What are the layers of the earth

How is the earth structured?

In the beginning, young earth was a hot ball of molten matter. All components were initially well mixed, just as they were distributed when the earth was formed: Metals, rocks, trapped water and gases and much more - a big mess.

But in the course of time that changed: The heavier substances sank down to the center of the earth - especially metals. Rocks, on the other hand, were a little lighter and rose, the lightest to the surface of the earth. There they slowly cooled down and froze.

So the material of the earth separated into the three spherical layers that we know today. You can imagine the structure of the earth like a peach: on the outside a wafer-thin “shell” made of light, solid rock - the Earth crust. On average, it is only 35 kilometers thick.

Under the crust is the "pulp" - the almost 3000 kilometers thick Mantle made of heavy, viscous rock. And inside the earth lies that Earth core from the metals iron and nickel.

The core of the earth itself consists initially of an outer layer about 2200 kilometers thick, the outer core. It is over 5000 degrees Celsius there, which is why the metal has melted and is as fluid as mercury.

That is right inside inner core, slightly smaller than the moon. At over 6000 degrees Celsius, it is a little hotter than the outer core - but surprisingly solid. This is because with increasing depth, not only does the temperature increase, but also the pressure. The outer layers that weigh on the earth's core compress its material so unimaginably that it cannot liquefy.

How do you know how the earth is structured?

We can fly to the moon, but a trip to the center of the earth will always be science fiction. Every drilling rig becomes soft at a depth of just a few kilometers because it cannot withstand the enormous pressure and high temperature. Nevertheless, researchers know very well how the earth is structured - but where from?

Similar to an X-ray machine, geologists can look inside the earth without having to cut open the earth. Your "X-rays" are earthquake waves: if there is a strong tremor in one place, the vibrations spread through the entire earth body, similar to sound waves in the air.

However, these waves are not always equally fast: In dense and hard material, the vibrations are transmitted faster than in lighter and softer material. If they hit a layer of rock with a higher density, they can also be refracted or reflected back, like rays of light on a pane of glass. And some waves can only move in solid or viscous substances and liquids cannot pass through them at all.

When the earthquake waves finally arrive on the other side of the world, they are recorded by a global network of highly sensitive measuring devices - so-called seismographs. From the patterns in these diagrams, the researchers can read off the type of waves and their speed and trace the path of the waves through the globe.

In this way, the researchers learn a lot about the interior of the earth - for example, at what depth there are layers of rock or metal and whether these are solid, viscous or thin.

The beginnings of the earth

We would not recognize the earth immediately after its formation. It was an extremely uncomfortable planet: there were neither continents nor oceans, but a seething surface of glowing hot, viscous magma. Why couldn't the earth's crust form for a long time?

A good 4.5 billion years ago comets, asteroids, gas and dust condensed to form our planet. Its own gravity pressed these individual parts together so that they were subjected to strong pressure. This pressure was of course highest in the earth's core, on which the weight of the entire outer layers weighed. As a result of the high pressure, the rock was heated up and melted. Outwardly, the pressure and thus also the temperature became less. Even so, the surface of the earth remained very hot for several hundred million years and could not cool down and solidify.

In order to understand the reason for this, the scientists had to look at the moon: Ancient lunar craters from the time the solar system was formed tell us that the moon was hit by numerous meteorites when it was young. It is therefore assumed that the earth was also exposed to a real rock bombardment from space at the same time. The lumps fell to the earth at high speed - and the impacts were correspondingly violent: Even lumps of a few hundred tons could easily cause an explosion the strength of an atomic bomb!

So the earth's surface continued to heat up for a long time, stirred up again and again and remained so fluid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the earth's surface drop. The rock could slowly solidify and form an earth crust that grew thicker and thicker over the course of millions of years. But to this day it is only a very thin layer that floats on a viscous, hot interior of the earth.

What is happening inside the earth?

The lava lamp - cult from the 70s: thick bubbles rise slowly in a viscous liquid, sink back to the ground and bubble up again. A similar circular motion of hot, viscous rock also takes place directly under our feet in the interior of the earth. But what is the reason for this?

Regardless of whether it is a lava lamp, water in a saucepan or the earth's mantle, the reason is always the same: When a liquid is heated, warm bubbles rise to the top. This is because the tiny particles that make it up move back and forth more and more as the temperature increases. To do this, they need more space and no longer huddle together so closely. There are now fewer particles in the same volume than in the vicinity, so it is lighter and rises upwards. There this bubble cools down again and the particles take up less space. The volume piece becomes heavier than the surroundings, sinks again and the cycle starts all over again. When a liquid flows in a circle due to a temperature difference, it is also called convection.

In a lava lamp, the heat from the lamp sets the liquid in motion. In the interior of the earth, the hot, solid inner core of the earth is the source of heat. It heats the overlying liquid metal of the outer core of the earth. This rises up and transfers its heat to the earth's mantle, which gradually cools it down. Then it sinks back down, where it heats up again.

A second, similar cycle takes place in the earth's mantle: its heated rock moves upwards from the core towards the earth's crust, to which it in turn gives off heat. After it cools down, it flows down to the Earth's core, where the cycle begins again. Because the earth's mantle rock is very tough, the convection current only moves a few centimeters per year - a cycle lasts a long time.

