island arcs

It's chains volcanic islands above the subduction zone (the place where oceanic crust sinks into the mantle) that occur when one oceanic plate sinks under another. Island arcs are formed when two oceanic plates collide. One of the plates is below and is absorbed into the mantle, on the other (upper) volcanoes form. The curved side of the island arc is directed towards the absorbed plate; on this side, there is a deep-water trench. The basis for the island arcs are underwater ridges from 40 to 300 km, with a length of up to 1000 km or more. The arch of the ridge protrudes above sea level in the form of islands. Often, island arcs consist of parallel mountain ranges, one of which is often external (facing deep sea trench), is expressed only by an underwater ridge. In this case, the ridges are separated from each other by a longitudinal depression up to 3-4.5 km deep, filled with a 2-3 km sediment layer. In the early stages of development, island arcs are a zone of thickening of the oceanic crust, volcanic structures planted on the crest. For more late stages development, island arcs form large massifs of insular or peninsular land, the earth's crust here approaches the continental type in structure.

Island arcs are widely developed on the margins of the Pacific Ocean. These are the Commander-Aleutian, Kuril, Japanese, Mariana and others. Indian Ocean the most famous is the Sunda arc. AT Atlantic Ocean- Antilles and South Antilles arc.

deep sea trenches

These are narrow (100–150 km) and extended deep depressions (Fig. 10). The bottom of the gutters has V-shape, rarely flat, the walls are steep. The inner slopes adjacent to the island arcs are steeper (up to 10–15°), while the opposite slopes facing open ocean, gentle (about 2-3°). The slope of the trench is complicated by longitudinal grabens and horsts, and the opposite slope is complicated by a stepped system of steep faults. The slopes and the bottom are covered with sediments, sometimes reaching a thickness of 2-3 km (Yavan Trench). The sediments of the trenches are represented by biogenic-terrigenous and terrigenous-volcanic silts, deposits of turbidity flows and edaphogenic formations are frequent. Edafogenic formations are unsorted products of collapses and landslides with blocks of bedrock.

The depth of the trenches ranges from 7000-8000 to 11000 m. The maximum depth recorded in the Mariana Trench is 11022 m.

Troughs are observed throughout the periphery of the Pacific Ocean. In the western part of the ocean, they stretch from the Kuril-Kamchatka Trench in the north, through the Japanese, Izu-Bonin, Mariana, Mindanao, New British, Bougainville, Novogebrida to Tonga and Kermadec in the south. The Atakama, Central American and Aleutian trenches are located in the eastern part of the ocean. In the Atlantic Ocean - Puerto Rican, South Antilles. In the Indian Ocean, the Java Trench. In the Northern Arctic Ocean gutters were not found.

Deep-sea trenches are tectonically confined to subduction zones. Subduction develops where the continental and oceanic plates (or oceanic with oceanic) converge. When they move in the opposite direction, the heavier plate (always oceanic) moves along the other and then sinks into the mantle. It has been established that subduction develops differently depending on the ratio of plate motion vectors, the age of the subducting lithosphere, and a number of other factors.

Since during subduction one of lithospheric plates is absorbed at depth, often carrying with it the sedimentary formations of the trench and even the rocks of the hanging wing, the study of subduction processes is associated with great difficulty. Geological research also hampered by the deep waters of the ocean. Therefore, the results of the first detailed mapping of the bottom section in the trenches, which was carried out under the Franco-Japanese Kaiko program, are of great value. Off the coast of Barbados, and then on the slope of the Nankai trench, during drilling, it was possible to cross the displacement zone of the subduction zone, located at the drilling point at a depth of several hundred meters below the bottom surface.

Modern deep sea trenches extend perpendicular to the direction of subduction (orthogonal subduction) or under acute angle to this direction (skew-oriented subduction). As mentioned above, the profile of deep-sea trenches is always asymmetric: the subducting limb is gentle, while the hanging limb is steeper. The details of the relief vary depending on the stress state of the lithospheric plates, on the subduction regime, and on other conditions.

Of interest are the relief forms of the territories adjacent to the deep-water trenches, the structure of which is also determined by the zones of subduction development. On the ocean side, these are gentle marginal ridges that rise 200–1000 m above the ocean floor. Judging by geophysical data, marginal ridges represent an anticlinal bend in the oceanic lithosphere. Where the frictional cohesion of the lithospheric plates is high, the height of the edge swell is perpendicular to the relative depth of the adjacent section of the trench.

With opposite side, above the hanging wing of the subduction zone, high ridges or submarine ridges stretch parallel to the trough, having a different structure and origin. If subduction is directed directly under the margin of the continent (and the deep-sea trench adjoins this margin), a coastal ridge and a main ridge separated from it by longitudinal valleys are usually formed, the relief of which is complicated by volcanic structures.

