Formation of igneous rocks
is an interesting process.
It is often seen as
the "beginning" of the rock cycle.
This is because it's the biggest change in the
rock cycle.
While sedimentary
rocks
form under pressure of sediment layers and metamorphic
rocks
form under
pressure and/or due to changing temperatures, none of them actually
melts and turns into hot, liquid magma. Igneous rocks do.
By Greg Bishop via Flickr.com
As part of the rock cycle, rocks are built, eroded and some end up
being pushed deep down
into the
interior of the Earth. The deeper down
they go, the hotter are temperatures - the core of the Earth is very
hot.
As the conditions (pressure and temperature) change, chemical reactions
start happening, because rocks, as other things, consist of chemicals.
All chemicals strive to reach stability so chaniging conditions make
them to react to become more stable in the new conditions.
As the
chemical reactions happen, the mineralogy of the rock changes. If that
happens in the solid state - metamorphic rock is formed.
Igneous rock, on the other hand, is formed after the previous rock has
undergone such extreme temperatures and pressures, that it has melted
and turned into hot liquid. When that liquid is under the
Earth's
surface, it is called magma. When it sprinkles out of a volcano, it is
called lava.
Igneous rock is formed
when that
magma, or lava, freezes
and turns into a rock. That happens when the rock is
lifted
closer to
the Earth's surface again, and the temperatures and pressures decrease;
or, when lava sprinkles out of a volcano and turns from a liquid to a
solid substance.
That is also when the minerals in the rock are formed. Different
minerals form at different pressures and temperatures. That's why
different rocks have different mineralogical composition.
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