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Submarine topography

Surface morphology of solid earth under water cover

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

The surface morphology of the solid earth covered by water. Due to the cover of sea water, it is difficult to directly observe the undulation of seabed topography. The early plumb sounding method is time-consuming and low precision. Since the 1920s, ships have used Echo sounder , can quickly measure the seabed depth, combined with accurate positioning, can reveal the truth of the seabed topography. Continental margin, ocean basin and mid ocean ridge

Chinese name: Submarine topography

Foreign name: submarine relief
Basic unit: continental margin , ocean basin and mid ocean ridge

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brief introduction

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Submarine topography (2 sheets)

From 1925 to 1927, the German ship "Meteor" investigated the South Atlantic Ocean and revealed for the first time that the undulation of the ocean floor terrain was no less than that of land. Since 1953, precision Echo sounder More and more ocean bottom topographic profiles are obtained. From 1967 to 1969, three-dimensional geomorphic maps of the Atlantic Ocean, the Pacific Ocean and the Indian Ocean came out one after another.

There are towering seamounts, undulating sea hills, long ridges, deep trenches, and broad Abyssal plain 。 Transoceanic Mid ocean ridge It stretches 80000 kilometers and is hundreds to thousands of kilometers wide. Its total area is comparable to that of global land, and its length and width are unparalleled by any mountain system on land. The deepest point of the ocean is 11034m deep, located in the Pacific Ocean Mariana Trench This depth exceeds the highest peak on land Mount Qomolangma (8848.86m)^[1] 。 On the Hawaiian island in the central Pacific Mauna Loa Volcano At an altitude of 4170 meters, while the ocean floor near the island is five or six kilometers deep, Mauna Loa volcano is actually a mountain with an ocean floor of about ten thousand meters high.

Basic unit

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Submarine topography

On the surface of the earth, the continent and the ocean floor present two different terraces. The altitude of most parts of the land is 0~1km, and the depth of most parts of the ocean floor is 4~6km. The whole seabed can be divided into three basic Topographic unit ： continental margin 、 Oceanic basin And the mid ocean ridge. The term "ocean basin" has two meanings: in a broad sense, it refers to the whole ocean beyond the continental shelf and continental slope; In a narrow sense, it refers to the deep ocean floor between the mid ocean ridge and the continental margin. The latter meaning is used here. The three major topographic units can further divide some sub level submarine topographic units.

continental margin

Evolution of passive continental margin

It is a broad transition zone between the two major terraces of the mainland and the ocean floor. It accounts for about 22% of the total ocean area, and generally divides the continental margin into Atlantic continental margin (Also called Passive continental margin ）And Pacific type continental margin (Also called Active continental margin ）。 The former is composed of three units of continental shelf, continental slope and continental uplift, with wide and gentle terrain, and is found in the Atlantic Ocean, Indian Ocean arctic ocean And most of Antarctica Periphery zone; The latter has narrow continental shelf, steep continental slope, undeveloped continental uplift and is replaced by trench, which can be divided into two types: trench island arc marginal basin series and trench close to continental margin Andean continental margin , mainly distributed in the Pacific Rim, but also in the northeastern edge of the Indian Ocean and other places. The continental shelf is a shallow sea zone with coast and gentle slope towards the sea. The water depth at the outer edge of the shelf is mostly 100~200 meters, where the gradient has a significant turning point and extends downward to a steep continental slope. The continental slope is the longest and most spectacular slope on the earth Submarine canyon , mainly caused by turbidity current Terrigenous sediment It is an important channel to enter the deep seabed, and there is often a submarine fan formed by sediment accumulation outside the canyon mouth. The continental slope descends or transits to a continental uplift (on the Atlantic continental margin), or drops sharply to a deep-sea trench (on the Pacific continental margin). The continental uplift is at the foot of the continental slope. It is a gentle slope formed by sediment accumulation and transits to the ocean side and becomes a deep-sea plain with a slower slope. The trench is about 2-4km deeper than the adjacent ocean basin, and its cross section is asymmetric V-shaped. Its continental slope is steep, and the oceanic slope is gentle. The ocean side slope transits to an ocean basin, and sometimes a wide and gentle outer edge uplift extending parallel to the trench is developed, which is about 500 meters higher than the abyssal plain. The land side of the island arc is Back arc basin (also called marginal basin), whose water depth is shallower than that of ocean basin, forms a unified trench arc basin system with adjacent island arcs and trenches. In addition, some continental margins have complex terrain, which are alternating basins and ridges, called continental margins, such as the south California Off shore. Steps with deep water outside the continental shelf are called marginal sea platforms, such as those off the southeast coast of the United States Black Haitai.

