Ocean floor, mid-ocean ridges and transition zones
There are still different points of view on the issue of education time The Pacific Ocean in its modern form, but, obviously, by the end of the Paleozoic era, a vast body of water already existed in the place of its basin, as well as the ancient continent of Pangea, located approximately symmetrically with respect to the equator. At the same time, the formation of the future Tethys Ocean began in the form of a huge bay, the development of which and the invasion of Pangea subsequently led to its disintegration and the formation of modern continents and oceans.
Bed The modern Pacific Ocean is formed by a system of lithospheric plates bounded on the ocean side by mid-ocean ridges, which are part of the global system of mid-ocean ridges of the World Ocean. These are the East Pacific Rise and the South Pacific Ridge, which, reaching a width of up to 2 thousand km in places, connect with each other in the southern part of the ocean and continue westward into the Indian Ocean. The East Pacific Ridge, extending northeast to the coast of North America, in the Gulf of California region, connects with the system of continental rift faults of the California Valley, the Yosemite Trench and the San Andreas Fault. The middle ridges of the Pacific Ocean themselves, unlike the ridges of other oceans, do not have a clearly defined axial rift zone, but are characterized by intense seismicity and volcanism with a predominance of emissions of ultrabasic rocks, i.e., they have the features of a zone of intensive renewal of the oceanic lithosphere. Throughout the entire length, the middle ridges and adjacent plate sections are intersected by deep transverse faults, which are also characterized by the development of modern and, especially, ancient intraplate volcanism. Located between the median ridges and limited by deep-sea trenches and transition zones, the vast floor of the Pacific Ocean has a complexly dissected surface, consisting of a large number of basins with a depth of 5000 to 7000 m or more, the bottom of which is composed of oceanic crust covered with deep-sea clays, limestones and silts of organic origin. . The bottom topography of the basins is mostly hilly. The deepest basins (about 7000 m or more): Central, Western Mariana, Philippine, Southern, Northeastern, East Carolinian.
The basins are separated from each other or crossed by arches uplifts or blocky ridges, on which volcanic structures are planted, within the intertropical space often crowned with coral structures. Their tops protrude above the water in the form of small islands, often grouped into linearly elongated archipelagos. Some of them are still active volcanoes, spewing streams of basaltic lava. But for the most part these are already extinct volcanoes, built on with coral reefs. Some of these volcanic mountains are located at a depth of 200 to 2000 m. Their peaks are leveled by abrasion; the position deep under water is obviously associated with the lowering of the bottom. Formations of this type are called guyots.
Of particular interest among the archipelagos of the central Pacific Ocean are the Hawaiian Islands. They form a chain 2,500 km long, stretching north and south of the Tropic of the North, and are the tops of huge volcanic massifs rising from the ocean floor along a powerful deep fault. Their visible height is from 1000 to 4200 m, and their underwater height is approximately 5000 m. In terms of their origin, internal structure and appearance, the Hawaiian Islands are a typical example of oceanic intraplate volcanism.
The Hawaiian Islands are the northern edge of a huge island group the central part of the Pacific Ocean, which bears the general name “Polynesia”. The continuation of this group to approximately 10° S. are the islands of Central and Southern Polynesia (Samoa, Cook, Society, Tabuai, Marquesas, etc.). These archipelagos, as a rule, extend from northwest to southeast, along transform fault lines. Most of them are of volcanic origin and are composed of strata of basaltic lava. Some are topped by wide and gently sloping volcanic cones 1000-2000 m high. The smallest islands in most cases are coral structures. Similar features have numerous clusters of small islands located mainly north of the equator, in the western part of the Pacific lithospheric plate: the Mariana, Caroline, Marshall and Palau Islands, as well as the Gilbert Archipelago, which partially extends into the southern hemisphere. These groups of small islands are collectively called Micronesia. All of them are of coral or volcanic origin, mountainous and rise hundreds of meters above ocean level. The coasts are surrounded by surface and underwater coral reefs, making navigation very difficult. Many small islands are atolls. Near some islands there are deep-sea ocean trenches, and to the west of the Mariana Archipelago there is a deep-sea trench of the same name, belonging to the transition zone between the ocean and the Eurasian continent.
In the part of the Pacific Ocean bed adjacent to the American continents, small single volcanic islands: Juan Fernandez, Cocos, Easter, etc. The largest and most interesting group is the Galapagos Islands, located at the equator near the coast of South America. This is an archipelago of 16 large and many small volcanic islands with the peaks of extinct and active volcanoes up to 1700 m high.
Transitional from the ocean to the continents, the zones differ in the structure of the ocean floor and the characteristics of tectonic processes both in the geological past and at the present time. They surround the Pacific Ocean in the west, north and east. In different parts of the ocean, the processes of formation of these zones proceed differently and lead to different results, but everywhere they are distinguished by great activity both in the geological past and at the present time.
On the side of the ocean floor, transition zones are limited by arcs of deep-sea trenches, in the direction of which lithospheric plates move and the oceanic lithosphere subsides under the continents. Within the transition zones, the structure of the ocean floor and marginal seas is dominated by transitional types of the earth's crust, and oceanic types of volcanism are replaced by mixed effusive-explosive volcanism of subduction zones. Here we are talking about the so-called “Pacific Ring of Fire,” which encircles the Pacific Ocean and is characterized by high seismicity, numerous manifestations of paleovolcanism and volcanogenic landforms, as well as the existence within its boundaries of more than 75% of the planet’s currently active volcanoes. This is mainly mixed effusive-explosive volcanism of intermediate composition.
All the typical features of the transition zone are most clearly expressed within the northern and western margins of the Pacific Ocean, that is, off the coast of Alaska, Eurasia and Australia. This wide strip between the ocean bed and the land, including the underwater margins of the continents, is unique in the complexity of its structure and in the relationship between the land and the water area; it is distinguished by significant fluctuations in depths and heights, and the intensity of processes occurring both in the depths of the earth’s crust and on the water surface.
The outer edge of the transition zone in the North Pacific Ocean is formed by Aleutian deep sea trench, stretching for 4000 km in a convex arc to the south from the Gulf of Alaska to the shores of the Kamchatka Peninsula, with a maximum depth of 7855 m. This trench, towards which the movement of lithospheric plates of the northern part of the Pacific Ocean is directed, borders the underwater foot of the Aleutian island chain from the rear, most of them They are volcanoes of the explosive-effusive type. About 25 of them are active.
