Igneous
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 rock type.
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Intrusive
(plutonic)
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Intrusive igneous rocks are formed from magma that cools and
solidifies deep beneath the Earths 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.
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Extrusive
(volcanic)
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Extrusive igneous rocks are formed from magma that cools and
solidifies at or near the Earths 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
forms.
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Sedimentary
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earths surface.
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Clastic
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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 size.
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Chemical
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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.
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Biologic
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Biologic sedimentary rocks form from
once-living organisms. They may form from accumulated carbon-rich
plant material or from deposits of animal shells.
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Metamorphic
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.
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Foliated
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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.
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Non-foliated
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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.
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