Mid-ocean ridges. Tectonic structure of the median ridge

The structure and development of the earth's crust is determined not only by the development, but also by the origin of the overall relief of the ocean floor. Two groups are distinguished here: the oceanic plateau as a phenomenon of the transitional type of the structure of the earth's crust and the median ridge with abyssal plains and gutters.

Attempts to classify

A unified planetary system has been established to generalize information on the structure of the ocean floor. The mid-oceanic ridges are located almost in the middle of the main oceanic spaces, dividing them into equal parts. There are several attempts at classification. Menard, for example, distinguishes them in this way:

• Wide underwater ridges with pronounced seismicity (eg East Pacific);
• Narrow underwater ridges with steep slopes and seismic activity (for example, the Mid-Atlantic Ridge);
• Narrow and steep, but not having seismic activity underwater ridges (eg Mid-Pacific and Tuamotu).

According to GB Udintsev, the mid-oceanic ridges have no analogues on land. D. G. Panov refers the underwater ridges in the Pacific to the corners of the platform - internal and external - and treats them as analogs of the continental platforms. Nevertheless, the tectonic structure of the median ridge can not be classified as ground tectonics. The amplitude of the tectonic shifts and the grandiose extent relative to the continental-terrestrial structures are too great.

Formation

One of the most common forms of mountain formations in the oceans is the oceanic shafts. Most of all they are represented by the Pacific Ocean. There are two varieties:

• Anticlinal type of uplifts with the most ancient rocks in the nucleus;
• Oceanic shafts with occurring volcanic cones, including extinct volcanoes (guyots).

Time of formation

The age of the middle ridge is determined by the structure of the cortex - it is continental or oceanic. Many areas can be considered in connection with alpine structures, highly fragmented and deeply descended into the ocean. For example, the area adjacent to the sea in Fiji.

The mid-oceanic ridges of the anticlinal type - gentle slopes, separate and quite rare underwater volcanoes - are almost not dissected. These are newly formed and the simplest types of deformation of the ocean floor in the form of fragmentation of platforms and intense seismicity and volcanism. As you know, all this began during the Cenozoic-Quaternary. Anticlinal formations - mid-ocean ridges - are being formed and are growing at the present time.

The second type of mountain formations in the oceans - oceanic shafts - is distinguished by a greater height and length. Extensively linear uplifts with gentle slopes have a much smaller thickness of the crust. Such a structure has many middle-oceanic ridges. Examples: South Pacific, East Pacific and others.

These are more ancient formations, volcanoes were formed on them in the Tertiary time, and later the formation of the seamounts continued. The fragmentation of deep faults was repeated many times.

Structure of the median ridge

The oceanic ridges in the zones of crushing are the most complex relief. The most sharp division of the structure is found in those places where the Mid-oceanic ridges are formed, such as the Atlantic and Indian Oceans, the South of the Pacific, the Southern Ocean from Africa, the zone between Australia and Antarctica.

One of the most characteristic features of this type of structure is the grabens (deep valleys) bordering a series of high (up to three kilometers) peaks interspersed by sharply rising cones of volcanoes. Somewhat similar to the alpine character of the structure, but the contrasts are greater, the decomposition is more pronounced than in the continental structure of the mountain belts.

In the absence of secondary (and more fractional) dismemberment, which has a median ridge and all its slopes, one can speak of signs of recent relief formation. Then in the lower part of the slope there are even terrace-like surfaces with ledges separated from each other. These are former step faults. Noteworthy is the rift valley, which divides the median ridge in half.

How far the planetary oceanic fault extends is determined by the size of the crushing zones. This is the most pronounced form of manifestation of tectonics in the last sections of large geological time. The tectonic structure of the median ridge may be different. For example, Kamchatka is a region of active tectonic processes, volcanism is modern and constant there. The lithospheric plates of the Okhotsk block reprocess the oceanic crust, forming the continental and middle ridge of Kamchatka - an object of constant monitoring of this process.

