IGNEOUS ROCK
CLASSIFICATION
Prepared by Dr. F. Clark,
Department of Earth and Atmospheric
Sciences, University of Alberta
Oct. 05
FORMAL CLASSIFICATION SCHEMES
Proper classification of igneous rocks requires that you
identify the minerals present, as well as their proportions
by volume percent of the whole rock, and recognize on
the basis of textures whether the rock is intrusive or
extrusive. There are various diagrams and tables that
will allow you to do this with great precision, but for this
presentation, we will use a much simplified version
wherein we apply commonly used intrusive and extrusive
rock names for igneous rocks of three, or perhaps four,
broad compositional types.
IGNEOUS ROCK COMPOSITION
The chemical composition, which determines what minerals
will crystallize, can be expressed as the sum of a number
of oxides of common cations. For igneous rocks, the
most important oxide is SiO2, or silica, most usually not
in the form of the mineral quartz. The silica content
ranges between approximately 40% and 70%. We notice
that the total content of magnesium and iron is generally
in inverse proportion to the silica content. Thus rocks
deficient in silica are rich in Mg+Fe, and are called mafic,
or even ultramafic, whereas rocks high in silica are
generally depleted in Mg+Fe, and are called felsic. Rocks
between these extremes are intermediate.
COMPOSIITON AND COLOUR
We observe that the colour of most igneous rocks is
controlled by the composition, which in turn of course
controls the mineralogy. Thus, mafic rocks, rich in
Mg+Fe, have a higher proportion of mafic minerals,
which are typically dark, whereas felsic rocks are low in
Mg+Fe, and are therefore typically much lighter in
colour, being dominated by quartz and feldspars. This
gives rise to the concept of Colour Index, which is the
volume percent of the rock that is accounted for by
mafic minerals (e.g. olivine, pyroxene, amphibole, and
biotite). As we shall demonstrate, there is not
necessarily a direct correspondence between Colour
Index and the colour we observe for a given rock.
INTRUSIVE IGNEOUS ROCKS
The coarse grain size produced by slow cooling of magmas
makes the task of mineral identification so much easier
for intrusive rocks. We will present images of ultramafic,
mafic, intermediate, and felsic igneous intrusive rocks, in
turn. As one might expect, the general trend will be for
the rocks to be progressively lighter as we move from
mafic to felsic.
DUNITE
This is an
ultramafic
(very low silica,
very high
Mg+Fe)
igneous
intrusive rock.
The bulk of these specimens consists of glassy green olivine crystals on
the order of 1mm or slightly finer. The few black grains [black arrows]
are of a pyroxene group mineral. Not even plagioclase feldspar is found
in these ultramafic rocks.
GABBRO
This is a mafic
igneous
intrusive rock.
Mafic igneous rocks typically have mixtures of pyroxene [black arrows,
as well as the yellow arrows highlighting a megacryst] and calcium-rich
plagioclase feldspar, which shows up in this specimen as dark green
crystals [blue arrows] that are nevertheless lighter than pyroxene.
DIORITE
This speckled
coarse-grained
specimen is an
intermediate
composition
igneous
intrusive rock.
With intermediate composition, the Mg+Fe content of the magma is
now tied up in the crystallization of the black minerals biotite [more so
in this specimen] and amphibole. The white mineral is plagioclase
feldspar, and there is minor clear, glassy quartz [purple arrows].
GRANITE –
This is a felsic intrusive rock.
The coarse grain size and variety of minerals makes granite an
attractive choice for building stone, and less happily, grave markers as
well. The most abundant mineral is yellowish potassium feldspar
[turquoise arrows], followed by grey glassy quartz [mauve arrows],
then black, relatively dull amphibole [no arrows], and finally shiny black
biotite [green arrows].
GRANITE –
yet again.
In comparison with the previous specimens, this example of granite has
pink potassium feldspar [turquoise arrows; note the well-developed
cleavage faces], much less quartz [mauve arrows], much more
amphibole, and more biotite [green arrows] as well. It might not
technically be a granite [too little quartz, using rigorous classification
schemes], but the name will do for our purposes.
GRANITE
This is a
sample of the
Kuskanax
Intrusion, in
the Canadian
Cordillera,
recovered from
Nakusp, BC.
Such boulders
are abundant.
This granite is much more fine-grained than the previous examples,
and at first glance is dominated by yellowish potassium feldspar.
However, there is substantial quartz content [mauve arrows], whose
presence is masked by the feldspar. The black grains are amphibole.
GRANITE
This very
coarse-grained
granite may be
a pegmatite,
depending on
the minimum
grain size
required to
apply such a
term.
Again we have potassium feldspar, mostly yellow, with some turquoise
grains of amazonite, a potassium feldspar variant [turquoise arrows].