A continuation of this zone off the coast of Eurasia is the system deep sea trenches, which are associated with the deepest areas of the World Ocean and, at the same time, areas of the most complete and diverse manifestation of volcanism, both ancient and modern, both on island arcs and on the outskirts of the continent. In the rear of the Kuril-Kamchatka deep-sea trench (maximum depth over 9700 m) there is the Kamchatka Peninsula with its 160 volcanoes, of which 28 are active, and the arc of the volcanic Kuril Islands with 40 active volcanoes. The Kuril Islands are the peaks of an underwater mountain range that rises above the bottom of the Sea of Okhotsk by 2000-3000 m, and the maximum depth of the Kuril-Kamchatka Trench, which runs from the Pacific Ocean, exceeds 10,500 m.
The system of deep-sea trenches continues to the south with the Japan Trench, and the volcanogenic zone continues with the extinct and active volcanoes of the Japanese Islands. The entire system of trenches, as well as island arcs, starting from the Kamchatka Peninsula, separates the shallow shelf seas of Okhotsk and East China from the Eurasian continent, as well as the Sea of Japan depression located between them with a maximum depth of 3720 m.
Near the southern part of the Japanese Islands, the transition zone expands and becomes more complex, the strip of deep-sea trenches is divided into two branches, bordering on both sides the vast Philippine Sea, the depression of which has a complex structure and a maximum depth of more than 7000 m. From the Pacific Ocean it is limited by the Mariana Trench with its maximum depth World ocean 11,022 m and the arc of the Mariana Islands. The internal branch, limiting the Philippine Sea from the west, is formed by the trench and the Ryukyu Islands and continues further with the Philippine trench and the arc of the Philippine Islands. The Philippine Trench stretches along the foot of the islands of the same name for more than 1,300 km and has a maximum depth of 10,265 m. There are ten active and many extinct volcanoes on the islands. Between the island arcs and Southeast Asia, within the continental shelf, lie the East China Sea and most of the South China Sea (the largest in the region). Only the eastern part of the South China Sea and the interisland seas of the Malay Archipelago reach depths of over 5000 m, and their base is a transitional crust.
Along the equator, the transition zone within the Sunda archipelago and its island seas continues towards the Indian Ocean. There are a total of 500 volcanoes on the Indonesian islands, of which 170 are active.
The southern region of the Pacific Ocean transition zone northeast of Australia is particularly complex. It extends from Kalimantan to New Guinea and further south to 20° S, bordering the Sokhul-Queensland shelf of Australia to the north. This entire section of the transition zone is a complex combination of deep-sea trenches with depths of 6000 m or more, submarine ridges and island arcs, separated by basins or areas of shallow water.
Off the eastern coast of Australia, between New Guinea and New Caledonia, is the Coral Sea. From the east it is limited by a system of deep-sea trenches and island arcs (New Hebrides, etc.). The depths of the Coral basin and other seas of this transitional region (the Fiji Sea and especially the Tasman Sea) reach 5000-9000 m, their bottom is composed of oceanic or transitional type crust.
The hydrological regime of the northern part of this area favors the development of corals, which are especially common in the Coral Sea. On the Australian side, it is limited by a unique natural structure - the Great Barrier Reef, which stretches along the continental shelf for 2,300 km and reaches a width of 150 km in the southern part. It consists of individual islands and entire archipelagos, made of coral limestone and surrounded by underwater reefs of living and dead coral polyps. Narrow channels crossing the Great Barrier Reef lead to the so-called Great Lagoon, the depth of which does not exceed 50 m.
From the side of the Southern Basin of the ocean floor between the islands of Fiji and Samoa, the second arc of trenches, external to the ocean, extends to the southwest: Tonga (its depth of 10,882 m is the maximum depth of the World Ocean in the southern hemisphere) and its continuation Kermadec, maximum depth which also exceeds 10 thousand m. On the Fiji sea side, the Tonga and Kermadec trenches are limited by underwater ridges and arcs of the islands of the same name. In total, they stretch 2000 km to the North Island of New Zealand. The archipelago rises above the underwater plateau that serves as its pedestal. This is a special type of structure of the underwater margins of continents and transition zones, called microcontinents. They vary in size and are uplifts composed of continental crust, topped with islands and surrounded on all sides by basins with oceanic-type crust within the World Ocean.
The transition zone of the eastern part of the Pacific Ocean, facing the continents of North and South America, differs significantly from its western margin. There are no marginal seas or island arcs. A strip of narrow shelf with mainland islands stretches from the south of Alaska to Central America. Along the western coast of Central America, as well as from the equator along the outskirts of South America, there is a system of deep-sea trenches - Central American, Peruvian and Chilean (Atacama) with maximum depths of more than 6000 and 8000 m, respectively. Obviously, the process of formation of this part of the ocean and neighboring continents proceeded in interaction deep-sea trenches and continental lithospheric plates that existed at that time. North America moved onto the trenches along its path to the west and closed them, and the South American Plate moved the Atacama Trench to the west. In both cases, as a result of the interaction of oceanic and continental structures, folding occurred, the marginal parts of both continents were uplifted, and powerful suture zones were formed - the North American Cordillera and the Andes of South America. Each of these structural zones is characterized by intense seismicity and the manifestation of mixed types of volcanism. O.K. Leontiev considered it possible to compare them with the underwater ridges of the island arcs of the western transition zone of the Pacific Ocean.
Transition zone
Underwater continental margins
Geological structure and bottom topography
Map of the depths of the Pacific Ocean
The underwater continental margins occupy 10% of the Pacific Ocean. The shelf topography displays the features of transgressive plains with subaerial relict topography. Such forms are characteristic of underwater river valleys on the Java shelf and the Bering Sea shelf. On the Korean shelf and the shelf of the East China Sea, ridge landforms formed by tidal currents are common. Various coral structures are common on the shelf of equatorial-tropical waters. Most of the Antarctic shelf lies at depths of more than 200 m, the surface is very dissected, underwater tectonic elevations alternate with deep depressions - grabens. The continental slope of North America is heavily dissected by submarine canyons. Large submarine canyons are known on the continental slope of the Bering Sea. The continental slope of Antarctica is distinguished by its wide width, diversity and dissected relief. Along North America, the continental foot is distinguished by very large cones of turbidity currents, merging into a single inclined plain, bordering the continental slope with a wide strip.
The underwater margin of New Zealand has a peculiar continental structure. Its area is 10 times larger than the area of the islands themselves. This underwater New Zealand plateau consists of the flat-topped Campbell and Chatham rises and the Bunkie depression between them. On all sides it is limited by the continental slope, bordered by the continental foot. This also includes the Late Mesozoic underwater Lord Howe Ridge.
