What is the function of the rock cycle

Cycle of rocks

No rock on earth is made to last. It weathers on the surface, is removed and redeposited. When two plates collide, layers of sediment are compressed and unfolded to form high mountains. The rock of submerged plates melts in the earth's interior and forms the source of volcanoes. Lava that spits out from a volcanic crater cools down and solidifies again into rock.

It is an eternal cycle that ensures that even the hardest rock is constantly changing and new things are created from it. The transformation does not happen overnight, of course, but over millions of years. "Players" in this cycle are three groups of rocks, each of which is formed under different conditions:

When magma cools, the hot mass solidifies igneous rock. This can happen both on the surface of the earth and inside the earth. On the other hand, where layers of excavated rock pile up, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify Sedimentary rock. In turn, high pressure and great heat in the earth's interior ensure that rock is transformed and another is created. Then geologists speak of transformation or of metamorphic rock.

These three types of rock are closely related: each type can transform into any other. This rock cycle will continue as long as the earth exists.


It is a sensation for science: In the north of Canada, geologists have stumbled upon the oldest rocks ever discovered. They are part of the Nuvvuagittuq greenstone belt on Hudson Bay and are over four billion years old.

An international team of researchers has now dated the rocks in northern Canada to 4.28 billion years. That would make it just 300 million years younger than our solar system. The scientists are now investigating whether the ancient rocks are a remnant of the very first crust that was once separated from the earth's mantle. Then the discovery could help unravel some of the secrets of the very early history of the earth. Perhaps the rocks reveal something about where and when life began? The researchers also hope to be able to read in the rock how the atmosphere changed and when the first continent of our earth was formed.

Incidentally, the name greenstone belt comes from the color of its metamorphic rock. It is the minerals contained in the rocks that give them a greenish color in some places.

Age ranking of the "veterans"

They are all old and wrinkled. But the chunks of the oldest rocks are separated by millions of years. Until recently, when the rock from the Nuvvuagittuq rock belt was dated an impressive 4.28 billion years ago, another rock in northern Canada was considered the oldest rock on earth: the so-called Acasta gneiss in the northwest of the country. After all, it is 4.03 billion years old. With its old age, the old gneiss surpasses a formation from ribbon iron in Greenland: This now ranks third on the age scale of the rocks. The rocks in Greenland are “only” 3.9 billion years old!


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 a depth of 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 a number of puzzles 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.


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 over 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.

Igneous rocks

Biting granite means that something is hopeless. Because of its great hardness, granite can not only be used as a phrase, but also as a paving stone or for building walls. Granite is a rock that lies over two kilometers below the earth's surface and is common in the earth's crust.

Granite is formed when glowing magma solidifies when it cools. The dark spotted gabbro or the monzonite are also formed from slowly cooling magma. If this process takes place deep inside the earth, geologists speak of Deep rock, also Plutonite called.

If, on the other hand, the hot rock slurry penetrates outwards during a volcanic eruption and pours over the surface of the earth, it is from Effluent rock or Volcanite the speech. Vulcanites include light pumice stone, porous tuff or rhyolite, which is made from the same material as granite but has a different structure and is less hard because it cools faster on the surface of the earth than the granite in the depths. Basalt is also a volcanite. Sometimes it freezes into hexagonal, closely spaced columns that look as if they have been cast into shape. Basalt forms on the surface of the earth from the same mass as the gabbro in the depths.

Vulcanites weather immediately after their formation, plutonites only when the overlying rock layers have been eroded. Because both volcanites and plutonites became rock from cooled magma, both are classified as igneous rocks.

Metamorphic rocks

It happens inside the earth: Strong pressure and high temperatures ensure that the constituents of the rock, the minerals, react with one another and transform. In this way new rock is formed. Because the Greek word for metamorphosis is “metamorphosis”, geologists also speak of metamorphic rocks.

A correspondingly high pressure is created when two earth plates collide and one plate dips under the other. The rock is then squeezed together like in a huge press. A frequent result of such a rock metamorphosis is the blue slate. Its parent rock is basalt or a rock with a similar composition to basalt.

Extreme heat also causes rocks to transform. For example, it is baked near a magma stove like in an oven. Marble, for example, is nothing more than limestone that has been heated very strongly in the interior of the earth; During this process, new minerals are formed and the rock becomes harder. Sandstone also changes at high temperatures, because its quartz grains then stick together: the harder quartzite is made from the original sedimentary rock.

In contrast to the complete melting through volcanism, the rock remains solid during the metamorphosis. However, if the temperature continues to rise, the rock will eventually turn into liquid magma. If this mass cools down, it turns into igneous rock. The rock cycle is in full swing.

Sedimentary rocks

Some rocks look like they're striped. In the Dolomites, for example, such transverse bands can often be clearly seen. Sandstone or limestone quarries sometimes have similarly pretty patterns.

