The earth is getting hotter

Why is the earth warm inside?

The liquid interior of the earth bubbles under our feet. Volcanic eruptions and geysers show the heat there - over 6000 degrees Celsius in the earth's core. But why is it so hot in the earth?

Much of the heat comes from the Earth's childhood days when dust and rocks condensed into a planet. The word “condense” sounds a little too harmless, however: In reality, you have to imagine how many large meteorite impacts - each impact a gigantic explosion that heated up the young planet and melted the material.

Since then it has become a little quieter and the earth is cooling down again. However, it does this extremely slowly, the heat in the interior of the earth can only very slowly escape into space. Hot magma flows in the tough earth mantle transport the heat upwards. There it remains enclosed under the rigid earth's crust as if under a lid. The crustal rock only slowly releases its heat into space.

In addition, heat is still being produced inside the earth. This is because the core of the earth contains a lot of radioactive substances such as uranium. Since the formation of our planet, they have been disintegrating and giving off heat over a very long period of time. This “fuel” will last for billions of years.


No one has ever penetrated deeper into the earth: on the Kola peninsula, a Soviet team of researchers has drilled a hole over 12 kilometers deep in the earth's crust. Because of the unexpectedly great heat underground, the action was stopped after 12,262 meters.

The former Soviet Union began drilling on the Kola Peninsula in the north of the country as early as 1970. The aim of the “super-deep Kola borehole SG-3” was to reach the boundary between the earth's crust and mantle and to take rock samples. The choice fell on Kola because the rock here is more than two billion years old. This is where the “Uralmasch-4E” drilling machine comes in, which was also used for oil drilling. It was later replaced by a device that was supposed to penetrate up to 15 kilometers into the ground.

On the stony path underground, 45,000 rock samples were taken, various fossils were discovered and even gold was found. Copper and nickel deposits that are useful for industry could also be located. The biggest surprise, however, was: From a depth of 10 kilometers there was unimagined heat. At 180 degrees Celsius, the temperatures were much higher than expected. At 12,262 meters it was finally the end of the line. Technical breakdowns prevented the drilling work from proceeding. There is even talk of hellish noises underground. Whether the horrible sounds are a horror story, or whether the earth's crust is groaning here? In any case, the super-deep Kola borehole still poses some riddles for science.

US record low broken

With the Kola borehole, the Soviet Union pursued another interest besides a scientific one:

They wanted to outdo the USA, which with its “Berta Rogers” well had reached a depth of 9,583 meters. This borehole, which was considered the deepest in the world from 1974 onwards, is located in the US state of Oklahoma. But the record only lasted five years. On June 6, 1979, the USSR broke the American record for depth with the Kola well. Despite all the records: With an earth radius of 6371 kilometers, both holes are just a small prick on the earth's surface.


30 kilometers south of Iceland, an island was born out of the sea. A young volcano has been spewing fire and ashes here since November 14th. Its lava masses have already given rise to an island 40 meters high and 500 meters long.

White-gray ash clouds hang in the sky and darken it. Fine volcanic rock patters the area, every lava discharge is accompanied by the rumble of thunder. The smoke column caused by the volcanic eruption rises 10 kilometers into the air. And an island off Iceland's south coast continues to grow.

The eruption of the underwater volcano came unexpectedly, but not without its harbingers. Seismologists had already measured smaller earthquakes in the capital Reykjavik a week earlier - signs that a lot is happening at the plate boundary of the Mid-Atlantic Ridge. In addition, a research vessel had found that the sea was warmer than usual. And residents of the nearby coastal region believed they smelled hydrogen sulfide. When the volcano erupted on the seabed at a depth of 130 meters, it initially went unnoticed. Its explosions were weakened by the water pressure. But as it grew, it approached sea level and finally broke through it, spitting wildly. That was the birth of an island in Iceland.

The new island off the south coast already has a name: it is called "Surtsey" after Surt, the giant of fire. A Nordic legend tells of him that he hurls fire and destroys all life with his glowing sword.

How Iceland came into being

Iceland is actually nothing more than the climax of a huge mountain range in the Atlantic: The mid-Atlantic ridge, which stretches from north to south through the entire Atlantic, is almost 20,000 kilometers long. At the height of Iceland, the North American and Eurasian plates drift apart, by about two centimeters every year. Where they spread, hot magma penetrates from the interior of the earth to the surface. These volcanic eruptions have been piling up mountains underwater for millions of years and caused Iceland to appear above sea level 17 to 20 million years ago. These volcanoes are still active today. And now they have born another island: Surtsey.

Witnesses of volcanism

Even if a volcano hasn't erupted for a long time, you can tell that it exists. Because its magma chambers will be preserved for a long time. These chambers still give off heat and gases that escape into the earth's crust.

This heat heats the groundwater, it rises and escapes on the earth's surface as steam or hot water. This is how steam and thermal springs are created. In Germany, the cities of Wiesbaden and Baden-Baden are known for their thermal springs and baths. Such springs can be very hot, for example in Tuscany there are steam springs with 230 degrees Celsius. Geothermal power plants also generate electricity from this heat.

