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 rather thin crust is the uppermost layer of the Earth's solid body. The boundary between the crust and the mantle is called the Mohorovichic surface. The depth of its occurrence is not the same in different regions: under the continental crust it can reach 70 km, under the oceanic - only about 10. What is this boundary, what do we know about it and what do we not know, but we can assume?
Let's start with the history of the issue.
Opening
The beginning of the 20th century was marked by the development of scientific seismology. A series of powerful earthquakes that had devastating consequences contributed to the systematic study of this formidable natural phenomenon. The cataloging and mapping of the sources 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 elasticityenvironment, which makes it possible to obtain information about the properties of rocks in the bowels of the planet.
Openings were not long in coming. In 1909, the Yugoslav (Croatian) geophysicist Andrija Mohorovichic processed data on an earthquake in Croatia. It was found that the seismograms of such shallow earthquakes, obtained at stations remote from the epicenter, carry two (or even more) signals from one earthquake - 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 speed. The scientist associated this phenomenon with the presence of a certain boundary separating the layers of the subsoil with different densities: the mantle located deeper, containing dense rocks, and the crust - the upper layer, composed of lighter rocks.
In honor of the discoverer, the interface between the crust and the mantle was named after him and has been known as the Mohorovichic (or simply Moho) boundary for more than a hundred years.
The density of the rocks separated by the Moho also changes abruptly - from 2.8-2.9 to 3.2-3.3 g/cm3. There is little doubt that these differences are indicative of different chemical compositions.
However, attempts to get directly to the bottom of the earth's crust have so far failed.
Mohole Project - Starting Across the Ocean
The first attempt to reach the mantle was made by the US 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 ran into serious difficulties, funds were overspent, and after the completion of the first phase of work, Mohol was closed. Results of the experiment: five wells were drilled, rock samples were obtained from the bas alt layer of the oceanic crust. We were able to drill into the bottom at 183 m.
Kola Superdeep – drill through the continent
To this day, her record has not been broken. The deepest research and deepest vertical well was laid in 1970, work on it was carried out intermittently until 1991. The project had many scientific and technical tasks, some of them were successfully solved, unique samples of rocks of the continental crust were mined (the total length of the cores was over 4 km). In addition, during the drilling, a number of new unexpected data were obtained.
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 stopped at a depth of 12,262 m and has not resumed.
Modern projects are still across the ocean
Despite the additional challenges of deep sea drilling, current programs plan to reach the Moho boundary through the ocean floor, as the Earth's crust is much thinner here.
Currently, no country can carry out such a large-scale project as ultra-deep drilling to reach the roof of the mantle on its own. Since 2013 within the framework of the International ProgramIODP (International Ocean Discovery Program: Exploring the Earth Under the Sea) is implementing the Mohole to Mantle project. Among his scientific goals is to obtain samples of the mantle matter by drilling an ultra-deep well in the Pacific Ocean. The main tool in this project is the Japanese drilling ship "Tikyu" - "Earth", capable of providing a drilling depth of up to 10 km.
We can only wait, and if all goes well, in 2020 science will finally have a piece of the mantle mined from the mantle itself.
Remote sensing will clarify the properties of the Mohorovicic boundary
Since it is still impossible to directly study the subsoil at depths corresponding to the occurrence of the crust-mantle section, ideas about them are based on data obtained by geophysical and geochemical methods. Geophysics provides researchers with 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 intruded into the earth's crust during various processes.
So, it has been established that the Mohorovichic boundary separates two media with different density and electrical conductivity. It is generally accepted that this feature reflects the chemical nature of Moho.
Above the interface, there are relatively light rocks of the lower crust, which have the maincomposition (gabbroids), - this layer is conventionally called "bas alt". Below the boundary are rocks of the upper mantle - ultramafic peridotites and dunites, and in some areas under the continents - eclogites - deeply metamorphosed mafic rocks, possibly relics of the ancient ocean floor, brought 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 connected with the relief of the earth's surface, mirroring it: under the depressions the border is raised, and under the mountain ranges it bends deeper. Consequently, the isostatic equilibrium of the crust is realized here, as if immersed in the upper mantle (for clarity, let us recall an iceberg floating in water). Earth's gravity also "votes" for this conclusion: the Mohorovichic boundary is now globally mapped in depth thanks to the results of gravity observations from the European GOCE satellite.
It is now known that the boundary is mobile, it can even collapse during major 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 needed
Scientists' interest in Moho is not accidental. In addition to the great importance for fundamental science, it is very important to clarify this issue for applied areas of knowledge, such as hazardous natural processes of a geological nature. The interaction of matter on both sides of the crust-mantle section, the complex life of the mantle itself, have a decisive influence on everything that happens on the surface of our planet - earthquakes, tsunamis, various manifestations of volcanism. And to understand them better means to predict more accurately.
