Types of Igneous Rocks
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An igneous rock is formed by the cooling and crystallization of molten
rock. The term igneous is derived from ignius, the Latin word for
fire. Scientists have divided igneous rocks into two broad categories
based on where the molten rock solidified. Volcanic
rocks (also called extrusive igneous rocks) include all the products
resulting from eruptions of lava (flows and fragmented debris called
pyroclasts). Plutonic rocks (also
called intrusive igneous rocks) are those that have solidified
below ground; plutonic comes from Pluto, the Greek god of the underworld. The initial distinction
between volcanic and plutonic rocks is made on the basis of texture
(fine-grained volcanic vs. coarse-grained plutonic). Volcanic and plutonic
rocks are divided further on the basis of
chemistry and mineral composition. The classification scheme below is based
on chemistry, and is perhaps the simplest method; there
are many other classification methods for igneous rocks.
These rock types all have different
characteristics, including temperature when fluid, viscosity (resistance
to flow), composition, explosiveness, and types, amounts, and sizes of
minerals.
Classification and major oxide compounds of igneous rocks
Classification and flow characteristics of volcanic rocks
Components of igneous rocks
Major Chemical Elements Forming Igneous
Rocks
Illustration by J.
Johnson
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Volcanic rocks are typically divided into four basic types
according to the amount of silica (SiO2) in the rock (see figure
at bottom, Components of Igneous Rocks, for the plutonic
equivalents of these rock types):
- basalt consists of about 48-52% silica
- andesite consists of about
52-63% silica
- dacite consists of about 63-68% slica
- rhyolite consists of more than 68% silica
Other major elements in varying proportion include titanium (TiO2),
aluminum (A2O3), iron (FeO or Fe2O3),
manganese (MnO), magnesium (MgO), calcium (CaO), sodium (Na2O),
potassium (K2O, and phosphorous (P2O5). The
bar graph shows the average concentration of each major element for the four
basic types of volcanic rock.
Classification and Flow Characteristics
of Volcanic Rocks
Illustration by J.
Johnson
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The behavior of a lava flow depends primarily on its viscosity
(resistance to flow), slope of the ground over which it travels,
and the rate of lava eruption. Because basalt contains the least
amount of silica and erupts at the highest temperature compared to
the other types of lava, it has the lowest viscosity (the least
resistance to flow). Thus, basalt lava moves over the ground easily,
even down gentle slopes. Dacite and rhyolite lava, however, tend to pile up
around a vent to form short, stubby flows or mound-shaped domes.
Components of Igneous Rocks
Illustration by J.
Johnson
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When molten rock erupts onto the Earth's surface, it cools
quickly, freezing the growth of existing minerals and preventing
the development of new minerals. Such rapid cooling will
typically produce lava rocks with a few small minerals suspended
in a groundmass of volcanic glass. Molten rock that remains
below the ground, however, cools very slowly so that existing
minerals continue to grow and many new minerals develop. A
slow rate of cooling will produce a coarse-grained plutonic rock that consists entirely
of large crystals. Different names are given to such
slow-cooling plutonic rocks on the basis of chemical composition and mineral
proportions (for example, plutonic rocks of basaltic composition are called
gabbro).
This graph shows the volume percent of minerals present in a plutonic
rock that consists entirely of crystals. For example, a granite with 70% SiO2
might have 22% quartz, 38% alkali feldspar, 28% plagioclase feldspar, and
12% biotite.
The volume percent of minerals present in volcanic rocks typically varies
0-50%. To calculate the relative amounts of the crystals likely to be
present, multiply the volume percent in the graph by the actual volume
percent in the rock. For example, a rhyolite volcanic rock with 10%
crystals, is likely to 2.2% quartz, 3.8% alkali feldspar, 2.8% plagioclase,
and 1.2% biotite.
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