Rocks are aggregates
of minerals. Geologists divide rocks into three groups: igneous,
metamorphic, and sedimentary. Igneous rocks crystallize from
magma. Metamorphic rocks form by the deformation and/or recrystallization
of pre-existing rock by changes in temperature, pressure, and/or
chemistry. Sedimentary rocks form by weathering and erosion
of preexisting rock to make sediment, which is lithified into
Igneous rocks form when molten rock (magma) originating
from deep within the Earth solidifies. The chemical composition
of the magma and its cooling rate determine the final igneous
||Intrusive igneous rocks are formed from magma
that cools and solidifies deep beneath the Earth's surface.
The insulating effect of the surrounding rock allows the magma
to solidify very slowly. Slow cooling means the individual
mineral grains have a long time to grow, so they grow to a
relatively large size. Intrusive rocks have a characteristically
coarse grain size.
||Extrusive igneous rocks are formed from magma that cools
and solidifies at or near the Earth's surface. Exposure to
the relatively cool temperature of the atmosphere or water
makes the erupted magma solidify very quickly. Rapid cooling
means the individual mineral grains have only a short time
to grow, so their final size is very tiny, or fine-grained
Sometimes the magma is quenched so rapidly that individual
minerals have no time to grow. This is how volcanic glass
Sedimentary rocks are formed from pre-existing rocks or
pieces of once-living organisms. They form from deposits
that accumulate on the Earth's surface.
||Clastic sedimentary rocks are made up of pieces (clasts)
of pre-existing rocks. Pieces of rock are loosened by weathering,
then transported to some basin or depression where sediment
is trapped. If the sediment is buried deeply, it becomes compacted
and cemented, forming sedimentary rock. Clastic sedimentary
rocks may have particles ranging in size from microscopic
clay to huge boulders. Their names are based on their grain
||Chemical sedimentary rocks are formed by chemical precipitation.
This process begins when water traveling through rock dissolves
some of the minerals, carrying them away from their source.
Eventually these minerals are redeposited when the water evaporates
away or when the water becomes over- saturated.
||Biologic sedimentary rocks form from once-living organisms.
They may form from accumulated carbon-rich plant material
or from deposits of animal shells.
Metamorphic rocks are rocks that have been substantially
changed from their original igneous, sedimentary, or earlier
metamorphic form. Metamorphic rocks form when rocks are
subjected to high heat, high pressure, hot, mineral-rich
fluids or, more commonly, some combination of these factors.
||Foliation forms when pressure squeezes the flat or elongate
minerals within a rock so they become aligned. These rocks
develop a platy or sheet-like structure that reflects the
direction that pressure was applied.
||Non-foliated metamorphic rocks do not have a platy or sheet-like
structure. There are several ways that non-foliated rocks
can be produced. Some rocks, such as limestone are made of
minerals that are not flat or elongate. No matter how much
pressure you apply, the grains will not align. Another type
of metamorphism, contact metamorphism, occurs when hot igneous
rock intrudes into some pre-existing rock. The pre-existing
rock is essentially baked by the heat, changing the mineral
structure of the rock without addition of pressure.