What is the mineral composition of 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 schist. 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 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.


It is a sensation for science: In the north of Canada, geologists have come across 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. Now the scientists are 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 of ribbon iron in Greenland: This now ranks 3rd on the age scale of the rocks. The rocks in Greenland are “only” 3.9 billion years old!

Where plates collide

When two vehicles collide, their sheet metal is crumpled together. Something similar happens when two plates of the earth's crust collide. Then their rock is pushed together and very slowly laid into huge folds - this is how fold mountains arise. What the crumple zone is in a car accident, the mountains are in a collision of plates - only that a car accident takes place in fractions of a second, whereas a plate collision takes many millions of years.

This is exactly how the Alps came into being: Africa pressed against the Eurasian continent and unfolded the mountains. The Himalayas in Asia and the Andes in South America also owe their origins to the collision of migrating crustal plates.

In such a crash, the rock of the lighter plate is pushed upwards, the heavier plate sinks into the depths. This process is called subduction, the area in which the plate descends, the subduction zone. There are often deep gullies along these zones, which is why they are easy to see. The deepest of them is the Mariana Trench in the Pacific Ocean. This deep-sea channel lies where the Pacific plate dips under the Philippine one.

The further the earth's crustal plate disappears, the hotter it gets. The rock melts and magma forms in the depths. Due to the increasing pressure, it can be pressed up again. Where it penetrates to the surface of the earth, volcanoes spew lava and ash. There are entire chains of such volcanoes around the Pacific Plate, for example in Indonesia. Because one volcano follows the other, this plate boundary is also called the “Pacific Ring of Fire”.

Not only do volcanoes erupt at such plate edges. The earth also frequently trembles because the movement of the plates creates tremendous pressure and increasing tensions. As soon as these discharge, quakes shake the earth's surface. In Japan, for example, three plates meet: the Pacific, the Filipino and the Eurasian. It is for this reason that violent earthquakes hit Japan so often.

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.


Whether green emerald, blue sapphire or red ruby: we know precious stones as sparkling and particularly valuable pieces of jewelry. Gemstones are simply minerals. However, they have to meet three requirements in order to be considered gemstones: They have to be particularly rare, transparent and at the same time very hard.

Gemstones are formed deep in the earth's interior under high pressure and at high temperatures. The hardest of them and at the same time the hardest known mineral is diamond. It is formed from a single element at a depth of around 150 kilometers at temperatures above 1200 degrees Celsius: carbon. Crystals usually develop from eight equilateral triangles, called octahedra. Other shapes such as cubes are also possible. The diamond gets to the surface of the earth by being thrown upwards together with the rising magma. The largest diamond ever found is the so-called "Cullinan". It was discovered in a South African mine in 1905 and weighed exactly 3106.75 carats in its raw state. This corresponds to a weight of 621.35 grams.

Whether diamond, amethyst, emerald or topaz - all gemstones differ from one another in terms of structure, composition and color. They only become particularly beautiful and shiny when they are cut. He lets the colors of the gemstones shine through a certain refraction of light.

In addition to precious stones, there are other gemstones in the earth's crust, such as blue lapis lazuli or green malachite. Although these are also very sought-after and beautiful, they are not see-through and are too common to be considered gemstones.

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 makes up the earth's crust and mantle. 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.

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.

Sedimentary rocks

Some rocks look like they're striped. In the Dolomites, for example, such transverse bands are often clearly visible. 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 made the sea level 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, is raised in mountain formation, or blasted 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.