Since any subduction zone goes obliquely to the depth, its effect on the hanging wing and its topography can extend to 600–700 km or more from the trench, which depends primarily on the angle of inclination. At the same time, in accordance with tectonic conditions, various forms relief when characterizing lateral structural rows above subduction zones.

Deep-sea trenches and associated marginal ridges are important morphological structures of active ocean margins, stretching for thousands of kilometers along island arcs and the eastern continental rim of the Pacific Ocean. Deep-sea trenches trace the exit to the surface of seismic focal zones, reflecting in relief the boundary between the oceanic and continental segments of the Earth's lithosphere. Ocean trenches are narrow, extended depressions ocean floor, which are the deepest zones of the oceans.

There are two types of ocean trenches:

• 1. Ocean trenches associated with island arcs (Marian, Japanese, Sunda, Kamchatka, etc.;
• 2. Ocean trenches adjacent to the continents (Peruan-Chilean, Central American etc.).

The trenches of the island arcs are usually deeper (the Mariana Trench - 11022 m). At high sedimentation rates, oceanic trenches can be filled with sediment ( South coast Chile).

Most of the trenches are arcuate, with their concave side facing the island arc or the continent. In the section, they look like regular asymmetric depressions (Fig. 6.28) with a relatively steep (up to 10 ° or more) slope adjacent to the land and a more gentle (5 °) oceanic slope of the trench. On the outer ocean edge of the trench

Rice. 6.28. Schematic structure deep-sea trench, an external dome-shaped uplift is observed, often rising almost 500 m above regional level adjacent ocean floor.

Gutters, even the deepest ones, have little to no precise V-shape.

The width of the oceanic trenches is about 100 km, the length can reach several thousand kilometers: the Tonga and Kermadec trenches are about 700 km long, the Peru-Chile - 4500 km. The narrow bottom of an oceanic trench ranging in width from a few hundred meters to several kilometers is usually flat and covered with sediment. In section, the sediments look like a wedge. They are represented in the lower part of the wedge by hemi-pelagic and pelagic (prefix hemi - semi) sediments of the oceanic plate, falling towards the land. Above them, they are unconformably overlain by horizontally layered sediments of turbidity flows (turbidites) formed due to erosion of the continent or island arc. The type and amount of precipitation, the axial zone of the trench are determined by the ratio between the rates of precipitation and the rate of convergence of plates. Sedimentary wedges in the axial zones of the island arc troughs are thinner than those in the troughs adjacent to the continents. This is due to the limited exposure above the ocean (sea) level of the arc surface, which is the main source of precipitation, compared to the continent.

Ocean trenches near continental margins may consist of a series of structurally isolated small depressions separated by sills. Within their limits, in the presence of a slight inclination of the axis, a channel can form, along which turbidity flows flow. The latter can create alluvial swells and erosion structures in the body of the sedimentary wedge and control the distribution of lithofacies in the trench. In areas with very rapidly sedimentation and low convergence rates (Oregon-Washington Trench) can produce extensive fans moving from the continent towards the ocean over the axial sedimentary clip.

Oceanic trenches are convergent plate margins where an oceanic plate is subducted either under another oceanic plate (under an island arc) or under a continent. The rate of convergence of plates ranges from zero to 100 cm/yr. When plates collide, one of them, bending, moves under the other, which leads to regular strong earthquakes with foci under the slope of the trench adjacent to land, the formation of magma chambers and active volcanoes (Fig. 6.29). In this case, the emerging stresses in the subducting plate are realized in two forms:

• 1. An external swell-shaped (dome-shaped) uplift is formed with an average width of up to 200 km and a height of up to 500 m.
• 2. Stepwise faults and large structures such as horsts and grabens are formed in the curved oceanic crust on the oceanic slope of the trench.

Rice. 6.29. Kamchatka Deep Trench: 1 - active volcanoes, 2 - deep water trough 3 - isolines 1" hollows of magma chambers

There are no folded deformations in the sedimentary strata at the bottom of the trench. Gently dipping thrusts are formed in the slope of the trench adjacent to the land. The underthrust zone (the Benioff - Vadati - Zavaritsky zone) plunges at a slight angle from the trench axis towards the land. It is within this zone that almost all earthquake sources are concentrated.

In the Central American, Peru-Chile and Yap trenches, young basalts were discovered by boreholes (Fig. 6.30). The intensity of magnetic anomalies of the ocean floor near the trench is usually lower. This is due to the presence of numerous faults and ruptures in the curving oceanic crust.