Submarine topography (3 sheets)

Oceanic basin

Abyssal plain

It is located between the mid ocean ridge and the continental margin, and its one side is connected with the gentle piedmont of the mid ocean ridge, and the other side is adjacent to the continental uplift (Atlantic continental margin) or trench (Pacific continental margin). It accounts for about 45% of the total sea area. The ocean basin is divided by positive topography such as seamounts, forming a number of submarine depressions with a slightly equiaxed shape and a water depth of about 4000~5000 meters, called sea basins. A long strip of seabed depression with large width and gentle slopes is called a trough. Deep sea plains Abyssal hills Etc.

The abyssal plain is undulating basalt The basement is covered by thick sediments, and the slope is less than one thousandth. In addition to the equatorial biological high-yield zone, the formation of deep sea plains is mostly related to those originating from continents or islands Turbidite deposition It is related to the covering of large areas of objects. It is usually distributed at the adjacent continental uplift. If there are few sediments at the bottom of the basin, it is a deep-sea hill composed of lava flow or rock cap, and some individuals are small Shield volcano And fluctuates for tens to hundreds of meters. Deep sea hills are often distributed on the side from the deep sea plain to the mid ocean ridge. Trenches are distributed on the edge of the Pacific Ocean. Terrestrial materials such as turbidity current deposits are difficult to transport across the trenches to the ocean basin area. The amount of pelagic deposits from the overlying water layer is generally limited, which is not enough to cover the deep-sea plain. Therefore, the deep-sea hills in the Pacific Ocean account for 80~85% of the ocean floor area. But the deep sea plain in the Atlantic Ocean is very developed.

Elongated Submarine highland It is called ridge or ridge. There is almost no seismic activity in the ridge in the ocean basin, called Aseismic ridge 。 Most seamounts are of volcanic origin, and some seamounts are scattered in ocean basins in isolation, with a large scale exposed to the water island 。 Some seamounts have flat tops Flat topped seamount The water depth on the top surface varies from several hundred meters to more than 2000 meters Marine erosion It is flattened and sunk. The three oceans are also scattered with wide and gentle submarine highlands, called seamounts, such as Bermuda Hailong. Some sea terraces with flat top and steep slopes around (also called Submarine plateau ）, or formed by lava accumulation, or with granite basement, which is also called micro landmass, such as in the Indian Ocean Seychelles Where Maas Karin Plateau. The Plateau is most developed in the Indian Ocean.

Mid ocean ridge

The longest and widest global oceanic system on the earth, accounting for 33% of the total ocean area. In the Pacific Ocean, the mountain system is located to the east and its fluctuation is less than that of the mid Atlantic ridge East Pacific Rise 。 The Mid Atlantic Ridge is S-shaped and parallel to the contour on both sides (Figure 2). Indian Ocean Ridge It is divided into three branches in the shape of "entering". The southern end of the mid ridges of the three oceans is connected in the southern hemisphere, and the northern end is buried into the continent through shallow sea or bay respectively. The axis of the mid ocean ridge is about 1~3 km higher than the bottom of the ocean basins on both sides. The water depth at the top of the ridge is generally 2~3 km, and some even outcrop the sea surface, such as Iceland 。 The mid ridge is staggered by a series of large faults perpendicular or slightly oblique to the mountain system trend, and along the fault zone, there are long and narrow grooves, ridges and cliffs. The seafloor on both sides of the fault zone is divided into steps with different depths. The mid ocean ridge is divided into ridge crest area and ridge wing area. The ridge top area is composed of several rows of nearly parallel ridges and valleys. The crest of the ridge is a new oceanic crust, with very thin or missing overlying sediments, and the terrain is very rugged. Along the axis of the Atlantic Ocean and the Indian Ocean mid ridge, there are generally 1~3 km deep rifts between the rift ridges on both sides. To the ridge wing area, as the age of the oceanic crust increases and the sedimentary layer thickens, the height difference between the ridge and the valley gradually decreases. Some valleys can be filled with sediment in the form of steps, and the wings far away from the top of the ridge can have relatively smooth terrain.

formation mechanism

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Submarine Topography Analysis

