The Igneous Rocks Page
All information on this page
was taken from the Geology 101 Laboratory Guide Fourth Edition - George
Mustoe
All pictures on this page
were taken from Earth, An Introduction to Physical Geology, Fifth Edition
- Edward J. Tarbuck and Frederick K. Lutgens
For individual credits for
the pictures or information, please consult these books
Before looking at each individual
rock, here are a few things you should know about Igneous rocks in general:
Kinds of Igneous Rocks
There are two kinds of Igneous
rocks, Extrusive and Intrusive. Extrusive rocks are formed outside
the Earth, (rapidly) either from a volcanic eruption or a fissure.
Intrusive rocks are formed inside the Earth, (slowly) either by a pluton
or inside the mantle.
Texture
Aphanitic: fine grained
(form extrusively)
Phaneritic: course grained
(form intrusively)
Pegmatic: very course grained
(larger than 2 cm)
- Pegmatic
rocks form intrusively, slowly with water
Vesicular: bubbles or air
pockets
Porphyritic: mixture of
aphanitic and phaneritic
Composition
Igneous rocks consist mainly of
Quartz, Feldspars, Olivine, Micas, Amphiboles and Pyroxenes
Color
Felsic: light color
Intermediate: medium color
Mafic: dark color
Ultra-Mafic: rocks that
are low in Silica, but very high in Iron and Magnesium
Intrusive Rocks
Granite
(Granite)
Contains mostly light minerals:
Quartz and Feldspar, with some Mica and Pyroxene
Diorite
Contains approximately equal amounts
of light and dark minerals: Quartz, Feldspar, Mica, Pyroxene and
Amphibole
Gabbro
Contains mostly dark minerals:
Amphibole and Pyroxene, with some Feldspar
Pegmatite
(Pegmatite)
A type of Granite that contains
very large crystals
Dunite
A green-colored Ultra-Mafic rock
composed mostly of Olivine.
Extrusive Rocks
Rhyolite
(Rhyolite)
Light colored
Andesite
(Andesite)
Grey colored
Basalt
Very dark grey or black
Obsidian
(Obsidian)
Glassy, black - Formed when the
lava cools extremely rapidly
Pumice
(Pumice)
A form of Obsidian that has a
foam-like texture caused by many tiny gas bubbles
Scoria
(Scoria)
Similar to Pumice, but the size
of the bubbles has increased - many people thing Scoria resembles Swiss
Cheese.
Tuff
(Tuff)
An accumulation of volcanic ash
particles - it is often difficult to tell the difference between Tuff and
a fine-grained Sedimentary rock without a microscope
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