Quartz [mauve arrows] shows up as typical grey, glassy grains without
any cleavage faces. The flaky black grains are biotite.
EXTRUSIVE IGNEOUS ROCKS
With the rapid cooling of lavas that occurs at the Earth’s
surface, much smaller crystals form, or in some cases,
no crystals form at all. This is a tremendous obstacle to
identification in hand specimen, and in practice one is
heavily dependent on the overall colour of the rock as a
guide to its composition, and thus probable mineralogy.
There may be useful clues in the rock that will point you
to the mineralogy, even if no specific minerals can be
identified with the tools at your disposal, and some of
these will be featured in the following images.
BASALT
This typically
dark, mafic to
ultramafic
volcanic rock is
apparently
featureless.
The presence
of olivine
suggests it is
ultramafic.
The dark colour suggests this is mafic, and we note the occasional
green glint from slightly larger olivine crystals [green arrows]. Olivine is
found only in ultramafic and mafic rocks, rarely with plagioclase but
frequently with pyroxene, which we therefore surmise is also present.
BASALT –
With or Without Useful Clues
As with the previous image, the sample on the left exhibits a few
slightly larger olivine crystals [green arrows], and probably has
pyroxene as the other mineral. In the right sample, the purple arrows
point to amygdules, which are minerals precipitated in once-empty
vesicles. Given that they may have formed well after the lava cooled,
and be unrelated to it, they are no indicator of the lava’s composition.
BASALT –
Clues in Vesicular Basalts
The scoria, or scoriaceous basalt, on the left has a few clues to its
chemistry. Many of the vesicles have a lining of pale pea-green material
[green arrows] often derived from the weathering of olivine, and the
iron content is confirmed by the reddish-brown oxidized (rusted)
weathered surface [red arrows]. The right sample has obvious olivine
crystals, perhaps phenocrysts, but no lining to the vesicles.
BASALT –
Pillow Basalt from a Mid-Ocean Ridge
Submarine eruption of basalt lavas leads to very rapid cooling of the
exterior surface, to produce a flexible skin that contains the lava.
Successive eruptions look like a stack of pillows, with rounded, convex
upper surfaces. On the left we see the fresh surface (broken and also
cut with a rock saw) with the yellow arrows pointing to the rapidly
weathered exposed surface, seen on the right.
COLUMNAR
BASALT
This outcrop is
the Giant’s
Causeway in
Ireland; the
Devil’s Post Pile
is a similar
exposure in
California.
The polygonal columns result from cooling, contraction, and cracking of
thin, widespread sheets of mafic rock. These occur either by subaerial
eruption of fluid basalt at rifts within continents, or by intrusion of
mafic magma as near-surface sills between layers of cooler sediments.
BASALT
This is a
porphyritic
basalt, whose
phenocrysts
tell us much
about the
composition.
The plagioclase phenocrysts [red arrows] suggest this is mafic, not
ultramafic. As such, the matrix between the phenocrysts is likely a
mixture of pyroxene and plagioclase, without any appreciable olivine.
TRACHYTE –
an Intermediate Volcanic Rock
The classic intermediate composition volcanic rock is andesite. These
samples, from the Crowsnest Pass, are alkali-rich (K, Na) intermediate
rocks, whereas true andesite would be higher in Ca. The black
phenocrysts are garnet [yellow arrows], with dodecahedral habit (the
mafic minerals in intermediate rocks are more typically amphibole or
pyroxene). The pink phenocrysts [blue arrows] are feldspar.
RHYOLITE
The lighter
colour is
indicative of
the felsic
composition of
this extrusive
rock, though
we cannot
distinguish the
minerals.
We expect, based on its felsic composition, that it would have quartz
and potassium feldspar, and perhaps some mafic mineral content. The
black spots [red arrows] are in fact not mafic minerals, but small blobs
of black volcanic glass, or obsidian, which itself is felsic.
RHYOLITE
This would
normally be
called pumice,
a term that
describes the
texture rather
than being a
proper rock
name used in
classification.
It represents a case in which gas-rich felsic lava has turned into a sort
of sticky froth that has eventually hardened. Flow of this taffy-like
material has produced many glassy strands spanning the vesicles [blue
arrows]. It is otherwise like the previous sample.
OBSIDIAN
This rock would
have erupted as a
rhyolite lava in the
form of pumice,
then collapsed.
Rocks have
minerals, which
are crystalline, yet
this is felsic
volcanic glass, and
is an exception to
the basis for
colour index.
There are no mafic minerals (or, technically, any other for that matter),
so this rock has a Colour Index of zero, even though it is completely
black on the fresh surface. The brown weathered surface is a clue to
the presence of iron, dispersed through the glass to make it black.