How do we know so much about the Earth’s core if we have barely reached a depth of 12 kilometers?

From where you are, no matter where you are when you read this text, to the center of the planet, there are 6,370 km. That is more or less the average radius of the Earth, an area of ​​enormous mysteries.

To reveal them, we have gone so far as to drill ambitious holes such as the 12,226 meter Kola Superdeep Well. That is, below the limit of this hole there are 6,358 kilometers of planet until we reach its center. So how do we know the architecture of the Earth if we have barely scratched its surface?

In the beginning, all that man had to enter the planet were the natural caverns that he found and explored while imagining myths of the underworld, from the domains of Hades and the River Styx to the monumental novel ‘Journey to the Center’. of the Earth’ by Jules Verne.

To know the Earth’s depths, the first approach, perhaps the most obvious, is to drill as much as possible. Mining operations were the first to begin to advance in what we now know to be the Earth’s crust, until in the 20th century, projects such as the Kola project were launched, started in 1970 and abandoned due to lack of funds in 1995. It reached to cover a third of the Earth’s crust in the Baltic, bringing to light rocks up to 2.7 billion years old, out of the 4.5 billion that the planet has.

Study of volcanic rocks

The second way to know the interior of the planet is the study of the rocks expelled by volcanoes and geological faults; That is, places where blocks of rock from the most superficial part of the Earth have broken, allowing us to see the composition of the different strata that form it. The further away from the surface a stratum is, the older it is, so we can read the history of the planet in them. Likewise, volcanoes expel from the depths of the mantle ancient rocks, xenoliths, and ancient crystals, xenocrystals such as diamonds, which are wrapped in magma and provide us with information about those places that we have not yet reached. Diamonds are formed in specific conditions at 150 kilometers or more, so what accompanies them (or what they are embedded in) comes from those depths.

Another way to learn about our planet is to study the seismic waves that expand from the site of an earthquake, like those that move in water when we throw a stone into a calm pond. The waves of an earthquake are altered by the different densities of the rock through which they travel, and by studying their variations using seismographs in different locations, scientists can infer what the rocks they pass through are like.

Thus, seismologist Inge Lehman was able to determine that the center of our planet was not made of liquid metal, but was formed by a solid core that reflected the seismic waves that collided with it. The enormous density and pressure to which the iron core -mainly- of our home is subjected prevents it from melting, as would be expected. Thus we discovered the furthest point from the Earth’s surface, the core, which begins at 2,900 kilometers and is divided into the solid inner core of about 2,400 kilometers in diameter and an outer core formed mainly by liquid iron, which is the layer responsible for the Earth’s magnetic field and which has a thickness of about 2,300 kilometers.

Magnetic field measurement

Measuring the Earth’s magnetic field is another way of looking into its interior, as it informs us of the greater or lesser presence of magnetic rocks. Since our magnetic field changes over time, the electrical conductivity of different areas of the surface can be measured to determine its composition. This has allowed us to discover large quantities of water in the Earth’s mantle.

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How do we know so much about the Earth’s core if we have barely reached a depth of 12 kilometers?

Much of what we know has been obtained in laboratories where the hypotheses derived from all these types of observations are tested by creating temperatures and pressures similar to those inside the planet to see how different materials behave. The behavior of matter under these extreme conditions is usually very different from what we know in our environment, and thus we have been able to know how the inner core remains in a solid state (or in a state similar to the solid that only exists at those pressures. ) and that its temperature is about 6,100 °C, in fact hotter than the photosphere or visible surface of the sun, which is about 5,500 °C.

From space we can also study our planet. Gravity is not completely uniform throughout it, as has been proven thanks to satellites such as those of NASA’s Grace mission, which for 15 years, since 2002, studied the Earth’s gravitational anomalies. Gravity depends on mass, and therefore these fluctuations show us areas of the planet where there are higher density materials, and have even allowed us to detect long-term changes in the Earth’s crust due to earthquakes.

From space we also receive meteorites with the elements with which the solar system and our planet were formed. By analyzing some elements, mainly radioactive, from meteorites and the Earth’s crust, it is possible to reconstruct part of the planet’s history and how, about 4.5 billion years ago, large amounts of dust and rocks coalesced into a fiery ball. collapsing under its own weight, where most of the heavy elements such as iron and nickel sank to the core and the lighter ones such as silicon, oxygen and carbon, rose to the surface to create the crust, remaining in middle of the earth’s mantle.

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The mantle is a layer of rock about 2,900 kilometers thick, whose upper zone, the asthenosphere, is semi-molten and on which the crust floats, so to speak, divided into large areas called ‘tectonic plates’, which are constantly in motion. movement, while the inner mantle is less fluid due to the pressures to which it is subjected.

And it is in that cortex where we develop. Especially in the continental crust, the thickest, which has a thickness of between 35 and 70 kilometers, while the oceanic crust can be barely 8 kilometers.

This view is greatly simplified, of course. The continuous movement and transition zones between these layers is still a matter of study and debate among geologists. But knowing our home is a fundamental task since, according to all indications, this is where our future is, bright or dark.

A changing core

The inner core of our planet is not static, but grows approximately 1 millimeter each year, and in the 1990s it was discovered that its eastern side is growing faster than its western side.

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