3.2 Igneous Rocks
Text pp 70- 74
• Different kinds of igneous rocks form when
magma – or lava – cools and hardens.
Formation of Igneous Rocks
• When magma hardens beneath the Earth’s
surface, intrusive igneous rocks form.
This is because they intrude into the existing
rocks.
• The SLOW cooling of the magma allows larger
crystals to grow in intrusive rock, such as in
this sample of granite.
• Intrusive rock results when
magma solidifies under the
Earth’s surface. The magma
can force itself into the
cracks of existing rocks.
We see intrusive rock at
the earth’s surface
because of erosion or
weathering.
• Intrusive rock crystals do not have to be the
same size. A finely textured matrix may form
around larger crystals which cooled more
slowly.
Porphryitic andesite
• When lava hardens above the ground, the
igneous rocks that form are called extrusive
rocks. (They EXIT the Earth.)
• The fast cooling of the lava only allows small
crystals to grow in extrusive rock.
Basalt, fresh
broken surface.
• Extrusive igneous rocks form when lava
flows onto the surface of the earth or floor
of the ocean through deep cracks or
fissures and at volcanic vents, and then
cools and hardens.
Classification of Igneous Rocks
• Texture and Composition are two
characteristics used to classify igneous rocks.
Texture
• Texture describes the appearance of an
igneous rock based on the size, shape, and
the arrangement of its interlocking crystals.
Composition
• Composition classes of igneous rocks are
based on the proportions of light and dark
minerals in the rock.
Coarse-Grained Texture
• Magma, since it is beneath the surface, has a
chance to cool very slowly. This results in the
formation of large crystals. Igneous rocks
with large crystals exhibit a coarse-grained
texture.
Syenite igneous rock
Fine-Grained Texture
• Quick cooling of magma or lava results in
rocks with small, interconnected mineral
grains. Igneous rocks with small grains are
said to have a fine-grained texture.
Rhyolite
Glassy Texture
• When lava pours onto Earth’s surface, there
may not be enough time for crystals to even
form before the lava hardens. Such rocks
have a glassy texture.
Obsidian
Porphyritic Texture
• Especially deep in the Earth, a large body of
magma may not cool at the same rate
throughout. Some minerals cool much more
quickly than others.
The result is large
crystals surrounded
by a matrix of finegrained materials.
This is called a
porphyritic texture.
feldspar mineral labradorite
Granitic Composition
• Granites are made up of almost entirely of
the light-colored silicate minerals quartz and
feldspar. In addition, most granites contain
about 10% dark silicate minerals. Granites
are the major rocks of the continental crust.
Basaltic Composition
• Basalts contain many dark silicates and
metal minerals, such as magnesium and
iron. They tend to be darker and denser
than granite rocks.
• The most common basaltic is basalt, and
makes up most of the oceanic plates.
Andesitic Composition
• Rocks with a composition between granitic
and basaltic rocks have an andesitic
composition. They contain at least 25% dark
silicate minerals.
Andesite