Submarine topography and Landform The same is the result of internal and external forces. However, the submarine topography is usually the direct product of internal force, and Seafloor spreading Plate tectonic activities are closely related. The axis of the mid ocean ridge is the center of seafloor expansion, and the magnificent mid ocean ridge terrain is actually the reflection of the upwelling thermal expansion mantle material. The sea floor is cooling and sinking in the process of spreading to both sides. The seafloor expands slowly and has sufficient time to cool and sink. The two slopes of the mid ridge are steep, such as the mid Atlantic ridge; If the seafloor expands rapidly, the two slopes are gentle, Rudong Pacific Rise. From the mid ridge axis to both sides, as the seafloor ages, the water depth increases, and the sedimentary layer thickens; Accordingly, the mid ocean ridge transited into an ocean basin, and the rugged terrain on the top of the mid ocean ridge was replaced by deep-sea hills or even deep-sea plains. The deep ocean floor lacks the land compressive Folded mountain system The formation of ridges and seamounts is mostly related to volcanism and fault blocking. From the ocean basin to the continental side, there are two situations: first, there is no plate subduction activity, forming a wide and gentle Atlantic continental margin; Second, the subduction of the plate forms a deep trench and the associated volcanic arc (Pacific continental margin), with a great difference in terrain elevation. The continental side of the volcanic arc can form a marginal basin due to back arc expansion. The external force also plays a role in shaping the seabed topography. Stronger Sedimentation It can transform the original rugged volcanic and tectonic terrain to form a deep-sea plain. The geomorphic difference between the abyssal plain and the abyssal hill actually depends on the size of the sedimentary thickness. The submarine canyon is the most spectacular manifestation of turbidity erosion. Wave, tide and ocean current It has a profound impact on coastal and shallow sea terrain. Submarine landslide , deep seabed flow, etc. will also cause small or micro topography such as submarine cliffs, flow marks, etc. Except for the continental margin, erosion is far less important than that on land in the process of shaping the ocean floor topography.

Submarine continental shelf

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continental shelf It is a natural extension of land to the sea. The original coastal plain was submerged by sea water, which became the shallow sea of the continental shelf. The shallow sea of the continental shelf encircles the land like a garland, but its total area is 27.5 million square kilometers, equivalent to the area of the African continent. Chinese the bohai sea 、 Yellow Sea And most of the East China Sea are on the continental shelf.

We eat fish shrimp Such marine products are mainly captured from the shallow sea of the continental shelf, which accounts for 80% of the total marine aquatic products.

There are abundant petroleum 、 natural gas , accounting for about 1/3 of the world. Many oil mines on the land are also located on the continental shelf seabed Generated in the environment. Comely shallow sea Scenery.

Seafloor spreading

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In the 1950s, geographers could use advanced technology to map out underwater world 。 The surveying and mapping results show that there is a quite tall ocean at the bottom of the sea“ ridge ”, forming an underwater "mountain range", stretching about 83683.6 kilometers, passing through all the oceans in the world. The "ridge" at the bottom of the ocean is also called Fault valley , magma constantly emerges from the fault valley. After the magma cools ocean The bottom creates strips Meandering and undulating The newborn of Undersea mountain range This process is called Seafloor spreading These new undersea mountains are called Hailing 。 As new rocks are added to the rift valley, the rocks on both sides of the rift valley gradually move away from the center of the ocean ridge. Therefore, the farther away from the "mountains", the older the rocks will be.

When the sea ridges and new submarine plains are formed, the magma of the rift valley will continue to erupt. They act as a "conveyor belt", pushing new sea ridges out of the crustal rock strata, and at the same time slowly pushing them down from the crustal rock strata, melting them again mantle To achieve the balance between renewal and growth and decline.

characteristic

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Submarine topography is the result of global geological evolution and has experienced a long period of Geological history period 。 According to the basic characteristics of submarine topography, it can be roughly divided into three major units: continental margin, ocean basin and mid ocean ridge. The so-called continental margin, that is, a broad transitional zone between the continental surface and the ocean floor, is a huge and complex slope zone, which is the marginal zone connecting the continent and the ocean. The global continental margin extends 350000 kilometers, with a total area of about 80 million square kilometers, accounting for about 15.9% of the global surface area^[2] 。 The continental margin topography can usually be divided into secondary topographic units such as continental shelf, continental slope, continental uplift, trench and island. The continental margin has changed the most in the whole earth history, and it is also the area with rich geological and mineral resources and the most favorable mining conditions. The ocean basin is the main part of the ocean. The terrain is broad and flat, accounting for about 45% of the ocean area. The oceanic basin has a small inclination, about 0 ° 20 ′~0 ° 40 ′, and its depth can extend from the continental uplift to about 6000 meters. In the wide ocean basin, because there is no light and the temperature is very low, the benthic fauna in the deep ocean is very rare, and its ocean floor sediment is mainly formed by the calcareous and siliceous skeleton of plankton breeding on the surface of the ocean sinking to the seabed and adding soft mud Calcareous ooze and Siliceous ooze 。 At the depth of more than 4700m Seafloor sediment The manganese nodule resources with wide distribution and large reserves have been found. A very important topographic feature of the ocean bottom is the submarine uplift which is distributed in vein shape and has a global scale. It is like a huge mountain range standing at the bottom of the ocean, extending Four oceans 。 These ocean floor "mountains", called mid ocean ridges, are larger than the largest valley on land, and their material composition is silico magnesian volcanic rocks. There are volcanoes and earthquakes.

