Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Igneous Rock Classification Table

PEGMATITIC TEXTURE (EXTREMELY COARSE-GRAINED)
Originates from water-rich intrusions, which cool and crystallize underground

Overall Color Most Common Minerals Rock Name
light (felsic) Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite pegmatite

PHANERITIC TEXTURE (COARSE-GRAINED)
Originates in deep intrusions, which cool and crystallize slowly underground

Overall Color (composition) Most Common Minerals Rock Name
light (felsic) Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite granite
medium-light (felsic) Na-plagioclase, quartz, amphibole, biotite, orthoclase granodiorite
medium-dark (intermediate) Na-plagioclase, amphibole, pyroxene, biotite diorite
dark (mafic) Ca-plagioclase, pyroxene, olivine, amphibole gabbro
APHANITIC TEXTURE (FINE-GRAINED)
Originates in lava flows (or very shallow intrusions), which cool rapidly
Overall Color Most Common Minerals Rock Name
light (felsic) Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite rhyolite
medium-light (felsic) Na-plagioclase, quartz, amphibole, biotite, orthoclase dacite
medium-dark (intermediate) Na-plagioclase, amphibole, pyroxene, biotite andesite
dark (mafic) Ca-plagioclase, pyroxene, olivine, amphibole basalt

FROTHY TEXTURE (POROUS, PUMICEOUS)
Originates in gas-charged volcanic eruptions, commonly pyroclastic

Overall Color Most Common Minerals Rock Name
light (felsic) glass (may contain a few minerals common in felsic rocks, listed above) pumice
dark (red or black) (mafic) glass (may contain a few minerals common in mafic rocks, listed above) scoria
Note: basalt with a smaller number of holes, known as vesicles, is called vesicular basalt.
GLASSY TEXTURE
Originates from cooling too rapid to allow crystal lattices to form
Overall Color Most common Minerals Rock Name
black (white, red) (felsic) glass (no minerals) obsidian
FRAGMENTAL TEXTURE, COARSE-GRAINED (CONTAINS LARGE ROCK FRAGMENTS)
Originates from pyroclastic (explosive) eruptions
Overall Color Most common Minerals Rock Name
any (felsic, intermediate, or mafic) variable (depending on rock fragments and ash content) volcanic breccia
FRAGMENTAL TEXTURE, FINE-GRAINED (MAINLY VOLCANIC ASH)
Originates from pyroclastic (explosive) eruptions
Overall Color Most common Minerals Rock Name
light (felsic) may contain a few minerals common in felsic rocks, listed above rhyolitic tuff
medium (intermediate) may contain a few minerals common in intermediate rocks, listed above andesitic tuff
dark (mafic) may contain a few minerals common in mafic rocks, listed above basaltic tuff

Igneous rock is solidified magma. Plutonic igneous rock comes from magma that intrudes within the earth, deep underground, and cools and crystallizes there. Volcanic igneous rock originates from magma that erupts, as either lava or as tephra, and turns solid on the earth's surface.

There are two categories of volcanic rock: (1) the effusive (lava flow) type of volcanic rock, which flows onto the earth's surface as lava and turns solid there, and (2) the pyroclastic (explosive) type of volcanic rock, which forms from volcanic material that explodes into the air and lands in pieces on the ground.


Geology of the Pacific Northwest
Igneous Rock Classification Table
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 6/21/2020