Rice. 6.30. Tectonic scheme of the Central American sector of the Pacific Ocean, according to Yu.I. Dmitriev (1987): I- deep sea trenches 2 - active volcanoes, 3 - wells that uncovered basalts

The accretionary prism of sediments in the lower part of the slope of the trench is deformed, crumpled into folds, and broken by faults and overthrusts into a series of plates and blocks.

Occasionally, an advancing continent or island arc rips away sediment from an axial trough and oceanic plate, forming an accretionary sediment wedge. This accretion process is accompanied by the formation of scaly thrust sheets, chaotic sedimentary bodies, and complex folds. Sedimentary-basalt mélange containing fragments and large blocks oceanic crust, sedimentary wedge and turbidites. This mass of accumulated unconsolidated sediments creates a large negative isostatic gravity anomaly, the axis of which is somewhat shifted to the land relative to the trench axis.

The structure of the cuts. The thickness of sediments above the basalt basement varies greatly. In the Central American trench in the well. 500 V, it is 133.5 m, in the well. 495 - 428 m, while sedimentary strata up to 4 km thick are known in other gutters. At the bottom of the trench, the presence of landslide facies and redeposited sediments is noted. Sedimentary and volcanic-sedimentary rocks are widely developed: volcanomictic siltstones, sandstones, gravelstones, clayey, siliceous-clayey rocks, edaphogenic breccias, and basalts in the outer zones. Basalts are characterized by petrochemical and geochemical characteristics that are transitional between typical oceanic and island-arc varieties (Dmitriev, 1987).

In scaly structures of accretionary prisms, these rocks alternate with gravitational olistostromes and landslide breccias. The fragments contain outliers of the oceanic crust: serpentinized ultramafic rocks and basalts. Metamorphic rocks of high pressure and low temperatures - glaucophane schists.

Minerageny. Oil and gas fields in weakly lithified strata. Deposits of antimony and mercury in paleoanalogues, in metasomatites along host rocks (jasperoids and listvenites) in zones of tectonic faults.

test questions

• 1. Determine the position of deep-sea trenches in the structure of the Earth.
• 2. Name the morphometric and structural features deep sea trenches.
• 3. Describe the structure and composition of rock associations that fill deep-water trenches.

An ocean trench is a long, narrow depression on the ocean floor, hidden deep under water. These dark, mystical recesses can be found at depths of up to 10,994 meters. In comparison, if Mount Everest were placed at the bottom of the deepest depression, its summit would be about 2.1 kilometers below the surface of the water.

Formation of ocean trenches

ocean trench

The world is full of high volcanoes and mountains, but deep oceanic trenches outshine any of the continental highlands. How are these depressions formed? The short answer comes from geology and the study of movements tectonic plates that relate to earthquakes as well as to volcanic activity.

Scientists have found that deep blocks earth's crust moving on the surface of the Earth's mantle. As a rule, oceanic crust is subducted under island arcs or continental margins. The border where they meet is the places that are deep ocean trenches. For example, the Mariana Trench, located at the bottom of the Pacific Ocean, next to the Mariana Island Arc, off the coast of Japan, is the result of so-called "subduction". The Mariana Trench formed at the junction of the Eurasian and Philippine plates.

Location of gutters

Ocean trenches exist throughout the world and are generally the deepest regions. These include: the Philippine Trench, the Tonga Trench, the South Sandwich Trench, the Puerto Rico Trench, the Peru-Chile Trench, and others.

Many (but not all) are directly related to subduction. Interestingly, the Diamantina Trench formed about 40 million years ago, when they demarcated. Most of the deepest ocean trenches known to be found in the Pacific Ocean.

The deepest point of the Mariana Trench is called the Challenger Deep, and it lies at a depth of almost 11 km. However, not all oceanic trenches are as deep as the Mariana Trench. With age, gutters can fill with sediment (sand, rocks, mud, and dead organisms that settle to the ocean floor).

Exploring ocean trenches

Most gutters were not known until the end of the 20th century. Their study requires specialized submersibles, which did not exist until the second half of the 1900s.

Bathyscaphe "Trieste"

These deep ocean trenches are not suitable for most living organisms. The pressure of the water at these depths will instantly kill a person, which is why no one dared to explore the bottom of the Mariana Trench for many years. However, in 1960, two explorers dived into the Challenger Deep using a bathyscaphe called the Trieste. And only in 2012 (52 years later) another person dared to conquer the deepest point of the oceans. It was the film director (known for the films "Titanic", "Avatar", etc.) and underwater explorer James Cameron, who made a solo dive using the "Deepsea Challenger" bathyscaphe and reached the bottom in the Challenger Basin of the Mariana Trench. Most other deep-sea research vehicles, such as Alvin (used by the Woods Hole Oceanographic Institute in Massachusetts), have not dived to great depths yet, but can still go down to about 3,600 meters.

