The deepest step of man
The distance from the earth's surface to the center of the earth is about 6360 kilometers. This distance is so deep that it is not possible for humans to enter this depth. In fact, even 0.1% of this depth could not be exceeded by humans. People want to go underground for two main reasons. First to collect mineral resources from underground. The deepest mine in the world at the moment is in South Africa, the Mponeng gold mine, which is about 4 km underground and plans to expand in the future. This is still the deepest place on earth where human feet have fallen. In addition to mining, holes are often made in the soil for scientific work, from which rocks are collected and from which rocks can be used to measure the temperature and pressure below the ground. This kind of hole is called borehole in the language of geology. Here the man does not reach directly, but the instrument reaches and picks up the stone. The deepest borehole in the world has gone up to about 12 kilometers. This borehole is located in the Pechangsky district of Russia and its name is Kola Superdip Borehole [1].
Although human endeavors are limited to 12 kilometers, we can find descriptions of different layers from the center of the earth to the surface of the earth, their properties, lithosphere, mantle, core, different discontinuity between them, e.g. , Mohorovičić discontinuity, Gutenberg discontinuity, etc. All details are available in school textbooks. But how do scientists collect all this unknown information?
The beginnings of physics
The scientists who collect deep information about the earth are called geophysicists. There are many more thematic divisions in geophysics. In each case, they have seized it, despite obstacles we can scarcely imagine. " For example, the purpose of geodynamics or geology is to calculate the balls that cause volcanoes or mountains in the depths of the earth. The main purpose of those who study seismology is to determine the cause of an earthquake, to measure the magnitude of an earthquake. Many of us know that when there is an earthquake in the Himalayas, we also feel the tremors in West Bengal as a result. So where the earthquake starts, the vibration spreads far away from the epicenter. One of the main tasks of seismologists or seismologists is to determine the mathematical calculation of how these vibrations flow around from the epicenter in waves.
Long studies have shown that multiple waves are emitted from the epicenter of an earthquake. They can be divided into two types. A body wave and a surface wave. Surface waves only flow over the earth's surface, cannot penetrate inside. But body waves emanate from the center of the earthquake and penetrate deep into the earth. Geophysicists collect all the information about the depths of the earth by determining the speed and motion of these body waves. How exactly is this information collected?
The language of earthquakes
Any wave flow has a common feature. When a wave travels from one medium to another, some part of the wave is reflected back to the first medium and some part is refracted and enters the second medium (refraction). Just as we see in the case of light. When sunlight falls on a bucket of water, we see the reflection of the sun in that bucket of water. Again I put a hard iron rod in that bucket and saw the change in the length of the rod in the water. This apparent change in length is possible due to the change in the refractive index between air and water. Each medium has a specific refractive index. How fast a wave can flow through a medium depends on the refractive index of the medium. The wave velocity changes when the refractive index changes.
Figure 1: Different body waves flow from the source of an earthquake. When entering from one medium to another, the waves are reflected (blue isolated line) and refracted (black continuous line). These reflected and reflected waves are recorded on a seismograph device located on the surface.
After the waves are reflected and refracted, they flow from the source to different places. There are consumer instruments to measure this movement of the wave. If the distance from the source of the wave to the receiver is known and the calculation of exactly how long after the reflected and refracted wave reaches the receiver, it is possible to determine the wave velocity. The seismograph used to record earthquake waves is called a seismograph. There is a specific language for recording earthquake waves. This language or seismogram is captured on a seismograph. By reading this language, scientists can understand exactly which way the reflected or refracted wave travels and is captured by the consumer device. These seismograph stations are scattered in different parts of the world. Whenever there is an earthquake, the reflected and reflected waves from it are captured on all seismograph devices around the world (Figure 1). Seismologists decipher the meaning of seismicity in a complex mathematical way and thoroughly determine how the speed of a wave changes with the depth of the earth. The part where the wave velocity changes abruptly is called seismic discontinuity or seismic disconnection line. The middle part of the two separation lines is denoted as a single layer.