Some of these hot springs deliver a spectacular natural spectacle: suddenly a fountain of hot water shoots out of the ground. The most famous of these spring springs or Geysers is the "Old Faithful" in Yellowstone National Park in the USA. Every 90 minutes he spits a fountain of water almost 50 meters into the air for a few minutes. The reason: under the opening of a geyser, a long, thin gap leads into the depths, which is filled with water. It's boiling hot below, but the cooler water in the gap blocks the exit. Only when it is hot enough deep down is the pressure enough to throw all the water out in one fell swoop: the geyser “jumps”. After the eruption, the gap fills again with cold groundwater and the heating starts all over again.

The water and steam from the hot springs contain gases and salts, such as carbon dioxide and sulfur compounds. Some of these substances come from the magma, others were absorbed on the way through different rock layers. The gases are released on the surface, including hydrogen sulfide, a gas that smells like rotten eggs. Other minerals deposit as they cool on the surface of the earth. They then form a crust, which is also called sinter. In the Turkish city of Pamukkale, such sinters made whole terraces from lime.

There are also hot springs underwater, namely on the sea floor. Black clouds penetrate there from structures that look like chimneys. They are the so-called black smokers. What gushes out of them, however, is not soot, but mineral water at temperatures of up to 400 degrees. Large amounts of metals such as zinc, iron or copper are dissolved in this water as sulfur salts. It is these sulfur compounds in the water that turn the hot underwater springs black.

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 exposed to strong pressure. This pressure was naturally highest in the core of the earth, 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 became 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 are asteroids, meteorites and comets?

On some nights you can observe a special moment in the sky: it looks like a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star could wish for something. But what is really behind it and where do the shooting stars come from?

In our solar system there are not only the sun, planets and moons. Many small pieces of rock and metal have also been discovered. They are much smaller and not as nicely round as planets, hence they are called minor planets or Asteroids. Like their big siblings, they circle the sun in regular orbits. Most asteroids can be found in the "asteroid belt" between the orbits of Mars and Jupiter.

Every now and then two of these asteroids collide. A crash like this creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to the earth, are attracted to it and fall to the earth. These falling chunks are also called meteorite.

On earth they would literally fall like a stone from the sky - if it weren't for the atmosphere. Because the meteorites are so fast that the air cannot move to the side quickly enough. The air in front of the falling rock is compressed and therefore extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see that as a shining streak that moves across the sky - a shooting star.

Most meteorites are so small that they burn up completely as they travel through the air. The trail then simply ends in the sky. Larger debris also lose mass on the way, but does not completely evaporate. They reach the ground and strike there.

What these meteorites do to the earth depends on how big they are. Small meteorites a few centimeters in diameter, for example, just leave a dent in a car roof.

The largest known meteorite hit about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. As a result, plants and animals all over the world died out - this was the end of the dinosaurs.

Fortunately, such large meteorites are very rare so we don't have to worry. In addition, unlike the dinosaurs, we can observe the sky with telescopes and discover such large asteroids long before the impact.

While a shooting star burns up in a few seconds, another phenomenon remains visible longer: Comets with its tail there are days or weeks in the sky. In the past, people also attributed many properties to them - as divine signs, heralds of calamity or harbingers of happy events. But the truth is a little less spectacular.

Astronomers also call comets "dirty snowballs". They come from the outer solar system, far from the warming power of the sun. It's so cold there that water immediately freezes to ice. This is how lumps of ice and dust form - dirty snowballs.

Even a comet initially travels far away from the sun - until it is deflected by a collision and flies in the direction of the inner solar system. It gets closer to the sun and over time receives more and more light and warmth. This will cause the frozen surface to begin to thaw and even to evaporate. This creates an envelope of water vapor and dust around the comet.

At the same time, the comet gets to feel the “solar wind” - tiny particles that fly out of the sun at high speed. They hit the comet's vapor envelope. This will blow away the comet's vapor envelope, forming an elongated cloud that points away from the sun. When this cloud is then hit by sunlight, it appears as a glowing streak - the comet's tail.

The comet makes an arc around the sun and then moves away again. When it is far enough away from the sun, thawing and evaporation will also stop. The tail disappears and the comet moves like a dirty snowball through the vastness of the outer solar system. Depending on the comet's orbit, it will take many decades or even centuries before it comes close to the sun again.

What is happening inside the earth?

The lava lamp - cult from the 70s: thick bubbles slowly rise 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 is the source of heat. It heats the overlying liquid metal of the outer core of the earth. This rises 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.

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 bit lighter and rose, the lightest to the surface. 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. Even at a depth of a few kilometers, every drilling rig becomes soft because it cannot withstand the enormous pressure and high temperature. Nevertheless, researchers know exactly how the earth is structured - but from where?

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.