China Seabed

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The submarine topography of China's offshore and adjacent waters, especially the Bohai Sea, the Yellow Sea and the East China Sea, is generally characterized by tilting from northwest to southeast. from China Sea The five island chain from the south of Nanjiang Province to the west of Kyushu, Japan, via Taiwan Province, can connect the Bohai Sea, the Yellow Sea, the East China Sea and South China Sea The seabed terrain in the north is divided into two different areas: the seabed in the west has little fluctuation, small slope and flat terrain; The submarine terrain in the east turns straight down, and the slope suddenly becomes steep, with trenches, troughs and ridges. As for the South China Sea, it is a large basin with large depth, relatively closed, complex terrain, shallow surroundings and deep center. The average depth of the center of the basin is about 3000 meters, and there are several uplifted reef islands in the basin^[2] 。

The China Sea not only has a vast continental shelf, but also has a continental slope, a deep basin and a deep trough. The continental shelf is a shallow water zone surrounding the continent and gently dipping to the sea. Although it is submerged by sea water, it is actually a natural extension of the mainland. The water depth is generally within 200 meters, and its width extends from the low tide line to the sea until the slope increases significantly. The continental shelf is characterized by shallow depth, gentle slope and extension of continental topography. Bohai Sea and Yellow Sea It belongs to the continental shelf. About 2/3 of the East China Sea belongs to the continental shelf. The continental shelf of the South China Sea is also very broad.

The continental slope of the China Sea is mainly distributed in the South China Sea, except for a small area on the southeast side of the East China Sea continental shelf. Deep sea basins only exist in the South China Sea.

1. Submarine topography of the Bohai Sea

The Bohai Sea is a shallow sea from northeast to southwest. The submarine terrain inclines from the three bays to the center of the Bohai Sea and the Bohai Strait, with a gentle slope, and the average slope is only 0 ′ 28 ″. The water depth in the coastal area is within 10 meters. The water depth near the Liao River estuary and the Hai River estuary is about 5 meters, and the water depth at the shallowest part of the Yellow River estuary is less than half a meter. The average depth of the Bohai Sea is 18 meters, and the maximum depth is in the Bohai Strait Laotieshan Waterway Nearby, about 80 meters. The topography of Liaodong Bay inclines from the top of the bay and both sides to the center, and the east side is deeper than the west side, with the deepest depth of more than 30 meters. The terrain of the Bohai Bay also inclines from the top of the bay to the center of the Bohai Sea. The water depth in the bay is very shallow, generally less than 20 meters. To Laizhou Bay Yellow River Delta It protrudes toward the sea and is separated from the Bohai Bay. The terrain in the bay is flat, and it leans slightly toward the center of the Bohai Sea. The water depth is generally 10-15m, and the deepest is about 18m. The central basin of the Bohai Sea is a shallow depression that is narrow in the north and nearly triangular in the south. The terrain is relatively flat, low in the middle, slightly higher in the northeast, and 20-25 meters deep.

2. Submarine topography of the Yellow Sea

The Yellow Sea is a semi closed shallow sea in a north-south direction. The submarine terrain inclines from the north, east and west to the center and southeast of the Yellow Sea, but the gradient is not large, the average gradient is 1 ′ 21 ″, and the terrain is relatively flat. The depth gradually becomes shallow from southeast to north, like a dustpan with its mouth facing south. It has an obvious low trough from southeast to north - the Yellow Sea Trough. The depth of the trough is 60~80 meters. From the south of Jeju Island, it extends along the middle of the Yellow Sea to the northwest, entering the North Yellow Sea, Qingdao Outer Sea and Haizhou Bay respectively.

The nearshore water depth of the Yellow Sea is mostly within 60 meters, except that the central and southeast water depth of the South Yellow Sea is more than 80 meters, and the deepest water depth is 103 meters on the north side of Jeju Island. The average depth of the Yellow Sea is 44 meters, including 38 meters in the north and 46 meters in the south.

On the west bank of the Yellow Sea Northern Jiangsu The coast is a vast tidal flat, shallow water area, with water depth less than 20 meters, and some Underwater delta , such as the ancient Yellow River underwater delta and Yangtze River Underwater delta, so shoals Sandbar Many, such as Dasha, Beisha, Jinjiasha, Langjiasha, Do Nansha Islands Etc. East side of the Yellow Sea Korean Peninsula The water depth along the coast is greater than that along the west coast, and there are many near vertical Underwater sand ridge There are many islands in the south, and the underwater terrain is complex. The terrain of the southern Yellow Sea inclines to the southeast, but there are several small underwater reefs, such as Sappan reef , Hupi Reef, etc., they form a northeast reef line with Jeju Island, forming the nature of the Yellow Sea and the East China Sea Dividing line 。

3. Submarine topography of the East China Sea

Generally speaking, it is high in the northwest and low in the southeast. The average depth of the sea area is 349m, and the maximum depth is 2717m. According to the trend of submarine topography, it can be divided into two regions: the shallow water area of the western continental shelf and the eastern part Okinawa Trough Deep Water^[3] 。

