On earth, earthquakes and volcanic eruptions constantly occur in different places. There are such movements of the earth's surface that a person does not even feel them. These movements occur constantly, regardless of territory, season. Mountains grow and shrink, seas grow and dry. These processes are invisible to the human eye, as they occur slowly, millimeter by millimeter. All this happens due to such phenomena as spreading and subduction.
Subduction
So what is it? Subduction is a tectonic process of movement of the earth's crust. As a result of this process, when the plates collide, the densest rocks that make up the ocean floor advance under the light rocks of the continents and islands. At this moment an incredible amount of energy is released - this is an earthquake. Part of the rocks that have plunged to great depths, when interacting with magma, begins to melt, after which it splashes to the surface through volcanic vents. So there is an eruption of volcanoes.
Subduction of lithospheric plates is an integral part of the life of the planet. It is as important as breathing for humans. It is impossible to stop this process, although many people die every year because of such movements.
Subduction zone
The most massive lithospheric plates are located in the oceans. Subduction is a linear phenomenon. In the subduction zone, two edges of the plate are distinguished - this is the upper, or leading, and the lower immersion. Only during their interaction do eruptions and earthquakes occur. The most active subduction is the seismic focal zone. In this zone, the deepest earthquakes occur. According to seismic data, it was found that the concentration of earthquakes is located deeper from the deepwater trench towards the continent.
The subduction zone is constantly monitored on a seismotomographic profile. The borders of both the upper and lower mantle are clearly visible there. There is no earthquake at the immense depth where the plate is immersed, since the plate at this depth becomes liquid (melts). Accordingly, it no longer gives a jolt at which energy is released. Subduction is a slow process. Its speed is measured in centimeters per year. Basically, its speed is from two to eight centimeters per year.
Continental subduction and collision
The active boundaries of the plates are divided into two types - subduction and collision. The first longer ones are approximately forty-five thousand kilometers. The second is shorter - twelve thousand kilometers. As we have already discussed, collisions occur between two plates - oceanic and continental. But there are collisions between two oceanic or two continental plates. If the first collisions lead to earthquakes and eruptions, then collisions of continental plates work differently. The density of the earth's crust on land is lower than on the ocean floor, so they can not go deep underground. Continental plates in a collision are superimposed and wound on each other, this leads to the formation of mountains. The most famous example of such mountains is the Himalayas.
Why didn’t oceans disappear due to subduction?
During subduction, part of the ocean floor is submerged under a land plate. At great depths, this part melts and exits through the vents of volcanoes. Why have the oceans not disappeared from the face of the earth? This is due to spreading. Spreading is the process of forming a new, young ocean crust. It is formed equal in size to that absorbed by subduction. In subduction zones, accretion is also constantly happening - the continental crust is growing. Thus, due to large-scale sinking of the ocean floor and its melting, building up a new ocean crust and land, the ancient ocean - Tethys disappeared from the Earth. From it there were only pools, our modern seas: the Mediterranean, Azov, Caspian and Black. Recent studies have shown that processes similar to those that led to the death of Tethys occur in the Pacific.
The most famous and large-scale zones of spreading and subduction pass along the Japanese islands, the Kuril Islands, Kamchatka, the coasts of South and North America, the Aleutian Islands, the Caribbean, New Zealand, and the South Sandwich Islands.
Classification of subduction zones
Subduction zones are classified by structure. Types of subduction are divided into the main four.
- Andean type. This type is characteristic of the Pacific coast on the east side. This is a zone in which only the newly formed young crust of the ocean floor at an angle of forty degrees at great speed enters under the continental plate.
- Sonde type. Such a zone is located in places where the ancient massive lithosphere of the ocean plunges under the continental. She leaves at a steep angle. Typically, such a plate goes under the continental, the surface of which is much lower than ocean level.
- Marian type. This zone is formed by the interaction of two sections of the oceanic lithosphere or their movement.
- Japanese type. This is the type of zone where the ocean’s lithosphere advances under the island ensialic arc.
All these four types are conditionally divided into two groups:
- East Pacific (this group includes only the Andean type. This group is characterized by the presence of a vast margin of the continent);
- Western Pacific (all other three types are located in it. The hanging edges of the volcanic arc of the islands are characteristic of this group).
For each type where the subduction process occurs, the basic structures that necessarily exist in different variations are characteristic.
Pre-arc slope and deep-sea trench
The deep-sea trench is characterized by the distance from the center of the trench to the volcanic front. This distance is mainly one hundred to one hundred and fifty kilometers, it is associated with the angle at which the subduction zone is inclined. In the most active parts of the continent’s outskirts, such a distance can reach three hundred and fifty kilometers.
The pre-arc slope consists of two bases - a terrace and a prism. Prism is the bottom of the slope, it is in the structure and structure of the scaly type. Below it borders on the main slope, which comes to the surface, in contact and interacting with precipitation. Prism is formed due to the layering of sediments below. These sediments are superimposed on the oceanic crust and with it go downhill for about forty kilometers. Thus a prism is formed.
Large ledges lie in the area between the prism and the volcanic front. The terraces are separated by ledges. Sedimentation pools are located on the gentle sections of such terraces; volcanic and pelagic sediments are deposited on them. In tropical areas on such terraces reefs can develop, crystalline basement rocks or foreign blocks can be exposed.
Volcanic arc - what is it?
This article refers to the term island, or volcanic, arc. Consider what it is. The tectonically active belt, which coincides with the zones of the largest earthquakes, is designated as a volcanic island arc. It consists of arched chains of stratovolcanoes currently operating. Explosive eruptions are typical of such volcanoes. This is due to the large amount of fluid in the island-arc magma. Arcs can be double or even triple, and a special form is a bifurcated arc. The curvature of each arc is different.
Marginal pools
This term refers to a basin or a number of such basins. They are semi-closed and are formed between the mainland and the island arc. Such basins are formed due to the fact that the mainland is torn or a large piece is separated from it. Usually a young oceanic crust forms in such basins . This process of crust formation in basins is called back-arc spreading. The Bering Sea is one of the types of such pools, it is fenced off. In recent years, there is no new information that rifting occurs somewhere, it is usually associated with the fact that the subduction zone is redirected or abruptly jumps to another place.