The structure and development of the earth's crust determines not only development, but also the origin of the general relief of the ocean floor. Two groups are distinguished here: the oceanic plateau as a transitional type of structure of the earth's crust and the middle ridge with abyssal plains and gutters.
Classification Attempts
To summarize information on the structure of the ocean floor, a single planetary system has been established. Mid-ocean ridges are located almost in the middle of the main oceanic spaces, dividing them into equal parts. There are several classification attempts. Menard, for example, distinguishes them in this way:
- wide submarine ridges with pronounced seismicity (e.g. East Pacific);
- narrow underwater ridges with steep slopes and seismic activity (eg. Mid-Atlantic ridge);
- narrow and steeply sloping, but not seismically active, underwater ridges (e.g., Middle Pacific and Tuamotu).
According to G. B. Udintsev, mid-ocean ridges have no analogues on land. D. G. Panov attributes the underwater ridges in the Pacific Ocean to the corners of the platform — internal and external — and considers them as analogues of continental platforms. However, the tectonic structure of the middle ridge cannot be classified as terrestrial tectonics. The amplitude of tectonic shifts and the magnitude of the extension relative to the continental - terrestrial structures are too large.
Formation
One of the most common forms of rock formations in the oceans is oceanic ramparts. Most of all they are the Pacific Ocean. There are two varieties:
- anticlinal type of uplifts with the most ancient rocks in the core;
- oceanic shafts with volcanic cones encountered, including extinct volcanoes (guyots).
Education time
The age of the Middle Ridge is determined by the structure of the crust - whether it is continental or oceanic. Many areas can be considered in connection with alpine structures, highly fragmented and deeply lowered 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 rather rare underwater volcanoes - are almost not dissected. These are the recently formed and 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 present.
The second type of rock formations in the oceans - oceanic ramparts - is distinguished by a greater height and extent. Elongated linear uplifts with gentle slopes have a much smaller crust thickness. Such a structure has many mid-ocean ridges. Examples: South Pacific, East Pacific and others.
These are more ancient formations, volcanoes formed on them in the Tertiary time, and later the formation of seamounts continued. The fragmentation of deep faults was repeated more than once.
The structure of the middle ridge
Ocean ridges in crushing zones are the most difficult terrain. The sharpest division of the structure is found in those places where the Mid-Ocean ridges are formed, such as the Atlantic and Indian Oceans, the South Pacific, the Southern Ocean from Africa, the zone between Australia and Antarctica.
One of the most characteristic features of the structure of this type is grabens (deep valleys), bordering a series of high (up to three kilometers) peaks, alternating with sharply rising cones of volcanoes. A bit like the alpine nature of the structure, but there are more contrasts, the dissection is more pronounced than in the mainland structure of the mountain zones.
In the absence of a secondary (and more fractional) dissection, which has a median ridge and all its slopes, we can talk about signs of a 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 stepped faults. Noteworthy is the rift valley, which divides the middle ridge in half.
How far the planetary oceanic fault extends is determined by the size of the crushing zones. This is the most pronounced manifestation of tectonics in the last stretches of a large geological time. The tectonic structure of the middle ridge may be different. For example, Kamchatka is an area of active tectonic processes; volcanism there is modern and constant. The lithospheric plates of the Okhotsk block process the oceanic crust, forming the continental, and the middle ridge of Kamchatka - an object of constant monitoring of this process.
Location
The lithosphere plates are in motion, and with the expansion (the so-called divergence), their oceanic crust is transformed. The bed of 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.
Mid-ocean ridges have a total length of more than sixty thousand kilometers. Here you can start with the Gakkel ridge in the Arctic Ocean - from the Laptev Sea to Spitsbergen. Then continue without breaking his line to the south. There, the Mid-Atlantic Range extends to the island of Bouvet.
Further, the pointer leads to the west - this is the American-Antarctic Range, and to the east - along the African-Antarctic, which continues to the southwestern Indo-Ocean. There is a triple junction here again - the Arabian-Indian ridge follows the meridian, and the Southeast Indian Ocean extends to the Australian-Antarctic.
This is not the end of the line. Continuation of the South Pacific Rise, passing into the East Pacific Rise, which goes north to California, in the San Andreas Fault. This is followed by the middle ridge of Juan de Fuca - to Canada.
Having girdled the planet more than once, the lines laid by the pointer clearly show where the mid-ocean ridges are formed. They are everywhere.
Relief
Mid-ocean ridges are formed on the globe like a giant necklace up to one and a half thousand kilometers wide, but their height above the basins can be three or four kilometers. Sometimes gaps 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 dissection of the relief and the small block structure itself. A rift valley of thirty kilometers wide with an axial rift (four to five kilometer wide slit many hundreds of meters high) usually runs along the axis of the ridge.
At the bottom of the rift there are young volcanoes surrounded by hydrothermal hot springs that emit metal sulfides (silver, lead, cadmium, iron, copper, zinc). Small earthquakes are permanent here.
Under the axial rifts there are magma chambers connected by a kilometer-long, 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 - hundreds and hundreds of kilometers. They are covered with layers of lava sediment.
Not all links in the system are the same: some mid-ocean ridges are wider and more gentle, instead of a rift valley they have a ledge of the oceanic crust. For example, the East Pacific Rises, as well as the South Pacific and some others.
