So what is the origin of rocks and history of geology?
As solid as they seem, rocks are as dynamic
as the rest of the nature. They just change very slowly.
The oldest rocks formed with the formation of the Planet Earth 4.6
billion years ago.
Through the whole history of geology ever since, rocks have been
built, eroded and rebuilt in a process called the rock cycle. History
of geology seen in the field: plutonic
igneous rocks that have been uplifted and exposed by erosion. Poster by AllPosters.com.
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Rocks first (it's hard to say "first" because it's a cycle, but I have
to start somewhere) form by volcanic
activity. As the magma cools under the Earth's surface in
magma chambers, intrusive
igneous rocks and minerals form (one
of the examples is the very
common rock granite). As the lava gets out to the surface, it cools,
igneous rocks such as basalt form.
Through wind and rain activity those rocks erode. The material
that is eroded ends up on the ground, and finally often finds its way
to waterways. In the bottom of oceans, lakes and rivers, the sedimentary material
accumulates and older layers are covered by younger layers.
The older layers are finally buried so deep that they turn into rock
under all the pressure from younger layers. This is how sedimentary rocks
Some sedimentary rocks end up on the surface, by uplifting, and/or
erosion. Others keep getting buried deeper and deeper, where in the
Earth's interior temperatures and pressures increase.
It first metamorphoses into a metamorphic
rock, by pressures and temperatures - it is streched and
compressed but not melted. Some metamorphic rock ends up on the surface
by uplifting forces and erosion.
Others can get buried even deeper, where the temperatures are
getting so hot that the rock starts melting
- it turns into magma melt. Different forces may bring it closer to the
surface again, where the temperatures may get so low that it "freezes"
(this is still at a temperature of thousands of degrees Celcius) and
turns into an intrusive
igneous rock. Or, it may end up exploding in a
volcano and freeze on the surface of the earth to an extrusive igneous rock.
And the cycle continues by erosion, burial, uplift and so on.
All the rocks
we see at any
time on the Earth, are in some stage of that cycle.