A distinctive feature of the earth's lithosphere, associated with the phenomenon of global tectonics of our planet, is the presence of two types of crust: continental, which makes up continental masses, and oceanic. They differ in composition, structure, thickness and nature of the prevailing tectonic processes. An important role in the functioning of a single dynamic system, which is the Earth, belongs to the oceanic crust. To clarify this role, it is first necessary to turn to the consideration of its inherent features.
General characteristics
The oceanic type of crust forms the largest geological structure of the planet - the ocean bed. This crust has a small thickness, from 5 to 10 km (for comparison, the thickness of the continental-type crust is on average 35–45 km and can reach 70 km). It occupies about 70% of the total surface area of the Earth, but in terms of mass it is almost four times inferior to the continental crust. Average densityrocks is close to 2.9 g / cm).
Unlike the isolated blocks of the continental crust, the oceanic one is a single planetary structure, which, however, is not monolithic. The Earth's lithosphere is divided into a number of mobile plates formed by sections of the crust and the underlying upper mantle. The oceanic type of crust is present on all lithospheric plates; there are plates (for example, the Pacific or Nazca) that do not have continental masses.
Plate tectonics and crustal age
In the oceanic plate, such large structural elements as stable platforms - thalassocratons - and active mid-ocean ridges and deep-sea trenches are distinguished. Ridges are areas of spreading, or moving apart of plates and the formation of new crust, and trenches are subduction zones, or subduction of one plate under the edge of another, where the crust is destroyed. Thus, its continuous renewal occurs, as a result of which the age of the most ancient crust of this type does not exceed 160–170 million years, that is, it was formed in the Jurassic period.
On the other hand, it should be borne in mind that the oceanic type appeared on Earth earlier than the continental type (probably at the turn of the Catarcheans - Archeans, about 4 billion years ago), and is characterized by a much more primitive structure and composition.
What and how is the earth's crust under the oceans
Currently, there are usually three main layers of oceanic crust:
- Sedimentary. He was educated inmainly carbonate rocks, partly - deep-sea clays. Near the slopes of the continents, especially near the deltas of large rivers, there are also terrigenous sediments entering the ocean from land. In these areas, the thickness of precipitation can be several kilometers, but on average it is small - about 0.5 km. Precipitation is virtually non-existent near the mid-ocean ridges.
- Bas altic. These are pillow-type lavas erupted, as a rule, under water. In addition, this layer includes a complex complex of dikes located below - special intrusions - of dolerite (that is, also bas alt) composition. Its average thickness is 2–2.5 km.
- Gabbro-serpentinite. It is composed of an intrusive analogue of bas alt - gabbro, and in the lower part - serpentinites (metamorphosed ultrabasic rocks). The thickness of this layer, according to seismic data, reaches 5 km, and sometimes more. Its sole is separated from the upper mantle underlying the crust by a special interface - the Mohorovichich boundary.
The structure of the oceanic crust indicates that, in fact, this formation can, in a sense, be considered as a differentiated upper layer of the earth's mantle, consisting of its crystallized rocks, which is overlapped from above by a thin layer of marine sediments.
The "conveyor" of the ocean floor
It is clear why there are few sedimentary rocks in this crust: they simply do not have time to accumulate in significant quantities. Growing from spreading zones in the areas of mid-ocean ridges due to the influx of hotmantle matter during the convection process, lithospheric plates, as it were, carry the oceanic crust further and further away from the place of formation. They are carried away by the horizontal section of the same slow but powerful convective current. In the subduction zone, the plate (and the crust in its composition) plunges back into the mantle as a cold part of this flow. At the same time, a significant part of the precipitation is torn off, crushed, and ultimately goes to increase the crust of the continental type, that is, to reduce the area of the oceans.
Oceanic type of crust has such an interesting property as strip magnetic anomalies. These alternating areas of direct and reverse magnetization of bas alt are parallel to the spreading zone and are located symmetrically on both sides of it. They arise during the crystallization of bas altic lava, when it acquires remanent magnetization in accordance with the direction of the geomagnetic field in a particular epoch. Since it repeatedly experienced inversions, the direction of magnetization periodically changed to the opposite. This phenomenon is used in paleomagnetic geochronological dating, and half a century ago it served as one of the strongest arguments in favor of the correctness of the theory of plate tectonics.
Oceanic type of crust in the cycle of matter and in the heat balance of the Earth
Participating in the processes of lithospheric plate tectonics, the oceanic crust is an important element of long-term geological cycles. Such, for example, is the slow mantle-oceanic water cycle. The mantle contains a lotwater, and a considerable amount of it enters the ocean during the formation of the bas alt layer of the young crust. But during its existence, the crust, in turn, is enriched due to the formation of the sedimentary layer with ocean water, a significant proportion of which, partially in a bound form, goes into the mantle during subduction. Similar cycles apply to other substances, such as carbon.
Plate tectonics play a key role in the Earth's energy balance, allowing heat to move slowly away from hot interiors and away from the surface. Moreover, it is known that in the entire geological history of the planet gave up to 90% of the heat through the thin crust under the oceans. If this mechanism did not work, the Earth would get rid of excess heat in a different way - perhaps, like Venus, where, as many scientists suggest, there was a global destruction of the crust when the overheated mantle substance broke through to the surface. Thus, the importance of the oceanic crust for the functioning of our planet in a mode suitable for the existence of life is also extremely high.