Location:

The lithospheric plates are in motion, and when expanding (so-called divergence) their oceanic crust is transformed. The bed of the oceans rises, forming mid-ocean ridges. They were classified in the fifties of the twentieth century in the world system with the active participation of the Soviet Union.

The mid-oceanic ridges have a total length of more than sixty thousand kilometers. Here you can start from the Gakkel Range in the Arctic Ocean - from the Laptev Sea to Spitsbergen. Then continue without a break his line to the south. There, the Mid-Atlantic Ridge stretches to Bouvet Island.

Further, the pointer also leads to the west - the American-Antarctic Range, and to the east - along the African-Antarctic Continuing Southwest Indian Ocean. Here again triple junction - the Arabian-Indian ridge follows the meridian, and the Southeast Indian Ocean stretches to the Australasian Antarctic.

This is not the end of the line. Continuation along the South Pacific uplift, turning into the East Pacific uplift, which goes north, to California, in the fault of San Andreas. Then follows the median range of Juan de Fuca - to Canada.

Having girded the planet more than once, the lines laid by a pointer clearly show where the mid-ocean ridges form. They are everywhere.

Relief

The mid-oceanic ridges are formed on the globe as a giant necklace up to 1,500 kilometers wide, while the height of them over the basins is three and four kilometers. Sometimes the vaults protrude from the depths of the ocean, forming islands, most often volcanic.

Even the crest of the ridge reaches a width of one hundred kilometers. Particular beauty is given by the sharp dismemberment of the relief and the smallest block structure itself. Along the axis of the ridge is usually a rift valley thirty kilometers wide with an axial rift (a four-five-kilometer wide gap hundreds of meters high).

At the bottom of the rift there are young volcanoes, surrounded by hydrotherms - hot springs, which secrete metal sulphides (silver, lead, cadmium, iron, copper, zinc). Small earthquakes are constant here.

Under the axial rifts there are magmatic chambers connected by a kilometer, that is, rather narrow, channel with central eruptions at the bottom of this gap. The sides of the ridges are much wider than the ridge - for hundreds and hundreds of kilometers. They are covered with layers of lava sediments.

Not all links in the system are the same: some mid-ocean ridges are wider and more flat, instead of a rift valley, there is a protrusion of the oceanic crust. For example, the East Pacific uplifts, as well as the South Pacific and some others.

Each median ridge is dissected by transforms (i.e., transverse) faults in many places. On these faults, the axes of the ridges are displaced a distance of hundreds of kilometers. The intersection sites are blurred into the gutters, that is, depressions, some of which reach up to eight kilometers in depth.

The longest mountain underwater chain

The longest mid-oceanic ridge is located on the bottom of the Atlantic Ocean. It shares the North American and Eurasian tectonic plates. The Mid-Atlantic Ridge extends 18,000 kilometers. This is part of the system of oceanic ridges of forty thousand kilometers.

There is a middle ridge under the Atlantic from a number of slightly smaller ones: the Knipovich and Mona Ranges, the Iceland-Jannmaytsky and Reykjanes, and also from the very large-over-eight thousand kilometers North Atlantic Ridge and ten and a half thousand kilometers-the South Atlantic.

Here the mountains are so high that they formed chains of islands: these are Azores, Bermudas, and even Iceland, St. Helena, Ascension, Bouvet, Gough, Tristan da Cunha and many smaller ones.

Geological calculations indicate that this median ridge was formed during the Triassic period. Crosswise faults shift the axis to six hundred kilometers. The upper complex of the ridge consists of tholeiitic basalts, and the lower one consists of amphibolites and ophiolites.

Global System

The most outstanding structure in the ocean is the mid-ocean ridges stretching for sixty thousand kilometers. They divided into two almost equal halves the Atlantic Ocean, and the Indian into three parts. In the Pacific, the median slightly pumped up: the necklace of the ridges moved to the side, to South America, then to the isthmus between the continents to escape to the mainland of North America.

