It is the recycling
process by which the rocks are formed, broken down and reformed.
It is not
only one path though. There are different ways through the cycle.
First, say, that igneous
rock
is formed.
Magma that's lifted higher up by tectonic movements inside
the mantle, starts freezing as the temperatures cool and pressures
ease.
By orangems via Flickr.com
As it starts freezing, minerals start to form and finally solid
rock is formed.
But not all of that rock follows the
straightforward igneous
=>
sedimentary => metamorphic
pattern.
The movements in the Earth's interior may press the rock (or parts of
it) back down.
If it is pushed deep enough that the temperatures and
pressures make it melt again, it has turned into magma, which can build
new igneous rock when it freezes.
If it's pushed down not deep enough
for the rock to melt, the pressures, temperatures and shear stress will
turn that rock into a metamorphic
rock. Although it does not melt, new minerals and new rock is
formed.
By
Kiril Rusev via Flickr.com
If
some of that igneous, or metamorphic rock is uplifted, erosion starts
as soon as it is at the surface.
Wind, ice and water start
weathering the rock away. Some eroded particles are transported by
wind, ice or water, and deposited in a new place.
Others are deposited
where they eroded. Enough layers on the top and the lower layers will
start turning into sedimentary
rock
under pressures from upper layers.
That sedimentary rock can be
uplifted again, in which case it will reach the surface and start
eroding. Or, it may be pushed deeper down, where it can turn into
either metamorphic or igneous rock.
By
Kiril Rusev via Flickr.com
So the traditional igneous
=> sedimentary => metamorphic => igneous cycle is
a simplified
way to show how it works.
In reality, anything can happen next, and all
the rocks that we see (and the ones we don't) are in some stage in that
cycle.
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