Chapter 5.2 – Classification of Igneous
Rocks
• Two main types of igneous rock:
– Intrusive rocks form when magma cools and
crystallizes below Earth’s surface. Crystals large
enough to see without magnification
– Extrusive rocks form when magma cools and
crystallizes on Earth’s surface (lava flows). Small
crystals that need magnification to see
Types of Classification
• Properties
– Grain size
– Texture
– Mineral composition
• Basaltic rocks – mafic rocks, dark-colored, low
silica contents, and contain mostly plagioclase
and pyroxene
• Granitic rocks – felsic rocks, light-colored,
have high silica content, and contain mostly
quartz and feldspar
Types of Classification
• Intermediate rocks – rocks that have mineral
compositions in between the two others
• Ultrabasic – ultramafic, contains rocks with
only iron-rich minerals and are always dark
• Texture refers to size, shape, and distribution
of crystals/grains that make up a rock
– Examples: fine-grained, coarse-grained
Other Properties
• When lava cools on the surface, it cools so
quickly that only small crystals form
– Sometimes crystals don’t form at all
– This results in volcanic glass called obsidian
Other Properties
• When lava cools slowly under the surface,
large crystals form – granite, diorite, gabbro
Porphyritic Rocks
• Porphyritic texture – large, well-formed
crystals surrounded by finer-grained crystals
of the same mineral or different minerals
– Complex cooling history (example: slow cooling
then rapid cooling) causes different crystal sizes
Vesicular Rocks
• Vesicular texture – Spongy appearance
formed when gas is prevented from escaping.
– The holes left behind are called vesicles
– Examples are pumice and vesicular basalt
Thin Sections
• A Thin Section is a slice of rock 2 x 4 cm and
only 0.03 mm thick
– Light is able to pass through this sample
– These samples are viewed through a petrographic
microscope and geologists look for certain
properties
– Examples of properties: feldspar grains have
banding called “twinning,” quartz grains look wavy
as stage is rotated, etc.
Igneous Rocks as Resources
• Useful in many fields such as
– Construction
– Energy production
– Jewelry making
• Ores can be extracted from veins of lava flows
– Gold, silver, lead, copper
– These elements released at end of magma
crystallization process
Igneous Rocks as Resources
• Pegmatites are igneous rocks made of
extremely large-grained materials
– Found as igneous intrusions or veins
• Rare elements such as Lithium, Beryllium form
in these ores
• They also produce complex crystals
Igneous Rocks as Resources
• Kimberlites are ultrabasic rocks that were first
found in Kimberly, South Africa
– Only formed under very high pressure
– Valuable minerals such as diamonds
• Most diamonds in the world are found in
South African mines. Only one diamond mine
in the US in Colorado
Igneous Rocks as Resources
• Igneous rocks are strong building materials
– Durable
– Weather-resistant
– Countertops, floors, statues, etc.