Along the western edge of the Pacific Ocean there are transitional regions from the margins of the continents to the ocean floor: Aleutian, Kuril-Kamchatka, Japanese, East China, Indonesian-Philippines, Bonin-Mariana (with the deepest point of the ocean - the Mariana Trench, depth 11,022 m), Melanesian, Vityazevskaya, Tonga-Kermadec, Macquarie. These transitional regions include deep-sea trenches, marginal seas, and island arcs. Along the eastern edge there are transitional regions: Central American and Peruvian-Chilean. They are expressed only by deep-sea trenches, and instead of island arcs, young rocky mountains of Central and South America stretch along the trenches.
All transitional areas are characterized by volcanism and high seismicity; they form the marginal Pacific belt of earthquakes and modern volcanism. Transitional areas on the western margin of the Pacific Ocean are located in two echelons, the youngest areas in terms of development stage are located on the border with the ocean floor, and the more mature areas are separated from the ocean floor by island arcs and island land masses with the continental crust.
Photograph of the Pacific Ocean from space
11% of the Pacific Ocean floor area is occupied by mid-ocean ridges, represented by the South Pacific and East Pacific Rise. They are wide, weakly dissected hills. Side branches extend from the main system in the form of the Chilean uplift and the Galapagos rift zone. The Pacific mid-ocean ridge system also includes the Gorda, Juan de Fuca and Explorer ridges in the northeast of the ocean. The ocean's mid-ocean ridges are seismic belts with frequent surface earthquakes and active volcanic activity. Fresh lavas and metal-bearing sediments, usually associated with hydrotherms, were found in the rift zone.
The system of Pacific uplifts divides the floor of the Pacific Ocean into two unequal parts. The eastern part is less complexly built and shallower. The Chilean uplift (rift zone) and the Nazca, Sala y Gomez, Carnegie and Cocos ranges are distinguished here. These ridges divide the eastern part of the bed into the Guatemala, Panama, Peruvian and Chilean basins. All of them are characterized by complexly dissected hilly and mountainous bottom topography. In the area of the Galapagos Islands there is a rift zone.
The other part of the bed, lying to the west of the Pacific uplifts, occupies approximately 3/4 of the entire bed of the Pacific Ocean and has a very complex relief structure. Dozens of hills and underwater ridges divide the ocean floor into a large number of basins. The most significant ridges form a system of arc-shaped uplifts, starting in the west and ending in the southeast. The first such arc is formed by the Hawaiian ridge, parallel to it the next arc is formed by the Cartographer Mountains, Marcus Necker Mountains, the underwater ridge of the Line Islands, the arc ends with the underwater base of the Tuamotu Islands. The next arc consists of the underwater foundations of the Marshall Islands, Kiribati, Tuvalu and Samoa. The fourth arc includes the Caroline Islands and the Kapingamarangi seamount. The fifth arc consists of the southern group of the Caroline Islands and the Euripik swell. Some ridges and hills differ in their extent from those listed above, these are the Imperial (North-Western) ridge, the Shatsky, Magellan, Hess, Manihiki hills. These hills are distinguished by leveled summit surfaces and are covered on top with carbonate deposits of increased thickness.
There are active volcanoes on the Hawaiian Islands and the Samoan archipelago. There are about 10 thousand individual seamounts, mostly of volcanic origin, scattered across the Pacific Ocean floor. Many of them are guyots. The tops of some guyots are at a depth of 2-2.5 thousand m, the average depth above them is about 1.3 thousand m. The vast majority of the islands of the central and western parts of the Pacific Ocean are of coral origin. Almost all volcanic islands are fringed with coral structures.
The floor and mid-ocean ridges of the Pacific Ocean are characterized by fault zones, usually expressed in relief in the form of complexes of conformably and linearly oriented grabens and horsts. All fault zones have their own names: Surveyor, Mendocino, Murray, Clarion, Clipperton and others. The basins and uplifts of the Pacific Ocean floor are characterized by an oceanic-type crust, with a sedimentary layer thickness from 1 km in the northeast to 3 km on the Shatsky Rise and a basalt layer thickness from 5 km to 13 km. Mid-ocean ridges have a rift-type crust that is characterized by increased density. Ultramafic rocks are found here, and crystalline schists were uplifted in the Eltanin fault zone. Subcontinental (Kuril Islands) and continental crust (Japanese Islands) has been discovered under the island arcs.
Mid-ocean ridges occupy 11% of the Pacific Ocean floor area and have their own specific structural features. The South Pacific and East Pacific rises are broad and relatively weakly dissected elevations. Large forms of deep dissection—transverse narrow depressions or “ocean troughs”—are associated with zones of cutting transverse faults. The flank zones of mid-ocean ridges are very wide; the rift zone only in certain areas reaches such expressiveness as in the ridges of the Atlantic and Indian Oceans. A distinctive feature of the mid-ocean ridges in the Pacific Ocean are also lateral branches from the main system in the form of the so-called Chilean Rise and the Galapagos Rift Zone. The system of mid-ocean ridges in the Pacific Ocean also includes the Gorda, Juan de Fuca and Explorer ridges in the northeast Pacific Ocean.
The mid-ocean ridges of the Pacific Ocean are seismic belts, but unlike transition zones, earthquakes here are only superficial.
Active volcanic activity occurs mainly in the rift zone. Fresh lavas were discovered (during underwater photography), metal-bearing sediments, usually associated with hydrotherms inherent in areas of modern volcanism in the Pacific Ocean.
The system of the South Pacific and East Pacific rises divides the floor of the Pacific Ocean into two unequal parts that differ greatly in structure. The eastern part is shallower and less complexly built. The lateral branches of the mid-ocean ridge system - the Chilean and Galapagos - are located in this part. In addition to the Chilean uplift, the Nazca, Sala y Gomez, Carnegie and Cocos ranges stand out here. These underwater ridges divide the southeastern part of the bed into the Guatemala, Panama, Peruvian and Chilean basins. All of them are characterized by complexly dissected mountainous and hilly bottom topography.
A rift zone is also identified in the area of the Galapagos Islands.
The rest of the ocean floor, lying to the west of the East Pacific Rise and from the underwater margin of North America and occupying approximately the area of the floor, has a very complex relief structure. Dozens of underwater ridges and hills divide the ocean floor into a large number of basins. The most significant ridges of the western and central parts of the Pacific Ocean floor have one common pattern: they form a system of arc-shaped uplifts, starting in the west and ending in the southeast. The first such arc is formed by the Hawaiian Ridge. Approximately parallel to it stretches the next, largest “arc”, starting with the Cartographer Mountains and further including the Marcus Necker Mountains, the underwater ridge of the Line Islands and ending with the underwater base of the Tuamotu Islands.
