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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

Origin

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 http://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/volcanicforms.gif
Extrusive Rock
Extrusive rock forms on top of the surface of Earth
Image from http://photo2.si.edu/earthquakes/krafla500.jpg
Erosion exposes an Intrusive Rock
formation
Image fromhttp://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/Intrusive_volcanic_neck.jpg
Texture
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
crystals

Texture


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
Texture
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

Texture
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 http://www.es.ucl.ac.uk/schools/Glossary/basalt.jpg
Obsidian image from http://www.es.ucl.ac.uk/schools/Glossary/obsidian.jpg
Granite image from http://www.beg.utexas.edu/mainweb/publications/graphics/granite-400.jpg
Porphyritic texture image from http://www.ux1.eiu.edu/~cfjps/1300/p_rhyolite.jpg