|How do rocks form?
|Question Date: 2014-10-07|
Good question! There are three main type of
rocks on earth: igneous, metamorphic, and
sedimentary, and they all form in different
Igneous rocks form when liquid rock (magma)
cools and crystallizes. Most of the earth is solid
rock (even the mantle is mostly solid), but some
conditions can cause rocks to melt to form magma.
When the magma cools enough, certain chemical
components in the melt are no longer stable in the
liquid phase, and minerals (like olivine or
feldspar) begin to crystallize. An example of an
igneous rock is a Hawaiian basalt that was erupted
as lava from a volcano and crystallized at the
In the paragraph above, I mentioned that in
cooling magmas, certain components become unstable
in the melt and crystallize to a solid phase.
“Phase changes” also occur in solid rocks when
temperature and pressure changes.
Metamorphic rocks undergo mineral and textural
changes when they are solid. An example of
metamorphism is when shale, a sedimentary rock
made up mostly of clay minerals and quartz, gets
buried in the crust. As the shale gets heated up
and pressure increases, the clay minerals are no
longer “stable” and they change into minerals like
chlorite and biotite (a mica). The quartz usually
remains stable but may “recrystallize”. The
end result when a shale is metamorphosed are
metamorphic rocks known as schists and
gneisses. Other rock types can be
metamorphosed as well. Igneous, sedimentary, or
other metamorphic rocks can all be metamorphosed.
Sedimentary rocks form at the surface of the earth
by the “reworking” of older rocks. They form two
main categories: clastic and chemical.
Clastic sedimentary rocks form from pieces of
older rocks that have been “weathered”. When rocks
break down from weathering at the earth’s surface,
solid fragments (sediment) will be eroded and
transported (often in streams and rivers). Solid
fragments, like quartz grains and clay minerals,
accumulate in basins (like river beds, lakes, and
the ocean). As the sediments are buried by other
sediments, they eventually become
“lithified” which means that they turn into
a sedimentary rock. An example of a clastic
sedimentary rock is a sandstone, solidified from
loose sandy sediment. Chemical sedimentary rocks
form when chemical components dissolved in water
“precipitate” (solidify from a solution). A
simple example is the precipitation of halite
(salt) from water. This happens in the Great Salt
Lake in Utah, where many natural salt deposits
occur. Living organisms often catalyzes
precipitation. For example, coral reefs are made
when coral (small marine invertebrate animals)
precipitate the mineral calcite or aragonite from
calcium and carbonate (CO32–) that was
dissolved in seawater. Coral reefs are often
preserved in the rock record as limestone.
There are three ways that rocks can form. They can:
-crystalize (freeze) from a molten state
(these are igneous rocks like granite or basalt);
-precipitate chemically from water
(limestone and other chemo-sedimentary rocks);
-solidify from sand, mud, or other sediment
(clastic sedimentary rocks).
There are also metamorphic rocks like schist
or gneiss that form when another rock is
heated and pressurized until it changes into
another type of rock. This isn't really the
formation of new rock, though, as it had been rock
beforehand, just a different kind of rock.
There are 3 types of rocks, and they are each
formed in a different way. Igneous rocks form
from lava -- think of rocks exploding from
volcanic eruptions, or magma ('lava') cooling
underground to make crystals. This produces rocks
like granite, which you might recognize from
countertops in fancy kitchens.
If an igneous rock has been around a long time, it
could change shape color, etc., by being heated or
put under high pressure (ie. buried deep
underground). Pressure and temperature can reform
a rock into a new one. We call these
metamorphic rocks. An example of a
metamorphic rock is marble (before it was cooked
underground and metamorphosed into a marble, it
was a limestone).
The last type of rock is sedimentary. When
any rock is left outside for a long time it slowly
degrades by wind, rain etc. It gets degraded into
smaller and smaller pieces (think of mountains
getting eroded into sand). After small sediments,
such as sand, are buried underground for a long
time, cement forms between the different tiny rock
particles and it all hardens to form one big
Rocks form in three main ways depending on the
type of rock:
-Igneous rock comes from solidified or
crystallized magma, from inside or under the
earth' crust, or from other rocks melted together.
The easiest place to see these rocks are at
volcanoes, although they exist all over the world.
Basalt is an example.
-Sedimentary rocks are form from sediment,
such as sand. As the sediment clumps together, and
gets buried, the pressure of the material on top
forces the sand to clump together physically to
form rocks. This is probably the most common type
on the surface of Earth. An example is limestone.
-Metamorphic rocks were once sedimentary or
igneous rocks, but have been changed further by
extreme pressure or heat in the Earth's crust.
These are most often found in mountains.
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