The East China Sea continental shelf is specially developed, with a maximum width of 640 kilometers, which is one of the widest continental shelves in the world. The continental shelf area accounts for about 66% of the whole sea area, which is wide in the north and narrow in the south. The submarine terrain is gently inclined to the southeast with an average gradient of 1 ′ 17 ″. The average water depth is 72 meters, and most of the sea areas are 60~140 meters deep. The outer edge of the shelf is at a depth of 120~140 meters. The continental shelf of the East China Sea can be divided into two parts, the east and the west, with a depth of 50-60 meters. There are many islands in the west, with complex underwater topography and steep slope; The east is open and gentle, and there are only some underwater highlands on its southeast edge, on which China's Diaoyu Islands and other islands are located. On the continental shelf of the East China Sea, the drowned valley of the Yangtze River extends from the mouth of the Yangtze River to the southeast, across the continental slope, and into the Okinawa Trough.

The continental slope distributed along the outer edge of the continental shelf of the East China Sea extends from northeast to southwest and forms an arc belt to southeast, accounting for 33% of the total area of the East China Sea. The terrain is steep with a gradient of 3~10 °. The main body of the continental slope is the Okinawa Trough, which is a deep trough, shaped like a crescent, protruding southeast. The trough is deep in the south and shallow in the north: the water depth in the north is 600~800 meters, and the slope is small; The water depth in the south is 2000~2500 meters, and the slope is also large, with the maximum depth of 2717 meters. The trough is U-shaped in profile, with flat valley bottom, steep slopes on both sides, about 3 ° on the west slope and 10 ° on the east slope. To the east of Okinawa Trough Ryukyu Islands The underwater shelf of Kyushu and other islands. The island shelf is narrow, 30~50 nautical miles at Kyushu and 2~20 nautical miles near Ryukyu Islands. The terrain of the island shelf is complex, with many beaches and rocky beaches. Ryukyu Islands is a part of the isolated islands on the edge of the western Pacific Ocean, which is the natural boundary between the East China Sea and the Pacific Ocean.

4. Submarine topography of the South China Sea

The depth of the South China Sea is greater than that of the Bohai Sea, the Yellow Sea and the East China Sea. Except for the shallow depth near the mainland on the north, west and south sides, the water depth in the middle and east is more than 2000 meters. The average depth of the South China Sea is 1100 meters, and the maximum depth is 5567 meters.

South China Sea Submarine geomorphology It has a wide continental shelf, steep continental slope and vast deep-sea basin. The submarine terrain is high in the northwest and low in the east and middle. The continental shelf is distributed around the edge of the basin; Outside the continental shelf, there is a stepwise descending continental slope. The Dongsha, Xisha, Zhongsha and Nansha Islands Is the reef island distributed on the ridge of the continental slope; It enters the deep sea basin of the South China Sea at the end of the continental slope. In the east of the South China Sea Taiwan Island To Luzon, Palahuang Island and other places, a series of island arcs and troughs (trenches) are distributed together.

The South China Sea continental shelf is very broad, mainly distributed in the north, west and south. Among them, the southern continental shelf is the widest, followed by the northern continental shelf, and the western and eastern continental shelves are narrow. Northern and northwestern continental shelf, roughly from the southern tip of Taiwan, China to Hainan Island Southerly south China In the shallow water area along the coast and the northern coast of Vietnam, the average submarine slope is 3 ′ 40 ″. The continental shelf is 190-280km wide, generally more than 250km. Beibu Gulf It is a shallow sea with a water depth of less than 100 meters, and the average water depth is about 40 meters, all belonging to the continental shelf. The topography of the bay is quite similar to that of the Bohai Sea. It is shallow in the north and west (20~40m), and deep in the middle and southeast (50~60m). The submarine terrain of the bay inclines from northwest to southeast, and the deepest point is in the southwest offshore of Hainan Island, reaching more than 90 meters. The Vietnamese coastal continental shelf in the west of the South China Sea is relatively narrow, with a width of about 50 kilometers at the north and south ends and only 20 kilometers in the middle; The slope is large. The eastern part of the South China Sea is an island shelf, from Taiwan Island to Luzon Island The island shelf along the belt is very narrow, only 5-10km long, with a gradient of 50 '- 1 ° 40'. palawan The nearby island shelf is 30-60km wide, and the slope is generally 17 '. The southern and southwestern continental shelves of the South China Sea are Sunda shelf It is one of the widest continental shelves in the world, with a width of more than 300 kilometers. The western and eastern continental shelves of the South China Sea are erosion accumulation continental shelves dominated by erosion, while the southern and northern continental shelves are accumulation continental shelves.