The next arc consists of the underwater foundations of the Marshall Islands, Kiribati and Tuvalu. Perhaps the Samoan islands are connected with it. The fourth arc is much shorter than the previous ones, it includes the Caroline Islands and the Kapingamarangi submarine shaft or rise. The fifth arc consists of the southern group of the Caroline Islands and the Eauriapic swell. There are several more underwater ridges, which are also the bases of numerous islands, parallel to this system, but not included in it (for example, Phoenix, Tahiti, Tubuai). Some ridges and hills stand out sharply in their extent. This is the Imperial, or North-Western, ridge, the heights of Shatsky, Magellan, Hess, Manihiki. The latter are distinguished by leveled top surfaces and usually bear “caps” of carbonate deposits of increased thickness.
Hawaii and Samoa have active volcanoes that differ significantly in the composition of volcanic products from the volcanoes of the transition regions. Scattered along the bottom of the Pacific Ocean within its bed are a huge number of individual seamounts, mostly also of volcanic origin. Many of them have flattened tops - these are the so-called guyots.
The tops of some guyots are located at depths of 2-2.5 thousand m, the average depth above them is about 1.3 thousand m. It is assumed that the tops of the guyots were once much closer to the ocean surface, perhaps even were islands, and then after abrasion or denudation alignment turned out to be submerged to the depths at which they are now located.
The vast majority of the islands of the western and central Pacific Ocean are coral. If these are purely volcanic islands, then they are almost always bordered by coral structures. The large thickness of coralline limestones on modern coral atolls also indicates significant negative crustal movements within the Pacific Ocean floor during the Cenozoic. The oldest coral limestones discovered by drilling on atolls are Eocene in age. They occur at depths close to 1300 m from the surface, while reef-building corals can only live at depths of no more than 50 m.
A very striking feature of the relief and tectonic structure within the ocean floor and mid-ocean ridges are zones of oceanic faults, usually expressed in relief in the form of complexes of linearly and conformably oriented tectonic depressions (grabens) and block ridges (horsts). All known fault zones have their own names. For example, in the northern part of the ocean, the most significant in extent are the Surveyor, Mendocino, Murray, Clarion, and Clipperton fault zones.
The basins and rises of the Pacific Ocean floor are characterized by an oceanic-type crust, but it is quite different. For example, in the northeastern part of the ocean floor, the “second” and “basalt” layers of the oceanic crust are thin, respectively less than 1 and less than 5 km, with average values of 1 and 7 km. On the Shatsky Upland, the maximum thickness of the “second” layer is noted, together with the sedimentary layer - up to 3 km and the basaltic layer - up to 13 km.
Mid-ocean ridges in the Pacific Ocean have a rift-type crust, characterized by an overall increased density (compared to oceanic crust). With the help of dredging, as at other mid-ocean ridges, ultramafic rocks were discovered here, and crystalline shales were raised in the Eltanin fault zone.
Transitional regions have a very variegated, mosaic structure of the earth's crust. Along with suboceanic and even oceanic crust, characteristic of deep-sea basins and the bottoms of deep-sea trenches, subcontinental (Kuril Islands) and even continental crust (Japanese Islands) has been discovered under island arcs. It is this mosaic structure of the earth’s crust in transitional areas that makes it possible to distinguish the earth’s crust developed here into a special geosynclinal type of the earth’s crust (Fig. 3).
There are still different points of view on the issue of education time The Pacific Ocean in its modern form, but, obviously, by the end of the Paleozoic era, a vast body of water already existed in the place of its basin, as well as the ancient continent of Pangea, located approximately symmetrically with respect to the equator. At the same time, the formation of the future Tethys Ocean began in the form of a huge bay, the development of which and the invasion of Pangea subsequently led to its disintegration and the formation of modern continents and oceans.
Bed The modern Pacific Ocean is formed by a system of lithospheric plates bounded on the ocean side by mid-ocean ridges, which are part of the global system of mid-ocean ridges of the World Ocean. These are the East Pacific Rise and the South Pacific Ridge, which, reaching a width of up to 2 thousand km in places, connect with each other in the southern part of the ocean and continue westward into the Indian Ocean. The East Pacific Ridge, extending northeast to the coast of North America, in the Gulf of California region, connects with the system of continental rift faults of the California Valley, the Yosemite Trench and the San Andreas Fault. The middle ridges of the Pacific Ocean themselves, unlike the ridges of other oceans, do not have a clearly defined axial rift zone, but are characterized by intense seismicity and volcanism with a predominance of emissions of ultrabasic rocks, i.e., they have the features of a zone of intensive renewal of the oceanic lithosphere. Throughout the entire length, the middle ridges and adjacent plate sections are intersected by deep transverse faults, which are also characterized by the development of modern and, especially, ancient intraplate volcanism. Located between the median ridges and limited by deep-sea trenches and transition zones, the vast floor of the Pacific Ocean has a complexly dissected surface, consisting of a large number of basins with a depth of 5000 to 7000 m or more, the bottom of which is composed of oceanic crust covered with deep-sea clays, limestones and silts of organic origin. . The bottom topography of the basins is mostly hilly. The deepest basins (about 7000 m or more): Central, Western Mariana, Philippine, Southern, Northeastern, East Carolinian.
The basins are separated from each other or crossed by arches uplifts or blocky ridges, on which volcanic structures are planted, within the intertropical space often crowned with coral structures. Their tops protrude above the water in the form of small islands, often grouped into linearly elongated archipelagos. Some of them are still active volcanoes, spewing streams of basaltic lava. But for the most part these are already extinct volcanoes, built on with coral reefs. Some of these volcanic mountains are located at a depth of 200 to 2000 m. Their peaks are leveled by abrasion; the position deep under water is obviously associated with the lowering of the bottom. Formations of this type are called guyots.
Of particular interest among the archipelagos of the central Pacific Ocean are the Hawaiian Islands. They form a chain 2,500 km long, stretching north and south of the Tropic of the North, and are the tops of huge volcanic massifs rising from the ocean floor along a powerful deep fault. Their visible height is from 1000 to 4200 m, and their underwater height is approximately 5000 m. In terms of their origin, internal structure and appearance, the Hawaiian Islands are a typical example of oceanic intraplate volcanism.
