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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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"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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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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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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