sound, reflected from the surface, rises back, knowing the speed of sound propagation in water, you can calculate the depth and find out the nature of the deep relief;
- using underwater vehicles, which directly photograph the ocean floor;
- using spacecraft, which are also capable of taking pictures of the underwater bottom.
With the help of all these devices, relief maps of the bottom of the seas and oceans were compiled. The main unit of measurement on them is the depth scale. Such maps are necessary for everyone involved in navigation.
Rice. 1. Relief map of the bottom of the World Ocean
Underwater landforms and components of the ocean floor
Underwater, as on land, there are large landforms and small ones. You can see underwater plains and seamounts, as well as components of the ocean floor:
- underwater continental margins;
- ocean bed;
- mid-ocean ridges.
Brief characteristics of the underwater continental margins
The border between the continent and the ocean is not on land, but under water. That is why the part of the continent that is located under water is called the underwater outskirts of the continent. It is divided into two large parts:
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- continental shallows or shelf with shallow depths, up to a maximum of 200 meters;
- continental slope with depths of 2-3 thousand meters.
Rice. 2. Continental shelf and continental slope
At the point of the continental slope there are many deep-sea seas, which are separated from the ocean by chains of islands, which are the tops of deep-sea ridges, and deep-sea trenches that go 6 or more thousand meters deep.
The world's deepest deep-sea trench is the Mariana Trench. Its depth is more than 11 thousand meters.
Rice. 3 The Mariana Trench is the deepest place in the world's oceans
Brief characteristics of the ocean floor
The ocean bed, with depths from 4 to 6 thousand meters, is the central part of the ocean floor, occupying up to 70% of its territory. Here you can find such landforms as
- basins - giant underwater plains;
- seamounts and mountain ranges;
- underwater volcanoes, both active and extinct.
Volcanoes, rising above the water, form entire islands and island systems. Prominent examples are the Kuril Islands and the Hawaiian Islands.
Brief characteristics of mid-ocean ridges
Ridges are the results of uplift of the earth's crust, which occurred as a result of the collision of lithospheric plates. Most often, they are the boundaries of different plates. We can say that the mid-ocean ridge divides the ocean bed in half, forming a continuous chain stretching for 70 thousand km.
A fault runs along the axis of the uplift of the ridges, forming a gorge that seems to divide the ridge in half. The steeper slopes face the gorge, the gentler slopes face the ocean bed.
The gorge has a depth of up to 3 km, hot springs bubble up at its bottom, and the outpouring of basaltic magma is visible. There are volcanoes along the slopes. The height of the ridges is 3-4 km, width - up to 2 thousand km.
In those places where the tops of underwater ridges come to the surface, islands are formed. For example, Iceland.
There are also separate ridges and mountains on the ocean floor. For example, in the Arctic Ocean.
Changing the topography of the ocean floor
The topography of the ocean floor is still changing. This is primarily due to volcanic eruptions and earthquakes. You can learn more about changes in the topography of the ocean floor in 6th grade lessons.
What have we learned?
The profile of the bottom of the World Ocean is complex and rugged. Most of it is the ocean bed. The greatest depths are located in the trenches and at the bottom of the gorge of the mid-ocean ridge.
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Previous ideas about the bottom of the World Ocean as a single flat area were explained by the lack of factual data about the underwater part of our planet. As a result of a long study of the World Ocean, information has accumulated that allows us to assert that the ocean floor is no less complex than the continent. Just as on land, the topography of the ocean floor was greatly influenced by exogenous (external) and endogenous (internal) processes. Internal ones cause vertical and horizontal movements of sections of the earth's crust, earthquakes and volcanic eruptions. They create, as on land, large relief forms.
The external processes that form the ocean floor include sedimentation, that is, the subsidence and accumulation of rock destruction products. Their distribution and movement occurs under the influence of ocean currents in the World Ocean. Currently, the relief of the ocean floor includes the following parts:
Shelf or continental shelf
This is a flat or slightly inclined underwater part adjacent to the shore. The shelf ends with an inflection of the bottom - an edge. The depth of the shelf does not exceed 200 meters, and the width can be different: in the seas of the Arctic Ocean, off the northern coast of Australia, in the Bering, Yellow, East China and South China Seas, it is widest, and off the western coasts of North and South America it stretches a narrow strip along the coast. The shelf occupies about 9% of the area of the World Ocean. This is its most productive part, since it is here that 90% of seafood and many minerals are extracted, primarily oil and natural gas. In 1982, a UN convention established a 200-mile economic zone and the legal outer limit of the shelf, up to which the rights of the coastal state extend.
continental slope
This part of the ocean floor lies below the shelf limit (from the edge) to depths of 2000 meters. It has steep slopes of 15-20°, and sometimes up to 40°. The continental slope is strongly dissected by steps and lateral depressions. It contains depressions and hills. Under the influence of gravity, large masses of destroyed rocks move along the continental slope, often even in the form of huge landslides, and are deposited on the ocean floor. The continental slope occupies 12% of the area of the World Ocean. Its productivity is significantly lower than that of the shelf. The flora is poor due to lack of light. Animals lead a bottom-dwelling lifestyle. The continental slope passes into the ocean floor.
Bed of the World Ocean.
It is located at a depth of 2500 to 6000 meters and occupies 3/4 of the area of the World Ocean. The productivity of this area is the lowest, since climatic conditions and high salinity (up to 35%o) do not allow the rich flora and fauna to develop here.
The ocean bed has a complex topography. Its most interesting form is the mid-ocean ridges, the discovery of which occurred in the fifties of the 20th century. These are the largest forms of relief on the bottom of the World Ocean, forming a single system of mountain structures with a length of more than 60,000 km. They are swell-like uplifts of the oceanic crust. Their relative height is 3-4 km, width up to 2000 km. A fault, which is a gorge, usually runs along the axis of the uplift. It divides the rise into two parts, the slopes of which drop steeply towards the gorge and gently towards the ocean floor. At the bottom of the gorge, outpourings of basaltic magma and hot springs are found, and volcanoes are located on the slopes of the ridges. The ridges are composed of igneous rocks, almost not covered by sedimentary rocks. Mid-ocean ridges are broken by transverse faults, which are associated with volcanic activity and earthquakes, since the boundaries of lithospheric plates pass here. Where the tops of oceanic ridges come to the surface, islands are formed (for example, Iceland). There are also separate mountain ranges in the ocean (M.V. Lomonosov Ridge in the Arctic Ocean).
Vast deep-sea basins (more than 4000 meters) extend between the underwater ridges. The topography of their bottom is leveled by marine sediments. Basically, the surface of the basins is small hilly. High volcanic cones rise above the bottom of the basins. The active ones erupt lava, which is carried by water flows and settles to the bottom. The peaks of extinct volcanoes are aligned and have a flat shape. The alignment of the tops of these volcanoes occurs with the help of ocean currents. Rising above the water, the peaks of volcanoes form islands (for example, Hawaiian ones).
Continental sediments were formed by washing them off the land. They cover mainly the ocean shelf, and in some places their thickness reaches 4000 m. Pebbles and sand are often deposited here near the shore, and the smallest particles settle to form clay. Continental sediments cover approximately 1/4 of the entire surface of the seabed.
