The Earth’s mantle is a part of the geosphere located between the crust and core. It contains a large proportion of all the planet's matter. The study of the mantle is important not only in terms of understanding the internal structure of the Earth. It can shed light on the formation of the planet, give access to rare compounds and rocks, help to understand the mechanism of earthquakes and the movement of lithospheric plates. However, obtaining information on the composition and characteristics of the mantle is not easy. So far, people do not know how to drill wells so deeply. The Earth’s mantle is mainly being studied using seismic waves. And also by modeling in a laboratory.
Earth Structure: mantle, core and crust
According to modern concepts, the internal structure of our planet is divided into several layers. The upper one is the crust, then the mantle and core of the Earth lie. The crust is a hard shell, dividing into the oceanic and continental. The Earth’s mantle is separated from it by the so-called Mokhorovicic border (named after the Croatian seismologist who established its location), which is characterized by an abrupt increase in the velocities of longitudinal seismic waves.
The mantle is approximately 67% of the mass of the planet. According to modern data, it can be divided into two layers: upper and lower. In the first, a Golitsyn layer or middle mantle is also distinguished, which is a transition zone from the upper to the lower. In general, the mantle extends at a depth of 30 to 2900 km.
The core of the planet, according to modern scientists, consists mainly of iron-nickel alloys. It is also divided into two parts. The inner core is solid; its radius is estimated at 1300 km. External - liquid, has a radius of 2200 km. Between these parts allocate a transition zone.
Lithosphere
The crust and upper mantle of the Earth are united by the concept of “lithosphere”. It is a hard shell having stable and moving areas. The solid shell of the planet consists of lithospheric plates, which are supposed to move along the asthenosphere - a rather plastic layer, probably representing a viscous and highly heated liquid. It is part of the upper mantle. It should be noted that the existence of the asthenosphere as a continuous viscous shell is not confirmed by seismological studies. Studying the structure of the planet allows us to distinguish several similar layers located vertically. In the horizontal direction, the asthenosphere, apparently, is constantly interrupted.
Ways to study the mantle
Layers below the cortex are inaccessible to study. Huge depths, a constant increase in temperature and an increase in density are a serious problem for obtaining information on the composition of the mantle and core. However, it is still possible to imagine the structure of the planet. When studying the mantle, geophysical data become the main sources of information. The speed of propagation of seismic waves, the features of electrical conductivity and gravity allow scientists to make assumptions about the composition and other features of the underlying layers.
In addition, some information can be obtained from igneous rocks and fragments of mantle rocks. The latter include diamonds, which can tell a lot even about the lower mantle. Mantle rocks are also found in the earth's crust. Their study helps to understand the composition of the mantle. However, they will not replace samples obtained directly from the deep layers, because as a result of various processes occurring in the crust, their composition is different from the mantle.
Earth Mantle: Composition
Another source of information about what constitutes a mantle is meteorites. According to modern concepts, chondrites (the most widespread group of meteorites on the planet) are close in composition to the earth's mantle.
It is assumed that it contains elements that were in a solid state or were part of a solid compound in the process of planet formation. These include silicon, iron, magnesium, oxygen and some others. The mantle they combining with
silica to form silicates. Magnesium silicates are located in the upper layer; the amount of iron silicate increases with depth. In the lower mantle, these compounds decompose into oxides (SiO
2 , MgO, FeO).
Of particular interest to scientists are rocks that are not found in the earth's crust. It is assumed that in the mantle there are many such compounds (grospidites, carbonatites, etc.).
Layers
Let us dwell in more detail on the length of the mantle layers. According to scientists, the upper of them occupies a range of about 30 to 400 km from the earth's surface. Next is the transition zone, which goes deep into another 250 km. The next layer is the bottom. Its border is located at a depth of about 2900 km and is in contact with the outer core of the planet.
Pressure and temperature
As you move deeper into the planet, the temperature rises. The Earth’s mantle is under extreme pressure. In the asthenosphere zone, the effect of temperature outweighs, therefore, here the substance is in the so-called amorphous or semi-molten state. Deeper under pressure it becomes hard.
Studies of the mantle and boundaries of Mokhorovichich
The Earth’s mantle has haunted scientists for quite some time. In laboratories, over the rocks, presumably included in the upper and lower layers, experiments are conducted to understand the composition and characteristics of the mantle. So, Japanese scientists found that the lower layer contains a large amount of silicon. In the upper mantle are water supplies. It comes from the earth's crust, and also penetrates from here to the surface.
Of particular interest is the surface of Mokhorovichich, the nature of which is incomprehensible to the end. Seismological studies suggest that at a level of 410 km below the surface there is a metamorphic change in the rocks (they become denser), which is manifested in a sharp increase in the speed of waves. It is assumed that basaltic rocks near the border of Mokhorovichich turn into eclogite. In this case, the mantle density increases by about 30%. There is another version, according to which, the reason for the change in the speed of seismic waves lies in the change in the composition of the rocks.
Chikyu Hakken
In 2005, a specially equipped ship Chikyu was built in Japan. His mission is to make a record deep well at the bottom of the Pacific Ocean. Scientists suggest taking samples of the rocks of the upper mantle and the boundaries of Mokhorovichich to get answers to many questions related to the structure of the planet. The project is scheduled for 2020.
It should be noted that scientists did not just turn their eyes to the oceanic bowels. According to studies, the thickness of the crust at the bottom of the seas is much less than on the continents. The difference is significant: under the water column in the ocean, magma needs to be overcome only 5 km in some areas, while on land this figure increases to 30 km.
Now the vessel is already working: samples of deep coal seams have been obtained. Realization of the main goal of the project will help to understand how the Earth’s mantle is arranged, what substances and elements make up its transition zone, and also to find out the lower limit of the distribution of life on the planet.
Our idea of the structure of the Earth is far from complete. The reason for this is the difficulty of penetrating into the bowels. However, technological progress does not stand still. Advances in science suggest that in the near future we will know much more about the characteristics of the mantle.