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Geology 151j: Volcanic Rocks
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AN EASY GUIDE TO THE COMMON VOLCANIC ROCKS
MAGMA TYPE
SILICA CONTENT
(Variable)
(variable)
(most commonly intermediate to felsic)
VOLCANIC ROCK(S)
COLOR
COMMON MINERALS
ACCESSORY MINERALS
Obsidian
Pumice
black, brown no minerals - all or mostly glass; massive - looks like glass (it IS!)
light gray
no minerals - frothy glass, very light; often floats on water
Felsic
> 70%
Rhyolite
pink to
light gray
Intermediate
65-70%
Dacite
dark pink to Sanidine, Na-plagioclase (white)
medium gray pyroxenes (greenish black), quartz
magnetite (Fe3O4)
55-65%
Andesite
medium to
dark gray
biotite
Mafic
45-55%
Sanidine (K-feldspar)(white)
Na-rich plagioclase (white to pale gray)
Biotite and/or hornblende (black)
Quartz (clear - will look gray)
Ca-plagioclase phenocrysts (gray)
Na-plagioclase in groundmass
olivine (green), pyroxene (greenish
black) and/or hornblende (black)
NO quartz
Basalt
very dark
Olivine (bright green)
Scoria
gray to black Pyroxene (greenish black)
(spongy appearance)
Ca-rich plagioclase (dark gray)
zircon
magnetite
ilmenite (FeTiO3)
magnetite
ilmenite
General rules that will help put order in the system:
Silica content and mineral content:
1. Volcanic rocks low in silica will consist mostly of minerals at or near the top of Bowen's Reaction Series.
2. Volcanic rocks high in silica will consist mostly of minerals near the bottom of Bowen's Reaction Series.
3. Intermediate volcanic rocks will consist of minerals in the middle of Bowen's Reaction Series.
Crystal size (texture) and origins: (See reverse for illustrations and more detailed discussion.)
1. Igneous rocks with a coarse-grained texture (generally, all or almost all grains at least sand-sized) are
going to be plutonic in origin - having crystallized underground. Larger crystals imply more time to crystallize.
2. Igneous rocks with a fine-grained texture (grains generally tiny or too small to see) are volcanic in origin.
3. Igneous rocks with a porphyritic texture (large grains - phenocrysts - surrounded by a finer-grained
groundmass) cooled in a two-stage process, first slowly and then more rapidly. Occurs in both plutonic
and volcanic rocks; very common in volcanic rocks in particular.
Geology 151j: Volcanic Rocks
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TEXTURES IN VOLCANIC ROCKS
Aphanitic
Fine-Grained
Porphyritic
Pyroclastic
Glassy textures result from rapid cooling. There are few if any mineral grains; the rock is chiefly or
entirely comprised of natural volcanic glass, or obsidian. Most obsidian is felsic (high in silica), but
it also forms from other magma types as well. (For example, in Hawai'i, basaltic lavas flow into the
ocean and chill into basaltic glass.) Frequently, there are tiny phenocrysts, called microlites, in
glasses. Over time, these glasses are unstable and will recrystallize into aphanitic volcanic rocks.
Volcanic glass that is frothy as well, from trapped gas bubbles, is called pumice.
Aphanitic textures characterize volcanic rocks that have cooled rapidly but nonetheless have allowed
for mineral crystals to form. However, the individual crystals are too small to be seen or identified
with the naked eye. These can be differentiated from glassy textures by the fact that they are far less
"glassy" or shiny in appearance; the surface is dull.
Fine-Grained textures are very common in volcanic rocks, particularly basalts. In these, all the rock is
crystallized, but the individual grains are very small (typically in the range of 0.25-1.0 mm across,
the size of fine to medium sand, and commonly even smaller).
Porphyritic textures may be the most common volcanic texture, especially for volcanic rocks on the
continents. In these, large grains, called phenocrysts are embedded in a finer groundmass. Most
commonly, the phenocrysts are grains that began crystallizing while the magma was still
underground, then became entrapped in the fine-grained groundmass that formed following eruption
and rapid cooling. Phenocrysts are commonly minerals higher in Bowen's Reaction Series than
groundmass minerals, and may include minerals such as hornblende or biotite, which will not occur
in the groundmass. Plagioclase feldspar phenocrysts will commonly be more calcium-rich than the
plagioclase feldspars in the groundmass.
Pyroclastic textures result from the fusion of solid volcanic ejecta that have been welded into a solid,
coherent mass. These often include shards of glass and many irregular rock fragments, ash, etc.