The continental slope of the South China Sea is distributed in the water depth of 150~3600 meters, and falls in a ladder shape. Starting from about 150 meters, the submarine slope becomes significantly steeper, from a flat continental shelf to a steep slope, and is separated by deep trenches. At a depth of about 1000~1800 meters, the terrain slows down and becomes a flat surface intermittently connected, which is hundreds of kilometers wide. At the outer side of the flat surface, there is another steep slope, which ends at the continental slope near 3600m and reaches the South China Sea Deep Sea Plain.

The South China Sea continental slope can be divided into four regions around the basin: the northern continental slope, the western slope terrace, the southern continental slope and the eastern continental slope.

The northern continental slope is located at the outer edge of the continental shelf from the south of Taiwan, China to the mouth of the Pearl River. The continental slope is undulating plain with uplifted reefs. stay Dongsha Islands The water depth nearby increases to 1000~2000 meters, and the terrain bulges southward.

The west slope terrace, also known as the southern continental slope of Hainan Island, is 300 nautical miles wide, located between the deep sea depression outside the Pearl River estuary and the southern continental slope of Vietnam, with a water depth of 1000~1500 meters. It has obvious ladder shape, large slope (5~10 °), dense isobaths and north-south distribution. Xisha and Zhongsha Islands It is distributed on the west slope terrace. There are many on the west slope terrace Underwater canyon The stepped continental slope is divided into many blocks. There is a long and narrow depression at the foot of the west slope terrace, with a depth of 5567 meters, which is the deepest known South China Sea.

The southern continental slope is also a stepped continental slope, connecting the Sunda shelf in the south and the Palawan Trough in the southeast. There is a submarine plateau in the middle of the continental slope, with a water depth of 1000~2000 meters. China's Nansha Islands is located on the ridge of this plateau. Due to the complex terrain and changeable water depth, it has become a "dangerous area" for navigation.

The eastern continental slope is located at the outer edge of the island shelf on the west side of Luzon, Mindulo and Palawan. The land slope is narrow, steep (as much as 10 °), falling in narrow steps, and cut by many underwater canyons, forming many straits and channels. Between the continental slopes of Xisha, Zhongsha and Nansha Islands in China, there is a long and narrow sea basin from northeast to southwest, called the Central Basin of the South China Sea, which is 1600 kilometers long, 700 kilometers wide at the widest point, and about 3600 meters deep. Since most areas in the sea basin are relatively flat, it can be regarded as a "deep sea plain" (in fact, the terrain is still very complex), which can be generally divided into flat north and mountainous south. The north is shallow, about 3400m; The southern part is relatively deep, about 4200~4400 meters. The terrain of the sea basin inclines from north to south. There are a group of isolated hills and seamounts in the center of the abyssal plain, most of which are not exposed to the sea, and may be formed by submarine volcanic eruption.

In the eastern island slope of the South China Sea and at the foot of the slope, there are huge long depressions. From north to south, there are Lusong Trough manila trench And Palawan Trough.

The Luzon Trough is located in the middle of the island slope to the west of Luzon Island, extending south-north and parallel to the coast. The northern section is called the North Luzon Trough, and the southern section is called the West Luzon Trough. The latter is about 225km long and 50km wide on average, with flat bottom and water depth of about 2500m; The former is about 620km long and 3400m deep.

The Manila Trench is located in Manila Bay At the foot of the outer island slope, it also extends south-north. It is about 350km long, only 10km wide at the bottom, 4800m deep, and 5377m deep at most. The average gradient of the west wall of the trench is 1 ° 30 ', while the gradient of the east wall is 13 °.

Palawan Trough is located in Palawan Island To the southwest, it is located in the Nansha step uplift block and Palawan - Kalimantan Island Narrow depression between plots. It is 675km long, 65km wide at the bottom, 2800-3000m deep and 3211m deep at the maximum. The bottom of the trough is flat, and there are small hills locally.

Pacific seabed

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The submarine landform of the Pacific Ocean fluctuates greatly. It's in the eastern Pacific Ocean. There is a mid ocean ridge and submarine mountains running through the north and south, accounting for about 35% of the total area of the Pacific Ocean. The mid ocean ridge is a huge arc, starting from the Aleutian Basin in the north, passing through the Gulf of Alaska Gulf of California 、 Galapagos Islands It is connected with the East Pacific Sea area, and then connected with the Indian Ocean Ridge system westward. Its northern section is buried by the Pacific coast of the United States, and its southern section is relatively obvious East Pacific Ridge 。 The mid ocean ridge is a huge tectonic zone, which is cut by a series of fault zones thousands of meters long parallel to the latitude line.