The Hawaiian Islands are the northern edge of a huge island group the central part of the Pacific Ocean, which bears the general name “Polynesia”. The continuation of this group to approximately 10° S. are the islands of Central and Southern Polynesia (Samoa, Cook, Society, Tabuai, Marquesas, etc.). These archipelagos, as a rule, extend from northwest to southeast, along transform fault lines. Most of them are of volcanic origin and are composed of strata of basaltic lava. Some are topped by wide and gently sloping volcanic cones 1000-2000 m high. The smallest islands in most cases are coral structures. Similar features have numerous clusters of small islands located mainly north of the equator, in the western part of the Pacific lithospheric plate: the Mariana, Caroline, Marshall and Palau Islands, as well as the Gilbert Archipelago, which partially extends into the southern hemisphere. These groups of small islands are collectively called Micronesia. All of them are of coral or volcanic origin, mountainous and rise hundreds of meters above ocean level. The coasts are surrounded by surface and underwater coral reefs, making navigation very difficult. Many small islands are atolls. Near some islands there are deep-sea ocean trenches, and to the west of the Mariana Archipelago there is a deep-sea trench of the same name, belonging to the transition zone between the ocean and the Eurasian continent.
In the part of the Pacific Ocean bed adjacent to the American continents, small single volcanic islands: Juan Fernandez, Cocos, Easter, etc. The largest and most interesting group is the Galapagos Islands, located at the equator near the coast of South America. This is an archipelago of 16 large and many small volcanic islands with the peaks of extinct and active volcanoes up to 1700 m high.
Transitional from the ocean to the continents, the zones differ in the structure of the ocean floor and the characteristics of tectonic processes both in the geological past and at the present time. They surround the Pacific Ocean in the west, north and east. In different parts of the ocean, the processes of formation of these zones proceed differently and lead to different results, but everywhere they are distinguished by great activity both in the geological past and at the present time.
On the side of the ocean floor, transition zones are limited by arcs of deep-sea trenches, in the direction of which lithospheric plates move and the oceanic lithosphere subsides under the continents. Within the transition zones, the structure of the ocean floor and marginal seas is dominated by transitional types of the earth's crust, and oceanic types of volcanism are replaced by mixed effusive-explosive volcanism of subduction zones. Here we are talking about the so-called “Pacific Ring of Fire,” which encircles the Pacific Ocean and is characterized by high seismicity, numerous manifestations of paleovolcanism and volcanogenic landforms, as well as the existence within its boundaries of more than 75% of the planet’s currently active volcanoes. This is mainly mixed effusive-explosive volcanism of intermediate composition.
All the typical features of the transition zone are most clearly expressed within the northern and western margins of the Pacific Ocean, that is, off the coast of Alaska, Eurasia and Australia. This wide strip between the ocean bed and the land, including the underwater margins of the continents, is unique in the complexity of its structure and in the relationship between the land and the water area; it is distinguished by significant fluctuations in depths and heights, and the intensity of processes occurring both in the depths of the earth’s crust and on the water surface.
The outer edge of the transition zone in the North Pacific Ocean is formed by Aleutian deep sea trench, stretching for 4000 km in a convex arc to the south from the Gulf of Alaska to the shores of the Kamchatka Peninsula, with a maximum depth of 7855 m. This trench, towards which the movement of lithospheric plates of the northern part of the Pacific Ocean is directed, borders the underwater foot of the Aleutian island chain from the rear, most of them They are volcanoes of the explosive-effusive type. About 25 of them are active.
A continuation of this zone off the coast of Eurasia is the system deep sea trenches, which are associated with the deepest areas of the World Ocean and, at the same time, areas of the most complete and diverse manifestation of volcanism, both ancient and modern, both on island arcs and on the outskirts of the continent. In the rear of the Kuril-Kamchatka deep-sea trench (maximum depth over 9700 m) there is the Kamchatka Peninsula with its 160 volcanoes, of which 28 are active, and the arc of the volcanic Kuril Islands with 40 active volcanoes. The Kuril Islands are the peaks of an underwater mountain range that rises above the bottom of the Sea of Okhotsk by 2000-3000 m, and the maximum depth of the Kuril-Kamchatka Trench, which runs from the Pacific Ocean, exceeds 10,500 m.
The system of deep-sea trenches continues to the south with the Japan Trench, and the volcanogenic zone continues with the extinct and active volcanoes of the Japanese Islands. The entire system of trenches, as well as island arcs, starting from the Kamchatka Peninsula, separates the shallow shelf seas of Okhotsk and East China from the Eurasian continent, as well as the Sea of Japan depression located between them with a maximum depth of 3720 m.
Near the southern part of the Japanese Islands, the transition zone expands and becomes more complex, the strip of deep-sea trenches is divided into two branches, bordering on both sides the vast Philippine Sea, the depression of which has a complex structure and a maximum depth of more than 7000 m. From the Pacific Ocean it is limited by the Mariana Trench with its maximum depth World ocean 11,022 m and the arc of the Mariana Islands. The internal branch, limiting the Philippine Sea from the west, is formed by the trench and the Ryukyu Islands and continues further with the Philippine trench and the arc of the Philippine Islands. The Philippine Trench stretches along the foot of the islands of the same name for more than 1,300 km and has a maximum depth of 10,265 m. There are ten active and many extinct volcanoes on the islands. Between the island arcs and Southeast Asia, within the continental shelf, lie the East China Sea and most of the South China Sea (the largest in the region). Only the eastern part of the South China Sea and the interisland seas of the Malay Archipelago reach depths of over 5000 m, and their base is a transitional crust.
Along the equator, the transition zone within the Sunda archipelago and its island seas continues towards the Indian Ocean. There are a total of 500 volcanoes on the Indonesian islands, of which 170 are active.
The southern region of the Pacific Ocean transition zone northeast of Australia is particularly complex. It extends from Kalimantan to New Guinea and further south to 20° S, bordering the Sokhul-Queensland shelf of Australia to the north. This entire section of the transition zone is a complex combination of deep-sea trenches with depths of 6000 m or more, submarine ridges and island arcs, separated by basins or areas of shallow water.
Off the eastern coast of Australia, between New Guinea and New Caledonia, is the Coral Sea. From the east it is limited by a system of deep-sea trenches and island arcs (New Hebrides, etc.). The depths of the Coral basin and other seas of this transitional region (the Fiji Sea and especially the Tasman Sea) reach 5000-9000 m, their bottom is composed of oceanic or transitional type crust.
The hydrological regime of the northern part of this area favors the development of corals, which are especially common in the Coral Sea. On the Australian side, it is limited by a unique natural structure - the Great Barrier Reef, which stretches along the continental shelf for 2,300 km and reaches a width of 150 km in the southern part. It consists of individual islands and entire archipelagos, made of coral limestone and surrounded by underwater reefs of living and dead coral polyps. Narrow channels crossing the Great Barrier Reef lead to the so-called Great Lagoon, the depth of which does not exceed 50 m.
