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Igneous Rocks
Chapter 3, section 2
Characteristics of Igneous Rock
Most igneous rocks are made of
mineral crystals
 Igneous rocks are classified
according to their origin, texture,
and mineral composition
Geologists classify igneous rocks
according to where they formed:
Extrusive rock formed from lava
that erupted onto Earth’s surface
Intrusive rock is igneous rock
that formed when magma cooled and
hardened beneath the Earth’s surface
Intrusive Rock
Intrusive rock forms below the surface of Earth
Image from
Extrusive Rock
Extrusive rock forms on top of the surface of Earth
Image from
Erosion exposes an Intrusive Rock
Image from
The texture of an igneous rock
depends on the size and shape of its
mineral crystals
Texture can be fine-grained,
course-grained, glassy, or porphyritic
*Rapid cooling lava forms
fine-grained rocks with small crystals
*Slow cooling magma forms
coarse-grained rock with large
Extrusive rock cools quickly on Earth’s
surface and the crystals grow for a short
time and are small
Intrusive rock cools slowly under Earth’s
surface and the crystals grow for a long
time and are large
Glassy rock may form if extrusive rock
cools very quickly
 Porphyritic texture is a mixed
texture that has large crystals
scattered on a background of much
smaller crystals
Porphyritic texture forms if an intrusive
rock cools in two stages. Some
magma cools slowly then the
surrounding magma cools quickly
Fine-grained basalt
Course-grained granite
Volcanic glass
Porphyritic texture
Mineral Composition
Silica is a material made of oxygen and
silicon that is found in most minerals.
Silica content affects the color of rocks
Lava that is low in silica tends to form
dark-colored basalt while magma that is
high in silica forms light-colored granite
Basalt image from
Obsidian image from
Granite image from
Porphyritic texture image from