I. Magma: The Parent Material
of Igneous Rock
Igneous rocks form as molten rock cools and
A.The Nature of Magma
3 distinct parts of the magma:
(1.)liquid component,(2.)solid component &
(3.) gaseous phase
B. From magma to crystalline rock
Crystallization- is the formation and growth of a
crystalline solid from a liquid or gas.
II. Igneous Textures
The size, shape, and mutual relationships of the
minerals in an igneous rock is called texture.
A. Factors Affecting Crystal Size
(1.) the rate at which magma cools
(2.) the amount of silica present
(3.) the amount of dissolved gases in the magma
B. Types of Igneous rock Textures
(1.) Basic Igneous Rock Textures
Phaneritic- this term applies to igneous rock in
which the constituent minerals are megascopic in
Aphanitic – this term is used to describe te texture of
an igneous rock composed entirely of microscopic
Glassy - few , no mineral grains are present and
the main material present is volcanic glass.
Porphyritic- this term applies to an igneous rock in
w/c larger grains(phenocrysts) are embedded in a
matrix of distinctly smaller grains(groundmass) which
are either phaneritic or aphanitic.
• Porphyro-phaneritic - both the large phenocrysts as
well as the smaller crystals in the groundmass are
large enough to see without magnification.
• Porphryo-aphanitic- if the phenocrysts are large
enough to see but the groundmass is
Pyroclastic- this terms applies to igneous rocks
composed of fragments of volcanic material ejected
during an eruption. This material may include dust,
ash, lapilli, bombs, and blocks, as well as individual
(2.) Special Igneous Rock Textures
vesicles are spherical or subspherical cavities
in igneous rock produced by gas which was
present in the original magma. Igneous rocks
containing vesicles are termed vesicular
Pegmatite texture- applied to rocks consisting of
exceptionally large mineral grains.
A granite pegmatite composed mainly of quartz and feldspar (salmon color).
The elongated, dark quartz crystal on the right is about the size of a person’s
Aplitic Texture- applies to granitic dike rocks
consisting of fine-grained, sugar-sized crystals.
IV. Naming Igneous rocks
A. Felsic (granitic) Igneous rock
(1.) Granite –perhaps the best known of all igneous
(2.)Rhyolite- extrusive equivalent of granite and lightcolored silicates. Aphanitic and contains glass
fragments and voids.
(3.) obsidian- dark-colored glassy rock that usually
forms when silica-rich lava is quenched quickly.
(4.) Pumice- volcanic rock that has a glassy texture.
• B. Intermediate (Andesitic) Igneous rocks
(1.) Andesite- medium gray fine-grained rock. Its name came
from South America’s Andes Mountains.
(2.) Diorite- plutonic equivalent of Andesite. Course-grained
intrusive rock that looks somewhat similar to gray granite.
• C. Mafic (Basaltic) Igneous rock
• (1.) Basalt- Very dark green to black fine grained
volcanic rock composed primarily of pyroxene and
calcium-rich plagioclase feldspar, with lesser
amounts of olivine and amphibole present.
(2.) Gabbro- intrusive equivalent of basalt.
D. Pyroclastic Rock- composed of fragments ejected
during a volcanic eruption.
Bowen’s Reaction Series- A concept proposed by N. L.
Bowen that illustrates the relationships between magma
and the minerals crystallizing from it during
the formation of igneous rocks.