Igneous Rocks
Igneous rocks can form from a variety of materials. Some
form from volcanic ash and others form directly from magma.
Igneous rocks formed from underground magma are called
intrusive igneous rocks and those formed at Earth’s
surface are called extrusive igneous rocks.
The Starting Material
Magma can be classified as felsic, mafic, or an intermediate
form.
Felsic Magma
Mafic Magma
 thick and slow-moving
 mafic magma is hotter,
thinner, and more fluid
 contains large amounts
then felsic
of silica (SiO2)
 contains large amounts
 contain smaller amounts
of iron and magnesium
of the elements calcium,
iron, and magnesium
 contains lower amounts
of silica
 typically hardens into
rocks of light-coloured
 usually contain large
silicate minerals such as
amounts of dark silicate
quartz and orthoclase
minerals such as
feldspar
hornblende, augite, and
biotite
Underground Magma
Magma trapped deep in the Earth’s crust hardens very
slowly to form intrusive igneous rocks. Evidence suggests
that massive bodies of intrusive rock may take thousands of
years to cool underground. Intrusive rocks appear at Earth’s
surface when they are uplifted and the overlying rock is worn
away.
Intrusive rocks have a coarse texture because the magma
from which they form cooled very slowly. Slowly cooling
magma remains liquid for a long time, and its atoms move
quickly and freely. The longer the magma stays liquid, the
longer the atoms are free to move and the larger the crystals
become.
At the Surface
When magma pours from a volcano we call it lava. The
lava, when cooled and hardened, is known as volcanic rock
or extrusive igneous rock. Extrusive rock hardens rapidly,
sometimes within a few hours, days, or years in the case of
large lava flows.
The particles within rapidly cooling magma have little time to
move around. Crystals have such a short time to form that
extrusive rocks may have only microscopic crystals or no
crystals at all.
Sometimes underground magma begins cooling slowly, but
then it is suddenly forced to the surface. During the period
of slow cooling, large crystals form. However, once the
partially crystallized magma arrives at the surface, the
remaining liquid cools quickly. Microscopic crystals form
around the larger, original crystals. The result is porphyry.
Finally, magma may also reach Earth’s surface when
expelled forcefully into the air as ash. The ash settles onto
Earth’s surface and, if present in large enough amounts,
may eventually be buried and compressed into a rock called
tuff.
Igneous Rock Descriptions
Igneous rock is grouped into families according to mineral
composition.
Granite Family
Rocks in the granite family form from felsic magmas and are
intrusive igneous rocks. These rocks are usually coarsegrained. Members of this family typically contain quartz,
feldspar (orthoclase, plagioclase, or both), mica, and
hornblende.
Granite, for which this family is named, is one of the
coarsest-grained rocks in the family. Granite usually ranges
from white or gray to pink, depending on which colour of
feldspar is present. Granite is a very common continental
igneous rock found in many mountainous areas.
Obsidian, a volcanic rock with a glassy texture, is moderately
hard and brittle, with conchoidal fracture. Obsidian’s
chemical composition resembles that of granite and other
light-coloured rocks, so it is considered a member of the
granite family. Obsidian is usually dark brown or black due
to tiny amounts of dark-coloured iron oxides.
Pumice is formed from silica-rich lava that hardened as
steam and other gases bubbled out of it. It resembles a
sponge because of its many holes and air pockets. It is
often light enough to float on water.
The granite family also includes felsite (the general name for
any light-coloured, fine grained rock) and rhyolite (a finegrained rock that ranges from light gray to pink).
Gabbro Family
The gabbro family consists of mafic rocks. They are dark in
colour and denser than rocks in the granite family.
Pyroxene, olivine, and plagioclase feldspar, are the most
plentiful minerals in a gabbro rock. Other minerals found in
gabbros are amphibole and biotite. Gabbro, the rock, is a
coarse-grained rock and is very dark in colour.
The most common rock in the gabbro family is basalt. It is
similar to gabbro, but is fine-grained. Basalt is typically dark
gray or black and is the igneous rock that makes up the
ocean floor. On land, it is most commonly formed from lava
flows.
Other members of this family include diabase, basalt glass,
and scoria
Diorite Family
Members of the diorite family have an intermediate
composition that is neither felsic nor mafic, but has
characteristics of both. They tend to be medium grays and
greens (darker than granites and lighter than gabbros).
Diorite, a coarse-grained rock, has less quartz than granite
and less plagioclase feldspar than gabbro. Andesite is a
fine-grained member of the diorite family.