Download Igneous rocks guide

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Rock Types:
Igneous
Intrusive/plutonic
o Coarse grained or phaneritic texture
▪ Granite – coarse-grained, pink to reddish in color; orthoclase feldspar (pink),
quartz (white to gray), albite feldspar (white), white mica (muscovite) or black
mica (biotite).
▪ Diorite – coarse-grained, with mineral (and color) composition between granite
and gabbro; plagioclase feldspar, mica, amphibole
▪ Gabbro – dark, coarse-grained; plagioclase feldspar, augite, olivine,
orthopyroxene.
Extrusive/volcanic
o Glassy texture
▪ Obsidian – a volcanic glass without gas bubbles. Same mineral content as
gabbro.
▪ Pumice – volcanic glass filled with gas bubbles (vesicles). Same mineral content
as diorite.
o Fine-grained or aphanitic texture
▪ Rhyolite – high-silica, fine-grained rock. Same mineral content as granite.
▪ Andesite – fine-grained midway in color and mineral composition between
rhyolite and basalt. Same mineral content as diorite.
▪ Basalt – fine-grained rich in iron. If contains a large number of gas bubbles then
it is called vesicular basalt or scoria. Same mineral content as gabbro.
dark
colors
intermediate
colors
pastel
colors
very fast cooling,
extrusive
“volcanic”
rocks
erupted at
surface
cooled fast
basalt
covers ocean floor
andesite
Andes mountains
rhyolite
ignimbrite
eruptions
fine grained
(aphanitic)
intrusive
“plutonic”
rocks
cooled beneath
surface
cooled slowly
gabbro
coarse grains
diorite
white granite
glass
obsidian with no
gas
pumice with gas
porphyry has:
large-grained
crystals,
dispersed in a finegrained
groundmass.
granite
often contains pink
feldspar
coarse
grained
(phaneritic)
chart prepared by Steve Tyminski (10/4/07)
pegmatite has:
very large crystals
of quartz,
feldspar and mica