Mojave River Valley Museum
Mohahve Historical Society
How Rocks are Classified
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 rock type.
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 forms.
Sedimentary rocks are formed
from pre-existing rocks or pieces
of once-living organisms. They
form from deposits that
accumulate on the Earth's
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.
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 carbonrich
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
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 nonfoliated
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.
Source - NPS, USGS