The "stripe design" is created when the rock is formed. The starting material is weathered debris that is carried away by water or the wind. Rivers, glaciers and dust storms lose their strength at some point: the courses of rivers become slower and slower towards the mouth and finally flow into the sea or a lake. Glaciers are advancing into warmer regions and melting. Dust storms also subside at some point. Then they can no longer move dust, sand and rubble. The crushed rock that is dragged along settles out. Over time, the deposited material forms an ever higher layer - the sediment. Such sediments, including the remains of dead animals or limestone shells, collect particularly on the seabed and on the bottom of lakes, where rivers wash up a lot of material.

Gradually, different sediments are layered on top of each other. A layer can, for example, consist of sandstone: During the dry season, the wind blew desert sand here. If the sea level rises again, this layer is covered by water: the limestone shells of marine animals sink to the sea floor and deposit another layer over the sand. Over millions of years the climate changed again and again and caused the sea level to fluctuate. This allowed different layers to deposit.

Over time, the sediment cover becomes thicker and thicker. Under the weight of one's own weight, the initially loose sediments are compressed more and more, small cavities disappear, the mass condenses. Further layers are deposited over it, the sediment becomes more and more solid and finally becomes sedimentary rock under pressure. This process is also called diagenesis in geology. For example, if the shells of tiny marine animals are pressed into stone, limestone is created. Fine grains of sand made of quartz cement together under the high pressure to form sandstone.

In addition to rubble, dead animals also settled, for example fish on the ocean floor. Their bones and scales remained hermetically sealed and petrified. Such fossils are immortalized in the stone. Even after millions of years they reveal a lot about the time in which the sediment was formed. Therefore, geologists can read in the rock layers like a history book.

Usually only the top layer is visible to us. However, when a river digs its way through the sedimentary rock, lifts it up during mountain formation, or blasts it free in a quarry, we get a view of the cross-section. The individual layers of sediment can then be easily recognized as "stripes" or bands in the rock.

From rock to grain of sand - weathering

Today the north of Canada is a gently undulating landscape. However, many millions of years ago there was a mountain range here. In fact, even high mountains can turn into small hills over a very long period of time.

The reason for this transformation: The rock on the earth's surface is constantly exposed to wind and weather. For example, if water penetrates into cracks in the stone and freezes, it splits the stone apart. This process is called frost blasting. The rock also becomes brittle through temperature changes between day and night and through the force of water and wind. In other words: it weathers. This process can also be observed in buildings or stone figures. During the weathering, the rock breaks down into smaller and smaller components up to fine grains of sand and dust. Different rocks weather at different rates: Granite, for example, is much more resistant than the comparatively loose sandstone.

Some types of rock even completely dissolve when they come into contact with water, for example rock salt and lime. Rock salt is chemically the same as table salt - and that already dissolves in ordinary water. Lime is a little more stable, but limestone also dissolves in acidic water. Acid is formed, for example, when rainwater in the air reacts with the gas carbon dioxide. This “acid rain” attacks the limestone and dissolves it over time. The weathering leaves rugged limestone landscapes on the surface of the earth, and caves are formed below the surface.

But not only solution weathering, heat and pressure also wear down and crumble rock under the earth's surface. Wherever plants grow, roots dig in, break up the rock piece by piece and also ensure that it is removed millimeter by millimeter.

In this way, weathering not only works on individual rocks, it gnaws at entire mountain ranges. It will take a few million years for the Black Forest to be as flat as northern Canada.

What is rock?

In some places it peeps out from under a thin cover of plants, in other places it rises up as a steep rock face: the bare rock. It is the building material that the earth's crust and mantle are made of. However, rock is not a uniform mass. Similar to cake batter - only much harder - it is a mixture of different ingredients: the minerals.

Rock therefore consists of different minerals. Depending on their composition, the minerals combine to form certain types of rock. Granite, for example, is a rock made up of the minerals feldspar, quartz and mica. The fact that granite is made up of different minerals can already be seen from the fact that it is speckled: it contains lighter and darker parts, which owe their different color to three different minerals. The darker parts come from the mineral mica. The quartz mineral often appears whitish to gray. The third mineral, feldspar, can take on any color, even pink. Unlike the hard granite rock, the softer sandstone consists almost entirely of quartz. Because of this, sandstone looks more uniform than the speckled granite.

Almost all minerals are arranged according to a certain lattice pattern to form uniform shapes, the crystals. The mineral rock salt, for example, grows into a cube. The regular arrangement also results in other shapes with smooth surfaces, as can be seen well in a rock crystal. This consists of particularly pure and therefore transparent quartz. If, on the other hand, liquid is enclosed in the quartz, it turns milky in color. Then geologists speak of a milk quartz.

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.