From the side of the Southern Basin of the ocean floor between the islands of Fiji and Samoa, the second arc of trenches, external to the ocean, extends to the southwest: Tonga (its depth of 10,882 m is the maximum depth of the World Ocean in the southern hemisphere) and its continuation Kermadec, maximum depth which also exceeds 10 thousand m. On the Fiji sea side, the Tonga and Kermadec trenches are limited by underwater ridges and arcs of the islands of the same name. In total, they stretch 2000 km to the North Island of New Zealand. The archipelago rises above the underwater plateau that serves as its pedestal. This is a special type of structure of the underwater margins of continents and transition zones, called microcontinents. They vary in size and are uplifts composed of continental crust, topped with islands and surrounded on all sides by basins with oceanic-type crust within the World Ocean.
The transition zone of the eastern part of the Pacific Ocean, facing the continents of North and South America, differs significantly from its western margin. There are no marginal seas or island arcs. A strip of narrow shelf with mainland islands stretches from the south of Alaska to Central America. Along the western coast of Central America, as well as from the equator along the outskirts of South America, there is a system of deep-sea trenches - Central American, Peruvian and Chilean (Atacama) with maximum depths of more than 6000 and 8000 m, respectively. Obviously, the process of formation of this part of the ocean and neighboring continents proceeded in interaction deep-sea trenches and continental lithospheric plates that existed at that time. North America moved onto the trenches along its path to the west and closed them, and the South American Plate moved the Atacama Trench to the west. In both cases, as a result of the interaction of oceanic and continental structures, folding occurred, the marginal parts of both continents were uplifted, and powerful suture zones were formed - the North American Cordillera and the Andes of South America. Each of these structural zones is characterized by intense seismicity and the manifestation of mixed types of volcanism. O.K. Leontiev considered it possible to compare them with the underwater ridges of the island arcs of the western transition zone of the Pacific Ocean.
It is advisable to consider the megarelief of two planetary landforms of the Earth - the bed of the oceans (thalassocratons) and the mid-ocean ridges - together. This is mainly due to the peculiarities of the orography of each of the oceans and the World Ocean as a whole.
Let us recall that the ocean floor is characterized by the oceanic type of the earth's crust, characterized by low thickness (5-10 km) and the absence of a granite layer. Mid-ocean ridges are characterized by a special type of structure of the earth's crust - riftogenic, on the basis of which they are distinguished as a special planetary form.
The ocean floor corresponds structurally to oceanic platforms, or thalassocratons. When looking at a bathymetric map of the bottom of any ocean, the cellularity of its megarelief is striking. Giant basins with a relatively flat, often hilly bottom are separated by large ridges, ramparts, and hills. The most typical oceanic crust is found at the bottoms of basins. At higher elevations, as a rule, the thickness of the crust increases, and in some cases, a layer of increased density is found under the typical basalt layer and the Moho surface is indistinctly distinguished.
The great depth of the oceanic basins is noteworthy, which indicates, first of all, the predominance of negative vertical movements in these areas of the earth's surface. If continents with their inherent positive movements are predominantly areas of denudation, then ocean basins serve as areas of accumulation of a wide variety of sedimentary material, mainly coming from land.
Mid-ocean ridges morphologically represent the largest swellings of the earth's crust, elongated in the meridional or submeridional direction, forming, as it were. huge(up to 2000 km in width and up to 6 km in relative height) a vault with a complexly dissected relief of the slopes and especially its axial zone, where asymmetrical ridges are developed, separated by deep, sharply defined troughs (Fig. 31) with a flat bottom and steep sides, elongated in accordance with the general strike of the mid-ocean ridge. These relief forms are the result of discontinuous disturbances of the earth's crust such as rifts, therefore the axial zones of the middle ridges are called rift zones.
Mid-ocean ridges form a single planetary system(Fig. 32). One of the main geological and geophysical features of mid-ocean ridges, unique to them, is the high velocity of elastic waves in the earth’s crust. Another significant geophysical feature is the high heat flow value. Important features also include the high seismicity of the median ridges and the confinement of numerous island and underwater oceanic volcanoes to their ridges and slopes. All this, as well as the sharp dissection of the relief, indicates that mid-ocean ridges are areas of intense modern tectogenesis and, according to the concept of lithospheric plate tectonics, represent spreading zones.
The geological structure of the ridges and rift valleys of mid-ocean ridges involves ultramafic rocks, mainly various peridotites, which often compose entire blocks that form individual rift ridges. Large outliers and stocks of ultrabasic rocks in rift zones penetrate into the earth's crust from the upper mantle and mix here with blocks of basic rocks, forming the so-called melange. This significantly increases the overall density of the crust beneath the rift zones.
Data on the morphostructures of transition zones, the ocean floor and mid-ocean ridges, given in Chapter. 10 and 11, can be depicted in the form of a generalized profile of the ocean floor shown in Fig. 33.
Relief of the bed of the Arctic Ocean. Arctic mid-ridges and uplifts. Even thirty years ago, on physical-geographical maps, the bed of the Arctic Ocean (AO) within its Arctic Basin was depicted as a single basin with a flat, uniform bottom. The modern understanding of the structure of the bottom of this ocean, thanks to many years of Soviet and American research, is completely different. Now a whole series of underwater ridges and hills have been established, dividing the Arctic basin of the Arctic Ocean into several basins (Fig. 34).
Near the Pole, the Arctic Basin crosses rise of Lomonosov, starting in the American sector near Ellesmere Island and adjacent to the Siberian shelf north of the New Siberian Islands. Another rise extends from the Ellesmere Island shelf - Alpha plateau, which goes into Mendeleev's rise. In the Siberian sector of the ocean, this rise is adjacent to the shelf of the East Siberian Sea.
Between the uplifts there are flat-bottomed basins Makarova And Tolya with a maximum depth of about 4 km. Between the Mendeleev Rise and the Alaska shelf is the largest ocean basin - Beaufort, its maximum depth is 4680 m. Most of the bottom of the basin is occupied flat abyssal plain.
In the European-Siberian sector of the ocean there is Gakkel Ridge. The axial part of the ridge, in contrast to the Lomonosov and Mendeleev uplifts, has a highly dissected topography: a number of individual short ridges, separated by deep rift valleys, en echelon located along the axis of the ridge. Between the Gakkel Ridge and the Lomonosov Rise there is a basin Amundsen(The North Pole is located within this basin; the depth of the ocean below it is 4316 m). South of the Gakkel Ridge lies a basin Nansen. Its maximum depth is about 4000 m.