Each middle ridge is dissected by transform (i.e., transverse) faults in many places. Along these faults, the axis of the ridges are displaced by a distance of hundreds of kilometers. Crossing sections are eroded into gutters, that is, troughs, some of which reach up to eight kilometers in depth.
The longest mountain submarine chain
The longest mid-ocean ridge is located at the bottom of the Atlantic Ocean. It separates North American and Eurasian tectonic plates. The Mid-Atlantic Ridge lasts 18,000 kilometers. This is part of a system of oceanic ridges of forty thousand kilometers.
The middle ridge near the Atlantic consists of a number of slightly smaller ones: the Knipovich and Mon Monts, the Icelandic-Yanmayiet and Reykjanes ridges, as well as the very large ones — more than eight thousand kilometers long, the 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 the Azores, Bermuda, and even Iceland, St. Helena, Ascension Island, Bouvet, Gough, Tristan da Cunha and many smaller ones.
Geological calculations say that this median ridge was formed in the Triassic period. Transverse faults shift the axis to six hundred kilometers. The upper ridge complex consists of tholeiitic basalts, and the lower is amphibolites and ophiolites.
Global system
The most outstanding structure in the ocean is the Mid-Ocean ridges stretching for sixty thousand kilometers. They divided the Atlantic Ocean into two almost equal halves, and the Indian into three parts. In the Pacific Ocean, medianity was a little disappointing: the necklace of the ridges slid to the side, towards South America, then to the isthmus between the continents in order to escape under the mainland of North America.
Even in the small Arctic Ocean, there is the Gakkel ridge, where the tectonic structure of the middle ridge is clearly visible, which is equivalent to the mid-ocean uplift.
Huge swellings of the ocean floor are the boundaries of lithospheric plates. The Earth's surface is covered with plates of these plates, which does not lie in place: they constantly crawl onto each other, breaking edges, releasing magma and building up a new body with its help. So, the North American Plate covered two neighbors with its edge at once, forming the ridges of Juan de Fuca and Gorda. When expanding, the lithospheric plate usually infringes and absorbs the territories of plates lying nearby. The continents suffer from this most of all. They look like hummocks in this game: the oceanic crust leaves under the mainland, lifting it, crushing and breaking it.
Rift zones
Under the center of each section of the ridges, magma flows rise, stretching the earth's crust, breaking its edges. Pouring to the bottom, magma cools, increasing the mass of the ridge. Then a new portion of the mantle melt breaks and crushes the new base, and everything repeats. So the earth’s crust grows in the ocean. This process is called spreading.
The spreading rate (formation of the ocean floor) determines the changes in the appearance of the ranges from one section to another. And this is with the same structure. Where the speeds differ, the ridge in the relief also changes completely.
Where the spreading speed is low (e.g. the Tajur rift), huge underwater valleys with active volcanoes at the bottom are formed. Their immersion below the ridge is about four hundred meters, from where there is a gradual terrace-like elevation of steps one hundred to one hundred fifty meters each. Such a rift is in the Red Sea and in many parts of the Mid-Atlantic Ridge. Similar oceanic mountains grow slowly, a few centimeters a year.
At high spreading speeds, the ridges (especially in cross section) look like this: the central elevation is half a kilometer above the main relief and is formed by a chain of volcanoes. Such, for example, is the East Pacific Rise. Here the valley does not have time to form, and the rate of buildup of the earth's crust in the ocean is very high - 18-20 centimeters per year. Thus, it is possible to determine the age of the middle ridge.
A unique phenomenon - "black smokers"
The tectonic structure of the middle ridge allowed the appearance of such an interesting natural phenomenon as "black smokers". Hot lava heats the water of the ocean to three hundred and fifty degrees. Water would have escaped with steam if there had not been such an incredible ocean pressure many kilometers thick.
Lava carries various chemicals that, dissolving 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 (sulfides).
Sediment falls from them in a cone about seventy meters high, inside which all the above reactions continue. Upward on a cone, red-hot solutions of sulfides rise and break free into black clouds.
Very spectacular sight. True, approaching is dangerous. The most interesting thing is that the hidden and most actively working part of each cone is many hundreds of meters high. And much higher than the Ostankino tower for example. When there are many cones, it seems that there is an underground (and underwater) secret plant. Most often, they are found in whole groups.
The middle ridge of Kamchatka
The landscape of the peninsula is unique. The mountain range, which is a dividing ridge on the Kamchatka Peninsula - the Middle Ridge. 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 lava plateau, and separate mountain ranges, as well as isolated peaks covered with eternal glaciers. The most prominent are the Bystrinsky, Kozyrevsky and Malkinsky ranges.
The highest point - 3621 meters - Ichinskaya Sopka. Almost on a par with it are many volcanoes: Alnai, Khuvkhoytun, Sishel, Sharp Sopka. The ridge consists of twenty-eight passes and eleven peaks, most of which are in the northern section. The central part is distinguished by significant distances between the peaks, in the southern part - high dissection into asymmetric massifs.
The tectonic structure of the Middle Ridge of Kamchatka was formed during the long-term interaction of the largest lithospheric plates - the Pacific, Kula, North American and Eurasian.