Our planet consists of three main parts (geospheres). The core is located in the center, a dense and viscous mantle extends above it, and the quite thin crust is the uppermost layer of the Earth’s solid. The boundary between the crust and the mantle is called the surface of Mokhorovichich. The depth of its occurrence is not the same in different regions: under the continental crust it can reach 70 km, under the oceanic crust - only about 10. What is this border, what do we know about it and what do we not know, but can we assume?
Let us first turn to the history of the issue.
Opening
The beginning of the 20th century was marked by the formation of scientific seismology. A series of powerful earthquakes with devastating consequences contributed to the systematic study of this formidable natural phenomenon. The cataloging and mapping of foci of instrumentally recorded earthquakes began, and the features of seismic waves began to be actively studied. The speed of their propagation depends on the density and elasticity of the medium, which makes it possible to obtain information about the properties of rocks in the bowels of the planet.
The discoveries were not long in coming. In 1909, the Yugoslav (Croatian) geophysicist Andria Mokhorovich was processing data on the earthquake in Croatia. It was discovered: seismograms of such shallow earthquakes, obtained at stations remote from the epicenter, carry two (or even more) signals from one tremor - direct and refracted. The latter testified to an abrupt (from 6.7–7.4 to 7.9–8.2 km / s for longitudinal waves) increase in velocity. The scientist associated this phenomenon with the presence of a certain boundary separating the depths of the bowels with different densities: the deeper mantle containing the dense rocks, and the crust - the upper layer, composed of lighter rocks.
In honor of the discoverer, the interface between the crust and mantle was named after him and has been known for more than a hundred years as the border of Mohorovichich (or simply Moho).
The density of the rocks separated by Moho also changes stepwise - from 2.8–2.9 to 3.2–3.3 g / cm 3 . There is practically no doubt that these differences indicate a different chemical composition.
However, attempts to get directly to the bottom of the earth's crust have not yet been crowned with success.
Mohole project - start across the ocean
The first attempt to reach the mantle was made by the United States in 1961-1966. The project was named Mohole - from the words Moho and hole “hole, hole”. It was supposed to achieve the goal by drilling the ocean floor, produced from a test floating platform.
The project faced serious difficulties, the funds were overused, and after the completion of the first stage of work, “Mohol” was closed. Experimental results: five wells were drilled, rock samples were obtained from the basalt layer of the oceanic crust. They were able to drill into the bottom at 183 m.
Kola superdeep - drill the continent
And to this day her record is not broken. The deepest research and deepest vertical well was laid in 1970, work on it was carried out intermittently through 1991. The project had many scientific and technical problems, some of them were successfully solved, unique samples of rocks of the continental crust were extracted (the total core length was over 4 km). In addition, a series of unexpected new data was obtained during drilling.
Clarifying the nature of Moho and establishing the composition of the upper layers of the mantle were among the tasks of the Kola superdeep, but the well did not reach the mantle. Drilling froze at a depth of 12,262 m and no longer resumed.
Modern projects - all the same across the ocean
Despite the additional difficulties encountered during deep-sea drilling, modern programs plan to reach the Moho border precisely through the ocean floor, since the earth's crust is much thinner here.
Currently, no country can implement such a large-scale project as ultra-deep drilling with the achievement of the mantle roof, on its own. Since 2013, the project “Mohole to Mantle” has been implemented as part of the International Ocean Discovery Program: Exploring the Earth Under the Sea. Among his scientific goals is obtaining samples of mantle material by drilling an ultra-deep well in the Pacific Ocean. The main tool in this project is the Japanese Chikyu drilling ship, the Earth, capable of providing a drilling depth of up to 10 km.
We can only wait, and if everything goes well, in 2020, science will finally have at its disposal a piece of the mantle, extracted from the mantle itself.
Remote research will clarify the properties of the border Mokhorovichich
Since it is still impossible to directly study the subsoil at depths corresponding to the occurrence of the coromantine section, ideas about them are based on data obtained by geophysical and geochemical methods. Geophysics makes available to researchers deep seismic sounding, deep magnetotelluric sounding, gravimetric studies. Geochemical methods make it possible to study fragments of mantle rocks — xenoliths brought to the surface and rocks embedded in the earth’s crust during various processes.
So, it has been established that the Mokhorovichich boundary separates two media with different densities and electrical conductivities. It is generally accepted that this feature reflects the chemical nature of Moho.
Relatively light rocks of the lower crust, having the main composition (gabbroids), lie above the interface, this layer is conventionally called “basalt”. Below the boundary are the rocks of the upper mantle - ultrabasic peridotites and dunites, and in some areas under the continents eclogites are deeply metamorphosed basic rocks, possibly relics of the ancient ocean floor, entered into the mantle. There is a hypothesis that in such places, Moho is the boundary of the phase transition of a substance of the same chemical composition.
An interesting feature of Moho is that the shape of the border is associated with the relief of the earth's surface, mirroring it: under the troughs, the border is raised, and under the massifs it bends inland. Therefore, the isostatic equilibrium of the crust is carried out here, as if immersed in the upper mantle (for clarity, we recall an iceberg floating in water). Earth gravity also “votes” for this conclusion: the Mokhorovichich border is now globally mapped by the depth due to the results of gravimetric observations from the GOCE European satellite.
At present, it is known that the boundary is mobile, it can even collapse during large tectonic processes. At a certain level of pressure and temperature, it is formed again, which indicates the stability of this phenomenon of the earth's interior.
Why is it necessary
The interest of scientists in Moho is not accidental. In addition to the great importance for basic science, clarification of this issue is also very important for applied areas of knowledge - such as dangerous natural processes of a geological nature. The interaction of matter on both sides of the coro-mantle section, the complex life of the mantle itself have a decisive effect on everything that happens on the surface of our planet - earthquakes, tsunamis, various manifestations of volcanism. And to understand them better is to predict more accurately.