Even in the small Arctic Ocean there is the Gakkel Range, where the tectonic structure of the middle ridge is clearly traced, which is equivalent to the mid-oceanic uplift.

The huge swellings of the ocean floor are the boundaries of lithospheric plates. The surface of the Earth is covered with plates of these plates, which do not lie in place: they constantly creep on each other, breaking edges, releasing magma and increasing with its help a new body. Thus, the North American slab covered two neighbors with its own edge, forming the ridges of Juan de Fuca and Gorda. Expanding, the lithospheric plate usually infringes and absorbs the territories of plates lying next to each other. The continents also suffer from this most of all. They look like hummocks in this game: the oceanic crust leaves under the continent, lifting it, crushing it and breaking it.

Rift zones

Under the center of each part of the ridges, magma flows, stretching the earth's crust, breaking its edges. Pouring to the bottom, the magma cools down, increasing the mass of the ridge. Then a new portion of the mantle melt breaks and crushes the new base, and everything repeats. So in the ocean grows the earth's crust. This process is called spreading.

The spreading rate (formation of the ocean floor) determines the changes in the appearance of ridges from one site to another. And this is with the same structure. Where the velocities differ, the ridge in the relief also changes completely.

Where the spreading speed is low (eg, the Tazhur rift), huge underwater valleys with active volcanoes at the bottom are formed. Their immersion below the crest is approximately four hundred meters, from where there is a gradual, terraces rise of stairs to one hundred and fifty meters each. This rift is in the Red Sea and in many areas of the Mid-Atlantic Ridge. Such oceanic mountains grow slowly, several centimeters per year.

At high spreading speed, the ridges (especially in cross section) look like this: the central rise is half a kilometer higher than the main relief and is decorated with a chain of volcanoes. This, for example, is the East Pacific Rise. Here the valley does not have time to form, and the rate of building up the crust in the ocean is very high - 18-20 centimeters per year. Thus, we can determine the age of the median ridge.

A unique phenomenon - "black smokers"

The tectonic structure of the median ridge made it possible to appear such an interesting phenomenon of nature as "black smokers". Hot lava warms the ocean up to three hundred and fifty degrees. The water would have boiled up, if there had not been such an incredible ocean pressure many kilometers thick.

Lava carries a variety of chemicals that, when dissolved in water, form sulfuric acid upon interaction. Sulfuric acid, in turn, dissolves many minerals of the spilled lava, interacts with them and forms sulfur and metal compounds (sulphides).

The sediment of them falls a cone about seventy meters in height, inside of which all the above-described reactions continue. Up on the cone, heated incandescent sulfide solutions rise and break free into black clouds.

Very spectacular sight. True, it's dangerous to approach. The most interesting thing is that the hidden and most active part of each cone happens many hundreds of meters high. And much higher than the Ostankino Tower for example. When there are many cones, it seems that an underground (and underwater) secret plant operates there. Most often they are found in whole groups.

The middle ridge of Kamchatka

The landscape of the peninsula is unique. The mountain chain, which is a watershed ridge on the Kamchatka Peninsula - the Middle Range. Its length is 1200 kilometers, runs from north to south and carries a huge number of volcanoes - most often thyroid and stratovolcanoes. There is also a plateau of lava, and individual mountain massifs, as well as isolated peaks covered with eternal glaciers. The most prominent are Bystrinsky, Kozyrevsky and Malkin ranges.

The highest point - 3621 meters - Ichinskaya Sopka. Almost level with it are many volcanoes: Alnai, Khuvkhutun, Shishel, Ostra Sopka. The range consists of twenty-eight passes and eleven peaks, most of which in the northern sector. The central part is distinguished by considerable distances between the peaks, in the Southern part - a high dissection into asymmetric massifs.

The tectonic structure of the Median ridge of Kamchatka was formed with long-term interaction of the largest lithospheric plates - the Pacific, Kula, North American and Eurasian.