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Rocks are everywhere! They are large and small,
heavy or light, porous or dense, but rocks
in some shape or form can be found all over
the planet. Different types of rock are formed
in different ways. There are three main types
of rock: Igneous, Sedimentary, and Metamorphic.
Igneous is a word that means 'fiery.' Igneous
rock is formed when magma or lava cools. Sometimes
magma cools slowly underneath the earth's
surface. This forms intrusive igneous rock,
like granite. Other times lava cools when
it comes to the surface in a volcanic eruption.
This forms extrusive igneous rock. Some examples
of extrusive igneous rock are tuff, obsidian,
and pumice. Igneous rocks make up about 95%
of the earth's crust.
The next type of rock is Sedimentary rock.
Sediment is small particles of sand, mud,
and organic material that settle to the bottom
of water or land areas, often lakes or oceans.
Sedimentary rocks are formed when sediment
accumulates over time in deposits that form
layers. These layers become squeezed and compressed
over time until they consolidate into a rock.
Sedimentary rocks are the types of rocks in
which fossils may be found, since the process
of forming sedimentary rocks can preserve
plant and animal remains that are deposited
into the sediment layers. Some examples of
sedimentary rock are limestone, shale, and
sandstone.
The third type of rock is Metamorphic rock.
The word 'metamorph' means to change form,
and metamorphic rock is rock that has been
changed by extreme heat and pressure. Sedimentary
rock, igneous rock, or even other metamorphic
rocks can be changed by heat and pressure
into new kinds of rock. Metamorphic rocks
can be formed by being deep under the earth,
where pressure and temperatures are high,
or when rock near the surface is heated up
by the movement of tectonic plates or magma.
Different types of rocks become different
types of metamorphic rock when exposed to
heat and pressure. For example, shale becomes
slate, sandstone becomes quartzite, and limestone
becomes marble.
Rocks are slowly but constantly changing in
something known as the 'rock cycle.' The rock
cycle begins with magma, or hot melted rock,
deep beneath the earth's surface. This magma
becomes crystallized, becoming igneous rock.
These rocks begin to erode, or break down
into small pieces because of wind, water,
or other forces. The small fragments of rock
are carried away as sediment when water passes
over them and are deposited in layers which
eventually become sedimentary rocks. Then,
some sedimentary rocks are pushed below the
surface due to tectonic activity, where they
are exposed to heat and pressure, transforming
them into metamorphic rocks. If the rocks
are buried even deeper, they may melt and
form magma, starting the cycle all over again.
Of course, sedimentary and metamorphic rocks
can be eroded into sediment, and igneous rock
can become metamorphic rock or lava, but one
way or another, rocks all over the world keep
changing from one form to the next.