In the middle of the Pacific Ocean, there is a magnificent Undersea mountain range . Starting from the north Kamchatka Peninsula , via hawaiian islands 、 Lane Islands The Upper Amos Islands, stretching over 10000 kilometers, divides the Pacific Ocean into eastern and western parts. To the west of the Central Pacific Mountains, except for the Northwest Basin Central Pacific Basin and South Pacific Beyond the sea basin, there is a sea bed mountain scattered like stars. Some of these seamounts sink in the deep sea, and some stand on the sea as islands. Hawaii Island They are some peaks in the Central Pacific undersea mountains. They rise from the sea floor more than 5000 meters deep, and the main peak on the island is 4270 meters above the sea level, Absolute height It is more than 9270 meters, exceeding the height of Mount Everest, the highest mountain on the land. It can be seen that the scale of seamounts is very grand. To the east of the Central Pacific Mountains, except for the North Pacific Basin, the East Pacific Basin and Peru Outside the Chilean Basin, there are also the vast East Pacific Plateau and Albat Ross Haitai, etc.

Interestingly, the deepest part of the Pacific Ocean is not in the central area, but in the western continental shelf area. In this area, there are a series of huge island arcs and trench belts. The island arc and trench are close together, forming the most undulating zone on the earth's surface, with a terrain elevation difference of 15000 meters. There are a series of marginal seas between the inner side of the island arc and the mainland, and the outer side of the island arc is close to the deep trench. Four of them are more than 10000 meters deep. The deepest Mariana Trench (11034 meters) in the world is located here.

In the eastern Pacific Ocean and the coastal areas of North and South America, there are no island arcs, only trenches. There are more than 10 trenches with a depth of more than 6000 meters. Among them, Peru Chile Trench With a length of nearly 5900 kilometers, it is the longest trench in the world's oceans. The continental shelf, continental slope, island arc and trench on the edge of the Pacific Ocean account for about 10% of the total area of the bottom of the Pacific Ocean.

Atlantic bottom

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In the middle of the Atlantic Ocean, there is a Atlantic Ridge 。 It stretches southward from the coast of Iceland, across the south of the Atlantic Ocean, until near Antarctica. It is 15000 kilometers long from north to south. The ridge trend is basically consistent with the surface morphology of the Atlantic Ocean, and also slightly "S" shaped. The ridge width is generally 1500-2000km, accounting for about 1/3 of the total width of the Atlantic Ocean. The height is generally 200~4000 meters. The central area of the seamount is the highest and steepest. The mountain peak is only 1500 meters away from the sea, and some even rise out of the sea to become high islands, such as Azores The mountain rises more than 2000 meters above the sea from the bottom of the sea. There is a very steep and deep rift valley along the ridge of the Atlantic Ridge, with a depth of 2000 meters, a width of 30 to 40 kilometers, and a length of more than 1000 kilometers. It is a big crack in the earth's crust. The ridge is cut off by many transverse fault zones, which are geomorphically represented by a series of ridges and narrow linear grooves. Among them, those located near the equator Roman The fault zone is 350 kilometers long and 7864 meters deep. It is the main channel connecting the eastern and western ocean bottom currents. It clearly divides the Atlantic Ridge into two parts: north and south.

The Atlantic Ridge and the ocean floor highland divide the sea floor, and a series of deep sea basins are formed on the east and west sides respectively. The western European basin Iberia Sea Basin, Canary Basin Cape Verde Basin 、 Guinea Sea basin Angola Basin and Cape Town Sea basin. The west side mainly includes North American Basin ， Brazil Basin and Argentine Basin 。 In the south of the Atlantic Ocean, there is also the Atlantic Indian Ocean basin. These basins are generally about 5000 meters deep, with a wide and flat center, and a large amount of deep-sea ooze is accumulated in the basin. Between these basins, there are several ridge veins, highland protrusions, and some outcrops form islands. as Madeira Islands 、 Cape Verde Islands, etc. These basins account for about 1/3 of the total floor area of the Atlantic Ocean.

The Medy land acquisition at the edge of the Atlantic Ocean is very complex, including continental shelf, continental slope, continental uplift (platform), submarine canyon, underwater alluvial cone and island arc trench belt. The area of the continental shelf is second only to that of the Pacific, with 6.2 million square kilometers. It accounts for 8.7% of the total area of the Atlantic Ocean. The width of the continental shelf varies greatly. He ranges from tens of kilometers to 1000 kilometers. For example, along the Gulf of Guinea Brazilian Plateau East section Iberian Peninsula The continental shelf on the west side is very narrow. Generally not more than 50km; Around the British Isles, including the whole the north sea Region, and South America Patagonia The continental shelf to the east is often about 1000 kilometers wide. The continental slope of the Atlantic Ocean varies from sea to sea. It is steep and narrow along Europe and Africa, and wide and gentle along America. Between the Atlantic submarine continental slope and deep sea basin, there are some continental uplifts, the larger ones are Greenland An Icelandic uplift, an Icelandic Faroe uplift, a Buenlein uplift and a Malvinas uplift. In Greenland and Labrador Peninsula Between the mid Atlantic submarine canyon and Mississippi River 、 Amazon 、 Congo River 、 Rhine Near the estuary of other rivers, there are some semi conical underwater alluvial cones, with a scale of only a few hundred square meters. In addition, there are two island arc trench zones in the Atlantic Ocean, namely Lesser Antilles The double island arc trench belt of the South America and Antarctic Peninsula Between the island arc trench zone. among Da'an On the north side of the Les island arc Puerto Rico Trench It is 1550km long, 120km wide and 8648m deep, making it the deepest point in the Atlantic Ocean.^[4]