Due to the rock currents in the earth's interior, great heat and pressure act on the thin earth crust. It cannot always keep up: Every now and then it tears open in individual places and hot rock escapes through volcanoes to the surface of the earth.

The outermost shell of the earth

Like an egg from an eggshell, the earth is also surrounded by a hard shell. This outermost layer surrounds the earth's mantle and is called the earth's crust. If you compare the earth to a peach, the earth's crust is - in relative terms - as thick as its skin. Under continents it reaches an average of 40 kilometers deep, under the oceans it is only about seven kilometers.

Below is the outer part of the earth's mantle, which extends to a depth of around 100 kilometers. It is also solid, but consists of heavier rock. The earth's crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock has broken into slabs of different sizes, which slowly drift around on the hot, viscous mantle of the earth.

Where the rock melt penetrates upwards from the hot earth's mantle, the earth's crust can break up. Then lava flows out, which becomes the new crust of the earth. This mainly happens where the plates of the lithosphere adjoin one another, such as on the mid-ocean ridges.

In Iceland, for example, these plate boundaries are easy to recognize: cracks and furrows run through the earth's crust, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes in Italy and there are always earthquakes.

The crust is covered by the bottom. The soil of the land masses is formed from weathered rock and remains of animals and plants. The sea floor, on the other hand, develops from deposits such as clay and sunken remains of marine organisms. On the coasts, the sea floor also consists of deposited rubble that was removed from the mainland and washed into the sea.


For years, the drill bit has laboriously hammered and twisted into the hard earth's crust. Again and again he got stuck. Now the press spokesman for the deep drilling program in Windischeschenbach has announced the end of the scientific project: On October 12, 1994, the drilling rig and its measurement technology had to be switched off at a depth of exactly 9101 meters and at a temperature of 265 degrees Celsius. Reason: The research project's coffers are empty. Overall, the drilling program is very successful, but it is too costly to continue.

The drilling at the Windischeschenbach site near Weiden in the Upper Palatinate was started in 1987 to investigate the earth's crust and the processes taking place in it. Originally, the geologists wanted to drill to a depth of 14 kilometers. According to their calculations, the electronic devices would have withstood the high temperatures of an estimated 300 degrees Celsius by then.

The deepest hole in the world

Kola, a Scandinavian peninsula on the icy northwestern edge of Russia. Here, where hardly a human soul strays, the earth's crust is over three billion years old. Such old crust is rare, and so a scientific drilling started in 1970. Researchers wanted to bring rock samples from the interior of the earth to the surface. But at a depth of a good 12 kilometers at a temperature of almost 200 degrees Celsius, the drilling rigs went soft and the electronics failed. The Russian deep drilling program had to be discontinued in 1989. But at 12,262 meters, it is still the deepest borehole in the world today. Over 45,000 rock samples were taken from the earth's crust during this time. Your exploration will take decades.


British researchers have discovered a hole several thousand square kilometers in the earth's crust on the ocean floor. According to the scientists, the earth's mantle is exposed there. The research vessel RSS James Cook is now on its way to investigate the sites more closely. The first destination of the expedition is a hole between Tenerife and Barbados. The seabed is to be scanned and samples taken with the high-tech TOBI robot.

The vacancies are on the Mid-Atlantic Ridge - there, tectonic plates drift apart and new ocean floor is created. Holes and cracks are not uncommon at this point, but they usually fill up again quickly with lava from below and thus cover the mantle rock. It is still a mystery to scientists why there is no lava crust in this case. Was it torn away or was it not able to form in the first place? A test result is expected in the coming months.

Excavator lifts mantle rock

The excavator of the German icebreaker Polarstern shovels huge boulders out of the icy sea in the Arctic. Under the microscope, what the researchers had been hoping for a long time is confirmed: In the samples, they find pure mantle rock that was not filled in by volcanoes. A significant find, because the earth's mantle is difficult to access and is usually covered by a thick crust. The mantle rock was discovered on the Gakkel Ridge - a northern extension of the Mid-Atlantic Ridge. There, the earth's crust spreads more slowly than anywhere else in the world - less than one centimeter per year. That is why there is so little volcanic activity there that the mantle rock has been well preserved.


Four propellers fight the waves near the Mexican Pacific coast. They are trying to hold the drilling ship, the Cuss I, in position. Because 3500 meters below him, his drilling rig is said to turn into the seabed. After several weeks and persistent attempts, the American researchers have now succeeded in drilling a hole 183 meters deep.

The oceanic crust is much thinner than the continental one. Originally, it was believed that at a depth of 10 to 15 kilometers, the boundary with the earth's mantle could be reached - the so-called Moho boundary. At 183 meters, the researchers did not get very far, but the project is a success: It shows that deep drilling is possible on the high seas. It will only be a matter of time before you can reach the mantle.

The discovery of the Moho border

In 1909 the earth shook near the Croatian city of Zagreb. Andrija S. Mohorovičić - a Croatian scientist who researched the internal structure of the earth - took a closer look at the recordings of the earthquake waves. After many calculations, he found that some waves are broken about 30 to 50 kilometers below the surface of the earth on a thick layer of rock - as if they were hitting a hard rock. He discovered the transition between the earth's crust and mantle: the “Moho boundary”, which was named after him.