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A Closer Look: Igneous Rocks
What are igneous rocks?
Igneous rocks are the most common rocks on
Earth. All of Earth's ocean floor, its entire mantle, and much
of the continental crust
consists of igneous rock. Igneous rock forms as molten (liquid)
rock cools and solidifies. Most of this molten rock originates
under the
Earth’s surface in a zone within the upper mantle where it
is extremely hot but the pressure is not great enough to keep
the rock solid. There are two major types of igneous rock:
extrusive and intrusive.
How are different types of igneous
rocks formed?
Basalt.
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Extrusive, or volcanic igneous rock forms when
magma erupts, or extrudes, onto the surface of the Earth as lava.
This occurs
along active plate margins, such as mid-oceanic spreading ridges
and subduction
zones, as well as at intraplate settings, like hot spots. Lava
cools and hardens quickly at the surface of the Earth, as finely
grained
rock with many tiny crystals. In some cases, the lava cools so
quickly that the molten material does not have enough time
to arrange itself
into crystalline structures. This kind of igneous rock is called
volcanic glass. Another product of volcanic eruption is pyroclastic
debris,
which are particles formed from the gas and lava that explode
into the atmosphere. Pyroclastic debris includes fine particles
of volcanic
glass called ash, and larger pieces of rock called, depending
on their size, cinders and bombs. The most common extrusive
igneous rock is
basalt, which is the rock that comprises oceanic crust. Basalt
accounts for more than 90% of all volcanic rock on the planet.
Intrusive, or plutonic, igneous rock forms when when magma beneath
the Earth’s surface rises upward and pushes its way,
or intrudes, into pre-existing crustal rocks. Features associated
with this ascension
of magma include sills (nearly horizontal intrusion of magma
that is injected between layers of rock); dikes (nearly vertical
injection
of magma that cuts across layers of rock); laccoliths (when a
sill domes upward, looking like a blister); and batholiths
(immense, deep,
dome-shaped intrusions of igneous rock). Magma cools and solidifies
more slowly underground than at the Earth's surface, which produces
igneous rocks with coarse crystals that can easily be seen with
the naked eye. One of the most well known intrusive igneous
rocks is granite,
which comprises much of the continental crust.
In addition to
being categorized as extrusive or intrusive, igneous rocks
are also classified in other ways:
Mafic igneous rocks are dense,
and rich in iron- and magnesium-bearing minerals and are usually
dark in color.
Felsic igneous rocks are rich in less dense minerals,
such as quartz, and are often light in color.
Granular igneous
rocks consist of crystals that are large enough to be easily
seen, such as granite.
Aphanitic igneous rocks are made of tiny
crystals that cannot be seen with the naked eye, such as basalt.
Glassy igneous rocks are composed mostly of volcanic
glass. Obsidian is one example.
Porphyritic igneous rocks have larger
crystals embedded
in a finer grained matrix.
Pyroclastic igneous rocks
are volcanic rocks that form from explosive eruptions that
shatter magma
and can be either cemented together
or unconsolidated fragments or slivers. Pumice and
ash are examples.
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