Earth level layout
The wave velocity increases by a certain proportion up to about 30-35 km below the continental surface (Figure 2). Exceeding this distance, the speed of the body waves increases at a much higher rate than a certain ratio. However, below sea level, this increase can be observed in only 5-6 km.
Figure 2: Shows how the speed of an earthquake wave changes from the surface to the center. The bottom of the picture shows the whole world like a round disc. The layout of the layered surface has been highlighted by blasting a sample part of it. A white continuous line between the blasted sample parts is shown as a measure of wave velocity. The line speed increases when the line moves to the right and decreases when it moves to the left. The depth at which the velocity changes is marked on the left of the image and the important separation lines are written on the right. The names of each significant level appear in the middle of the image.
This is the first significant change in the speed of body waves inside the Earth. The part from the surface to this depth is called the crust and the first change in wave velocity is called the moho discontinuity line. Then again at a depth of about 100 km the speed of the body waves suddenly decreases. The 100-kilometer-thick layer, including the Earth's crust, is called the lithosphere. The idea of geologists is that the layer beneath the rock is relatively soft which causes the speed of the wave to decrease. This layer is called the asthenosphere. The importance of the asthenosphere behind the currently established plate tectonics theory is immense. The asthenosphere is about 200 km thick. After a depth of about 300 km below the surface, the body wave speed of the earthquake gradually increases again. The speed of the wave decreases drastically below about 2691 km. Here geologists imagine another line of separation, called the Gutenberg line of separation. The 2700 km thick layer from the Moho dissociation to the Gutenberg dissociation is called the Mantle. From the height of the Himalayas to the Deccan Plateau, all contributions to the creation of volcanoes, including any misfortune on earth, are at this level. At a depth of about 60 km in the mantle, the wave velocity increases slightly. That is why the upper mantle up to the first 60 km of the mantle and the part up to the next 2691 km are identified as the lower mantle or lower mantle.
Below the Gutenberg line of separation begins the core, which extends from 2691 km to the very center, that is, to 6371 km. The center is again divided into two parts. At a depth of 2691 to 5150 km, the velocity of body waves decreases drastically. Incidentally, there are two types of waves in body waves. A P wave and an S wave. The characteristic of S waves is that they cannot flow in a liquid medium, i.e. the velocity of any liquid medium (such as water) is zero. Surprisingly, between 2691 and 5150 km, the prescribed speed of the S wave is zero. According to scientists, metals like iron and nickel remain in molten liquid state due to excessive pressure and heat at this depth. This layer is called the outer core. This is the only liquid layer in the earth, all the rest is solid. Needless to say, the point of this layer is not the only contact with the volcanic lava even though it is liquid. However, the reason for the creation of the Earth's magnetic field is the turbulence of this layer of liquid iron. The wave velocity increases again from 5150 km to the center. The S wave returns again, meaning that the liquid layer of the earth is converted to a solid layer. The 5150 km long line of separation is called the Lehman line. At the center of the earth is a hard iron sphere with a radius of about 1200 kilometers. This part of the line from the Lehmann bias line to the center of the earth is called the inner core.
Looking for more mysteries
Over the past century, the combined efforts of physicists and geologists have uncovered some of the world's deepest mysteries. Numerous more subtle and subtle layers have been discovered in the last two decades. There are still many secrets hidden everywhere. For example, how the temperature and pressure in the depths of the earth are changing, why the plate moves on the surface of the earth, how is this plate tectonics, when did this movement start, why is there no leaf movement on any planet other than Earth , The source of life and how evolution depends on leaf circulation, where is the source of mantle convection current, where is the source of the earth's magnetic field, why does the magnetic field reversal every few thousand years: The question remains to be answered. The solution to all these mysteries is hidden deep in the earth. Geologists, like astronauts, have no way to get around the earth. However, using mathematical power and scientific insights, geologists close their eyes and reach the depths of the earth in a blink of an eye and are busy solving complex mysteries.
*Another disconnection line near 220 km disconnects Lehman