The bottom of the Indian Ocean

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In the middle of the bottom of the Indian Ocean, there is a "enter" shape in the middle Meiling 。 It is composed of Central Indian Ocean Ridge 、 Western Indian Ridge And an Australian sea hill in the Antarctic Rodrigues Intersection. The Central Indian Ocean Ridge is the northern branch of the Central Plum Ridge, which is composed of a series of ridges, generally 1300~2500 meters higher than the sea basins on both sides, and some islands, such as Rodrigues Island Amsterdam Island Etc. The Central Indian Ocean Ridge is called Arabian Indian Plum Ridge to the northwest, and then extends westward to enter Gulf of Aden , and Crimson Sea and East African Rift Valley System connection. The West Indian Ridge is the southwest branch of the Central Ridge Amsterdam It is adjacent to the Mid Indian Ocean Ridge Edward After the islands, it is called the Atlantic Indian Ocean Hill, which is connected with the southern end of the Atlantic Ridge. An Australian seamount in Antarctica is the southeast branch of the Central Plum Ridge, which is connected to the Central Indian Ocean Ridge near Amsterdam Island. The Central Meiling of the Indian Ocean is composed of a series of ridges parallel to the mid ridge axis. The ridge veins are rugged and jumbled, with a maximum width of 1500 kilometers. There are also many transverse fault zones distributed among them.

The central ridge in the shape of "entering" divides the Indian Ocean into three sea areas: the east, the west and the south. The eastern suburbs are covered by East Indian Ocean Ridge Separate as Central Indian Ocean Basin Western Australia Basin and South Australia Basin. These basins are relatively wide and deep. The western region is crisscrossed with ridges, separating a series of sea basins, mainly including Somali Basin , Mascarene Basin Madagascar The sea basin and the Egalus basin. These basins are small and shallow. The terrain in the south is relatively simple, including Crozet Sea basin, Atlantic Indian Ocean basin and Antarctic East Indian Ocean basin. These basins are generally 4500-5000m deep.

The continental shelf area of the shallow sea area around the Indian Ocean is 2.3 million square kilometers, accounting for about 4.1% of the total area of the Indian Ocean. It is the smallest of the four oceans in terms of continental shelf area. Moreover, the continental shelf is generally narrow, only Persian Gulf 、 Malacca Strait , northern Australia, western Malay Peninsula and Indian subcontinent The continental shelf on the western edge is wider. The continental slope is not wide, but there are some continental uplifts and underwater alluvial cones. The main continental uplifts include the Egalus Plateau along the coast of Africa Mozambique Plateau, Chagoslakdav Plateau, etc. Underwater alluvial cones are mainly distributed in Ganges and Indus River The area near Renhaikou. In addition, there is an island arc trench zone at the bottom of the Indian Ocean Andaman Islands Ezek, to sumatra 、 Java 、 Nusa Tenggara Islands To the south, the direction of India Australia plate Eurasian plate Formed by subduction. among Java Trench With a length of 4500km and a depth of 7729m, it is the deepest point in the Indian Ocean.^[5]

The bottom of the Arctic Ocean

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In the middle of the Arctic Ocean lie two ridges, namely Romonosov Ridge and Mendeleev Ridge 。 Romonosov Ridge is slightly northwest southeast, from Novosibirsk Islands From, through the central part of the Arctic, directly to Greenland Sea Shore. Mendeleev Ridge is roughly parallel to Romonosov Ridge East Siberia The sea area of Wrangell Island and the northernmost part of Canada Elsmere Island The scale is smaller than Romonosov Ridge. Two ridges divide the bottom of the Arctic Ocean into three basins, namely Nansen Sea basin Canadian Basin and Makarov Sea basin. The Nansen Basin is 5449m deep, the deepest part of the Arctic Ocean.

The most prominent feature of the submarine landform of the Arctic Ocean is that the continental shelf is very wide, with a total area of 4.4 million square kilometers, accounting for 33.6% of the total area of the Arctic Ocean. It is the ocean with the largest proportion of the continental shelf area in the total area of the oceans among the four oceans in the world. The continental shelf is distributed on the edge of the Arctic Ocean, but mainly in Eurasia East Siberia Mei, Laptiv Sea, Kara Sea Barents Sea , Norwegian Sea and Greenland Sea. In the continental shelf area, there are extremely rich oil and natural gas resources. Coastal islands have coal, iron, copper, lead, zinc and other mineral deposits.

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