What is the hardest metamorphic rock

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.

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.

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.

What happens in the event of a volcanic eruption?

It steams and bubbles, it smokes and hisses. Glowing hot rock shoots up from inside the earth. An ash cloud rises, lava gushes out of the volcano and flows over the surface of the earth. When a volcanic eruption occurs, enormous forces are at work. But how does a volcano actually erupt?

In the earth's mantle, the rock layer under the earth's crust, temperatures of over a thousand degrees Celsius and very high pressure prevail. If the heat and pressure are high enough, the rock melts and becomes a viscous mass called magma. This magma expands and rises to the top. There it first collects in cavities, the magma chambers. However, none of this happens overnight, but takes tens of thousands or hundreds of thousands of years.

When the magma chamber is full and cannot hold any more material, the hot magma makes its way out. It penetrates through channels and crevices to the surface and emerges there as glowing hot lava - the volcano erupts. The channel through which the magma swells up is called a chimney, and its exit is called a crater.

Some volcanoes regularly spit lava, for example the Stromboli in southern Italy. One can observe its eruptions every day. Other volcanoes remain quiet for centuries but are not actually extinct. Often their craters are clogged with lava and debris. That makes them very dangerous because if they break out there can be huge explosions; well-known for this are, for example, Vesuvius near Naples or Krakatau in Indonesia. Such explosive eruptions blow up millions of tons of rock. The ash cloud that rises from the eruption can stay in the air for a long time and be widely dispersed by the wind. This cloud then only slowly settles on the earth as a fine layer of ash.

Lava that is not thrown into the air flows down from the crater rim as a scorching stream of molten rock. When this lava flow cools, it solidifies into lava rock. Little by little, lava flows, ash and debris build a mountain around the crater - the volcanic cone.

What is the difference between lava and magma?

Magma and lava actually refer to the same thing, just in different places: Magma is inside the earth, lava is on the surface of the earth.

Magma arises where the heat and pressure in the earth's interior are very high. There the rock melts and a viscous rock pulp, the magma, is created. The magma collects in underground cavities and flows up to the surface of the earth when the pressure rises. As soon as the magma swells out of the earth during a volcanic eruption, it is called lava. Gases that were trapped in the magma can then escape into the air. Therefore, lava and magma differ in their chemical composition.

As long as the lava is hot, it is soft and malleable. On the surface of the earth, the lava cools slowly and solidifies. Then it can look very different, depending on where and how it flowed out of the earth: For example, if a volcano erupts underwater, the lava cools down very quickly. It forms into structures that look like lumps or pillows. This is why one speaks of pillow lava. Other lava flows look like long balls of wool and are therefore called knitted lava.

Over time, various rocks are formed from lava. Particularly thin lava turns into dark gray basalt after cooling. This rock is often used as a paving stone for roads and paths. When lava is thrown into the air during a volcanic eruption and puffs up like foam, it creates pumice stone. The trapped air makes pumice stone so light that a piece of it can float on the water. Volcanic ash and volcanic dust that solidify turn into tufa. Many houses in the Vulkaneifel, for example, are made of tuff.

Oceanic and continental crust

The earth's crust is not built up in the same way everywhere. The earth's land masses consist of continental crust, the sea floor of oceanic crust. One of the differences is that in addition to oxygen, the continental crust mainly contains silicon and aluminum. The oceanic crust, on the other hand, also has a high proportion of magnesium. But that is by no means the only difference:

Oceanic crust forms on the sea floor, where magma rises and solidifies along the mid-ocean ridges. Since the crust is constantly growing back here, the two lithospheric plates are pressed outwards. The oceanic crust is therefore getting older and older towards the coasts. Some of the oldest pieces are around 200 million years old. They are located in the Atlantic off North America and east of the Mariana Trench in the Pacific. The five to eight kilometers thick oceanic crust does not get any older: because it is heavier than the continental one, it submerges in the event of a collision and is melted again in the interior of the earth.

The continental crust is lighter, but thicker than the oceanic crust: on average, it extends 40 kilometers, under mountains it can even be up to 80 kilometers deep. When exactly it was formed is still a mystery even to science. Evidence of this is provided by the oldest known rock on earth: It was found in northern Canada, is over four billion years old and is believed to be a remnant of the very first crust of the earth.

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 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 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, lifts it up during mountain formation, or blasts it free in a quarry, we get a glimpse of the cross-section. The individual layers of sediment can then be easily recognized as "stripes" or bands in the rock.