Except Arctic Basin in the Arctic Ocean stands out Norwegian-Greenland basin. Here basins of the Greenland and Norwegian seas separate mid-ocean ridges Knipovicha, Mona And Icelandic. The maximum depth of the Greenland Basin is 5327 m, confined to the rift valley of the Knipovich Ridge. This is the maximum depth of the ocean. The greatest depth of the Norwegian Basin is about 4000 m. The bottom topography of both basins is complicated seamounts And hills. There are also several small flat plains, formed due to the accumulation of deep-sea sediments. On the Icelandic ridge there is an active volcano islands Jan Mayen.
Relief of the Atlantic Ocean bed. Mid-Atlantic Ridge. The core orographic element of the Atlantic Ocean bottom relief is Mid-Atlantic Ridge, which stretches within its borders from Iceland in the north to 65° south. w. in the south. The strike of the ridge is not constant, but in general it is close to meridional, with the exception of the equatorial section, where it becomes sublatitudinal for some distance. The width of the ridge reaches 2500 km in the South Atlantic, but north of Iceland it decreases to 300 km.
The relative height of the Mid-Atlantic Ridge is up to 4 km. Morphologically, it would be more correct to call it, like other mid-ocean ridges, not a ridge, but a mountainous country or highland, since it consists of individual ridges, mountain ranges, longitudinal valleys and depressions. The most dissected and contrasting relief is characteristic of the rift zone of the ridge, represented by a complex system of horst ridges and narrow grabens- rift valleys, Moreover, depths of the order of 5-6 km are often confined to the latter. Maximum depths are usually characterized by narrow transverse depressions associated with fault zones cutting the ridge. An example of such a depression is Romanche depression(7730 m). Transverse faults further complicate the topography of both the rift zone and the flanks of the Mid-Atlantic Ridge.
Like other mid-ocean ridges, the Mid-Atlantic Ridge the earth's crust is characterized by riftogenic type, characterized by increased density and the absence of a clearly defined Moho boundary. Along with basalts, ultramafic rocks - peridotites and dunites - are common in the rift zone of the ridge. The axial zone and flanks are characterized by alternating positive and negative magnetic anomalies, with the most pronounced positive anomaly noted in the central rift valley. Gravity anomalies in the Bouguer reduction (i.e., normalized to sea level) over the median ridge are usually positive, but for rift valleys they are often negative.
The epicenters of earthquakes are located in the rift zone. The greatest concentration of epicenters was noted in sections of the ridge intersected by latitudinal and sublatitudinal transform faults. One of these faults crosses the ridge in the Azores region. Active manifestations of modern volcanism are associated with it. A large number of transverse faults parallel to each other are noted in the equatorial part of the ridge. Individual segments of the ridge, cut off by these faults, are shifted relative to each other by many tens and even hundreds of kilometers (see Fig. 12). These shifts determine the general sublatitudinal extent of the Mid-Atlantic Ridge on its equatorial segment.
The flanks of the ridge also have sharply rugged mountainous terrain and are characterized by manifestations of modern volcanism of the central type. The most significant modern active volcanoes on the wings and in the rift zone of the ridge are volcanoes of the Reykjanes range(segment of the median ridge adjacent to Iceland), equatorial part Tristan da Cunha ridge. In the southern part of the ocean, the Mid-Atlantic Ridge passes into the African-Antarctic submarine ridge.
The bed of the Atlantic Ocean on both sides of the median ridge is composed of oceanic-type crust. The thinnest thickness of the earth's crust is observed under oceanic basins, separated by underwater hills and ridges with increased thickness of the earth's crust. The names of some basins and hills are shown in the attached diagram (Fig. 35).
Let us consider, as an example, the structure of one of the underwater hills of the ocean floor - Bermuda plateau, located in the central part North American Basin. It has the appearance of a horst-anteclise with steep southeastern and gentle northwestern slopes. Fault tectonics is clearly evident in the structure of the plateau. The steep slope is dissected by deep hollows such as submarine canyons, which are narrow grabens open towards the basin. A whole network of faults is also evident in the topography of the plateau. Underwater volcanoes rise at the intersections of faults. A group of the highest volcanoes forms the foundation of Bermuda, composed of coralline limestones.
The structure of the bottom topography of oceanic basins is quite monotonous. In almost every basin of the Atlantic Ocean there are two main types of relief. Most of the basin bottom area has hilly terrain with a vertical dissection of an average of 250-600 m, in some cases - up to 1000 m. This type of relief is called "relief of abyssal hills". A smaller part of the basin bottom area is almost perfectly leveled. These completely flat spaces with insignificant surface slopes are called flat abyssal plains. They usually occupy not the deepest parts of the basins, but those located closer to the continental slope and foot. Seismic studies have shown that on the plains the thickness of the sedimentary layer reaches 1.5 km, and within the abyssal hills the thickness of the sedimentary layer is measured in several hundred or even tens of meters.
The origin of the abyssal hills is associated with volcanic processes. With a very low thickness of the oceanic crust, the formation of a network of small faults during its subsidence, along which volcanic manifestations took place, is permissible. After the extinction of the magmatic process, laccoliths or shield volcanoes were partially buried under a layer of bottom sediments, transforming them into abyssal hills.
Relief of the bed and middle ridges of the Indian Ocean. There are several mid-ocean ridges in the Indian Ocean: West Indian, Arabian Indian, Central Indian, passing east of Amsterdam Island into Australasian-Antarctic(Fig. 36). All ridges, with the exception of the Australian-Antarctic Ridge, are relatively well studied and show great similarity in structure with the Mid-Atlantic Ridge. The Australian-Antarctic Ridge (uplift) has been less studied. It is distinguished by less dismemberment of flank zones, lower height and weak expression of the rift zone.
The middle ridges of the Indian Ocean, as in the Atlantic, are broken not only by longitudinal faults, which give the arch a rift structure, but also by transverse faults. However, faults of meridional or (less often) sublatitudinal, but not latitudinal, strike predominate. With one of these sublatitudinal faults (Vima fault), cutting through the southern part of the Arabian-Indian ridge, the maximum depth of the Indian Ocean is associated - 6400 m. A wide zone of tectonic fragmentation has been identified in the middle part of the Australian-Antarctic Rise. It is expressed by a complex system of short meridional ridges and depressions.
Along with the median ridges, the Indian Ocean has several large ridges with an oceanic type of crustal structure and a fault-block structure. The largest of them is East Indian Ridge, starting in the southern part of the Bay of Bengal and ending near the Central Indian Ridge. This huge mountain system (more extensive than the Urals) was discovered in the early 60s.
