Geoscience Reference
In-Depth Information
Classification of igneous rocks
The classification of igneous rocks is a vexed business that has kept igneous
petrologists amused, annoyed and employed for the best part of a century. The
discussion that follows is not rigorous but merely attempts to outline the argu-
ments for those who are not igneous petrologists.
The problem is that an igneous rock can be classified in many ways, each
being useful in some circumstances. Classification began in the obvious way
when geologists simply looked at igneous rocks and the minerals they contain.
One obvious difference is between coarse-grained plutonic rocks and fine-grained
or partly glassy volcanic rocks. Plutonic rocks are derived from magmas that
cooled slowly at depth in the interior of the Earth and therefore grew large
crystals. Volcanic lavas that erupted at the Earth's surface cooled quickly and
thus the rocks grew small crystals. Indeed, if a magma cools very rapidly, much
or all of the rock may be glassy. Of course, to confuse the issue, a magma
may carry to the surface some larger crystals that formed at depth (called
phenocrysts
).
In general, the colour of igneous rocks is a good guide to their chemical
composition, and it usually reflects the mineralogy of the rock (Table 9.1). Light-
coloured rocks, rich in silica and alumina, are
felsic
rocks. These include granite
(a plutonic rock) and rhyolite (the volcanic equivalent of granite). The detailed
nomenclature of the granitoid rocks is very complex:
granites
and
rhyolites
are
rich in quartz and alkali feldspar minerals; granodiorites (plutonic) and their
volcanic equivalents, dacites, are rich in plagioclase feldspar minerals and quartz
but have less alkali feldspar; tonalites are plutonic rocks that contain quartz and
plagioclase feldspar but very little alkali feldspar.
Igneous rocks that contain moderate quantities of dark minerals, such as pyrox-
ene and hornblende, are termed
intermediate
.Anexample is diorite, a plutonic
rock that contains plagioclase feldspar, is moderately rich in calcium, and has
some dark minerals such as hornblende, biotite and pyroxene. Its volcanic equiv-
alent is andesite, a rock that is typical of many volcanoes above subduction zones.
Igneous rocks that are rich in dark minerals are called
mafic
, shorthand for rich
in magnesium and iron (Fe). These rocks include gabbro, a plutonic rock con-
taining calcic plagioclase, pyroxene and often olivine or hornblende; its volcanic
equivalent is basalt.
The
ultramafic
igneous rocks (Fig. 8.10) are almost exclusively made of dark
minerals. They include peridotites, which are plutonic rocks rich in olivine,
with some pyroxene or hornblende. Dunite is a peridotite that is 90% or more
olivine. Harzburgite is mostly olivine, plus some orthopyroxene and perhaps a
little clinopyroxene; lherzolite contains olivine, orthopyroxene and clinopyrox-
ene. The mantle of the Earth is ultramafic. Ultramafic rocks have rarely erupted
as lavas in the relatively recent geological past, but they were more common in
Archaean strata. Ultramafic lavas are called komatiites; fresh komatiites have
small crystals of olivine and pyroxene set in a glassy matrix.
