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Translations of Encyclopedia about Geology

Igneous Rocks

Magma, molten rock, is also called plutonic rock after the Greek god of the underworld. Magma is formed by the heat. It flows as hot, molten mass in the depths of the earth. Magma is propelled to the surface by the movement of the earth's crust, where it solidifies and turns into igneous rock. Magma rises to the surface also in the form of lava.

The so-called plutonic rock forms within the earth's core and is classified as intrusive magmatite or intrusive igneous rock. During a gradual cooling process it creates large crystals. This results in a coarse-grained rock structure. Usually, we do not see this intrusive igneous rock. However, the forces of erosion or earth folding may bring this rock to the surface. An example is granite, which is used in construction because of its hardness. Other examples are obsidian or felsite, which have the same composition as granite, but their cooling process is faster.

Igneous rocks (volcanites) are formed on the earth's surface or immediately underneath, and are called extrusive magmatites or extrusive igneous rocks. They cool down rather faster than plutonic rocks. This results in the formation of fine crystal structures. The best known is basalt. This hard, generally black rock is one of the most common rocks.

When igneous rocks occur as a fill of a flaw, they are called gangue.

Magma that cooled quickly during an eruption so that it could not crystallise, forms natural (volcanic) glass. This kind of rock is called obsidian.

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Sedimentary Rocks

"Sediment" means deposit. Therefore we call sedimentary rocks deposited rocks or stratified rocks. They form from sands, mud, as well as organic stratified sediments. The pressure of the layers above compacts these sediments, which end up petrified.

Over time, the effects of wind and weather cause the disintegration of all types of rock, even of granite. Most of these small fragments are carried sooner or later by streams of rivers, by ice or by wind to the sea, where they settle on the bottom.

Lower layers are compacted by the weight of the upper sediments. Water, which these layers contain, is often rich in minerals. Over time, the minerals crystallise in the spaces between the layers and further solidify portions of the sediments. Rock-forming processes may bring the sedimentary rocks again to the earth's surface. By the repeated processes of erosion the cycle begins. When the erosion uncovers the individual layers, as it happened, for example, in the Grand Canyon, in the United States, we can reconstruct the landscape as it looked millions of years ago.

Depending on the origin and composition (loose and solidified rocks), we have classical, chemogenous , and organogenous or biogenous sediments. The classical sediments, which formed mainly from the accumulated rock fragments of various sizes, include sand or sandstone, clay (which can be used for the making of pottery, schistic soil, gravel, and similar material. Chemogenous rocks, which formed by precipitation from solutions are, for example, gypsum, salt of potassium, limestone, dolomite (brown limestone), and marl. Biogenous or organogenous sediments are chalk, peat, coal, or lime. They contain a substantial quantity of vegetal and animal organisms.

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Metamorphic Rocks

Metamorphose means "change" or "transformation." Sedimentary and igneous rocks change as a result of pressure and, especially, of temperature changes affecting the metamorphised rock. Rocks formed by melting do not belong to the category of metamorphised rocks.

Mineral crystals in a rock are crushed or fragmented. They assemble again and form a new rock. The cause of the pressure or heat, which make this happen, may be for example volcanic explosions. Rock which is in the proximity of a lava flow is not molten, but is exposed to high heat. The most common cause of these changes is the collision of the tectonic plates. As a consequence of these collisions, horizontally positioned masses of rock form mountain ranges.

We classify metamorphic rocks depending on the formation process: contact metamorphose or regional metamorphose. In the case of the contact metamorphose, the rocks form as a result of local heating by contact with molten lava.

Different rocks form from the original sedimentary rock. For example, quarzite derives from sandstone, marble from limestone, magnesite-quartzite rocks from dolomite. In the case of the originally magmatic rocks, they change mainly into cherts. In the case of regional metamorphose, rocks form as a result of lowering of large portions of the crust, as well as increased pressure and temperature. Then the sandstone changes into quartzite, the calcareous sandstone and marl change into calcareous micas, calcite, and calcareous quartz, the limestone changes into marble and micas, clay minerals change into gneiss and micas.

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Rock Cycle

All types of rocks, igneous, sedimentary, and metamorphic, are joined in the rock cycle. Due to constant changes, one rock forms from another rock.

The first one in the cycle is magma, which itself forms by melting of the existing rocks of all types. By melting in the earth's interior, all chemical elements become homonized [sic? homogeneous?]. New minerals form during cooling by means of crystallisation. New igneous rock forms. A major part forms on the boundary of colliding lithospheric plates.

In the mountain-forming process, when the plates collide, they are propelled upwards. Rocks which are now on top are exposed to erosion and weathering and are strewn loosely over the igneous rock. Now the igneous rocks are exposed to the weathering process.

Minerals containing iron change into iron oxide, other minerals mix with the soil. This partially transformed and partially unchanged detritus is borne by rivers to the ocean, where different sedimentary layers are formed from sand, soil, and dead organisms. Gradually, these layers are covered by new ones, and under their own weight they sink lower and lower.

When the minerals, which on the earth's surface are stable, are exposed in the earth's interior to temperatures of more than 300 degrees Celsius, they change. High pressure and high temperatures change the sedimentary rocks into metamorphic rocks. If they continue to be exposed to high temperature, new magma forms and the cycle starts all over again. This process never ends, it repeats itself in different stages and different places of the earth.

Naturally, individual phases of this cycle may be skipped. Every kind of rock may be loose on the earth's surface and, as a result of weathering or fragmentation, may become the beginning of new sediments. We also know from ocean drilling, that igneous rocks deep in the earth's interior never came to the surface and so they were never weathered.

The rock cycle is linked directly to the tectonic plate processes. When the plates descend, the rocks melt. When the plates collide, mountain ranges form. Metamorphic rocks originate as a consequence of high pressure and high temperatures. The top of the mountain ranges fractures and is eroded. Resulting sediments form deposits on the ocean floor, they are slowly covered and sink deeper, where they turn into magma and their cycle starts again.


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