Let us mention two more large blocky ridges - Maldivian And Mascarene, located in the western part of the ocean. The Mascarene ridge in the northern part (region of the Seychelles) has a continental type of crust. According to some researchers, this is a fragment of the once united continent of the southern hemisphere - Gondwana, which united all the southern continents of our planet at the beginning of the Mesozoic. According to others, it is an underdeveloped continent. Madagascar, Mozambican ranges And Agulhas Hill, located in the southwestern part of the ocean, they are composed of continental-type crust and should be considered as elements of the underwater margin of the African continent.
Of the largest orographic elements of the Indian Ocean, we also mention Crozet plateau- typical oceanic volcanic formation, Kerguelen plateau, representing a protrusion of the Antarctic continental platform projecting far to the north.
For basin bottoms Indian Ocean The most characteristic relief is abyssal hills. Flat abyssal plains occupy only a small area of the ocean floor.
Relief of the bed and mid-ridges of the Pacific Ocean. The Pacific Ocean, whose area makes up almost half of the entire World Ocean, has the greatest diversity of bed megarelief. The middle ridges of the Pacific Ocean (there are two of them - South And Eastern Pacific) in structure they resemble the Australasian-Antarctic: their wide flanks have a relatively weakly dissected relief, and the rift structure of the axial zone is not as pronounced as in the Mid-Atlantic or Arabian-Indian ridges. In the structure of the middle ridges of the Pacific Ocean, a significant role is played by powerful zones of oceanic faults cutting them across the strike. Along the faults, the middle ridge is divided into a number of segments with parallelepipedal outlines, shifted laterally relative to each other 1 . The geophysical features of the structure of the mid-Pacific ridges are similar to those described for other mid-ocean ridges.
Between 40 and 30° S. w. departs from the East Pacific Ridge to the southeast Chilean ridge, having a rift structure and characterized by seismicity and volcanism, therefore it can be considered a branch of the mid-ocean system. Note that the East and South Pacific Ridges, like the Australian-Antarctic Ridge in the Indian Ocean, as well as the Chilean Ridge, differ morphologically from other mid-ocean ridges in their large width and relatively small dissection of the rift zone. Proponents of the concept of plate tectonics associate these features with high spreading rates. But it is possible that these morphological features indicate the youth of the named morphostructures. Due to this morphological specificity, they are usually (on maps, in the literature) called not ridges, but uplifts.
Other linearly elongated orographic elements of the Pacific Ocean floor (Fig. 37) are characterized by the oceanic type of the earth's crust. They look like large shafts, on the arches of which volcanoes are planted, often forming entire volcanic chains. The most grandiose of them in terms of length, height and violent manifestations of volcanism oceanic type Hawaiian Ridge, crowned by islands of the same name. The volcanoes of these ridges are shield volcanoes. They erupt mafic magma.
In the Pacific Ocean, flat-topped seamounts are especially numerous - guyots(Fig. 38). Most common on Marcus Necker Seamounts, which stretch in a latitudinal direction from the southern part of the Hawaiian Islands to the west to the islands of Benin and Volcano. The depth above the peaks of many guyots reaches 2500 m (average 1300 m). As noted above, such a depth obviously indicates a sinking of the ocean floor, since there is no reason to assume such a significant decrease in its level in the past.
Many oceanic arched rises have mountain peaks crowned with coral structures - ring reefs, or atolls. According to geophysical surveys and drilling, the mountains that form the basis for the coral reefs are also volcanic formations. It is interesting that most of the oceanic arched ridges with volcanic chains, guyots, and coral reefs are confined to a wide strip crossing the Pacific Ocean from southeast to northwest, from the Easter Island area to the Northwest Basin inclusive. According to G. Menard, these oceanic rises are the remnants of an ancient mid-ocean ridge, which at the end of the Cretaceous - the beginning of the Paleogene was destroyed as a result of powerful tectonic processes. Violent volcanic eruptions occurred along deep faults, and then large sections of the ridge experienced subsidence. A labyrinth of basins, mountain rises, volcanoes, guyots and coral atolls emerged - an extremely complex topography of the central and northwestern parts of the Pacific Ocean floor. The scale of the volcanic processes of that time is evidenced by the total volume of ejected volcanic material. It, according to G. Menard's calculations, turned out to be tens of times greater than the total volume of effusives that make up the lava plateaus - the Columbian and Deccan. composed of volcanic material trains at the foot of underwater ridges (remains of the median ridge). They look like sloping abyssal plains, called "island plumes" or apronov. Sloping plains are one of the specific types of relief in the marginal parts of the basins of the Pacific Ocean floor.
The bed of the Pacific Ocean is almost everywhere separated from the continents by deep-sea trenches, so the supply of terrigenous material from land to the Pacific Ocean is small. As a result, the thickness of sediments in the basins of the Pacific Ocean is low. Relief dominates everywhere abyssal hills. Available only within the Gulf of Alaska vast flat plain formed by young and ancient fans of turbidity currents (see Chapter 20). Numerous guyots rise above the plain. The vast abyssal plain occupies most of the Antarctic basin of the Pacific Ocean- Bellingshausen basin. The widespread development of abyssal plains is also observed in the Antarctic basins of the Indian and Atlantic oceans. This is due to the significant supply of terrigenous material by floating ice - icebergs, formed due to the flow of ice from the Antarctic ice sheet.
For the bed of the Pacific characterized by zones of deep faults of latitudinal strike, traceable over several thousand kilometers. They are expressed in the relief of the bottom of the basins in the form of narrow blocky ridges-horsts stretching from west to east and accompanying troughs-grabens. Faults also cross the East Pacific and South Pacific rises, and individual segments of these rises, as already mentioned, are shifted relative to each other by hundreds of kilometers. Thus, in the Pacific and Atlantic oceans there are indisputable signs of significant lateral movements of the earth's crust. However, the main significance in the development of the megarelief of the bottom of the oceans in general and the Pacific in particular belongs, apparently, to the vertical movements of the earth's crust. For the middle ridges, the main role is played by positive, and for the ocean floor, negative movements play a major role. This is evidenced by the presence of guyots at depths tens of times greater than the possible range of ocean level fluctuations, and by the great thickness of the coral limestones that make up the oceanic atolls. Drilling on some atolls of the Pacific Ocean has shown that the total thickness of coral deposits, starting from the Eocene, reaches 1400 m, while reef-forming corals can live only at depths of up to 50 m. Natural fluctuations in ocean level due to melting ice sheets do not exceed 120 m. Data from deep-sea drilling also indicate significant vertical movements (mostly negative) of the ocean floor. Apparently, during the Cenozoic, the average subsidence of the ocean floor was about 1 km.