Is there life in deep sea trenches?

Surprisingly, despite high pressure water and the cold temperatures that exist at the bottom of the deep sea trenches, life thrives in these extreme conditions.

Tiny unicellular organisms live at great depths, as well as some types of fish (including), tube worms and sea cucumbers.

Future exploration of deep sea trenches

Exploring the deep sea is expensive and difficult process, although the scientific and economic rewards can be quite significant. Human intelligence (like Cameron's deep-sea dive) is dangerous. Future research can be well relied upon (according to at least partially) to automated unmanned aerial vehicles, just like astronomers use them to study distant planets. There are many reasons to continue exploring the depths of the ocean; they remain the least explored terrestrial environments. Further research will help scientists understand the workings of plate tectonics, as well as identify new life forms that have adapted to some of the most inhospitable habitats on the planet.

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Since I am a lover of everything unusual on our planet, I cannot pass by this question without sharing my knowledge. I will tell you about how the gutters are formed and describe the deepest of them - the Mariana.

What is a deep sea trench

Found in some parts of the ocean special forms bottom - deep-sea trenches. As a rule, they are a narrow depression, the slopes of which go down sheer for many kilometers. In fact, this transition region between the ocean and the mainland, located along the island arcs and, as a rule, repeating their outlines.

How are deep sea trenches formed?

The reason for the formation of such areas is the mobility of lithospheric plates, when the oceanic one goes under the continental one, which is much heavier. These areas are different increased seismicity and volcanism. Most of trenches is located in the Pacific Ocean, and there is also the deepest - Mariana. There are 14 such formations in total, but I will give an example of only the largest ones. So:

• Mariana - 11035 m., Pacific Ocean;
• Tonga - 10889 m., Pacific Ocean;
• Philippine - 10236 m., Pacific Ocean;
• Kermadec - 10059 m., Pacific Ocean;
• Izu-Ogasawara - 9826 m., Pacific Ocean.

Mariana Trench

Its length is more than a thousand kilometers, however, despite the enormous depth and impressive size, this place does not stand out on the surface. Despite the development of technology in our time, this is not enough to detailed study of this place and its inhabitants, and the reason for this is the gigantic pressure at the bottom. However, even superficial studies have shown that life is possible under such conditions. For example, huge amoebas were discovered - xenophyophores, the size of which reaches 12 centimeters. Presumably, this is a consequence of difficult conditions: pressure, low temperature and insufficient lighting.

This place is recognized as a national monument of the United States, and is also the largest marine reserve in the world. Therefore, any activity is prohibited here, whether it is fishing or mining.

In the marginal parts of the oceans, special forms of bottom topography have been discovered - deep-sea trenches. These are relatively narrow depressions with steep, steep slopes, stretching for hundreds and thousands of kilometers. The depth of such depressions is very great. Deep-sea trenches have an almost flat bottom. It is in them that the greatest depths of the oceans are located. Typically, the trenches are located on the oceanic side of the island arcs, repeating their bend, or stretch along the continents. Deep sea trenches are the transition zone between the mainland and the ocean.

The formation of trenches is associated with the movement of lithospheric plates. The oceanic plate bends and, as it were, “dives” under the continental one. In this case, the edge of the oceanic plate, plunging into the mantle, forms a trough. Areas of deep-water trenches are located in zones of volcanism and high seismicity. This is explained by the fact that the trenches are adjacent to the edges of the lithospheric plates.

According to most scientists, deep-sea trenches are considered to be marginal troughs and it is there that intensive accumulation of sediments of destroyed rocks.

The deepest on Earth is the Mariana Trench. Its depth reaches 11,022 m. It was discovered in the 1950s by an expedition aboard the Soviet research vessel Vityaz. The research of this expedition was very great importance to study the gutters.

Most of the trenches are in the Pacific Ocean.

Earth's deep sea trenches

Gutter name	Depth, m	Ocean
Mariana Trench	11022	Quiet
Tonga (Oceania)	10882	Quiet
Philippine Trench	10265	Quiet
Kermadec (Oceania)	10047	Quiet
Izu-Ogasawara	9810	Quiet
Kuril-Kamchatka Trench	9783	Quiet
Puerto Rico Trench	8742	Atlantic
Japanese chute	8412	Quiet
South Sandwich Trench	8264	Atlantic
Chile Trench	8180	Quiet
Aleutian Trench	7855	Quiet
sunda trench	7729	Indian
Central American Trench	6639	Quiet
Peruvian Trench	6601	Quiet

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