Download AN EASY GUIDE TO THE COMMON IGNEOUS ROCKS

AN EASY GUIDE TO THE COMMON IGNEOUS ROCKS
MAGMA TYPE
SILICA CONTENT
VOLCANIC ROCK(S)
PLUTONIC ROCK
COMMON MINERALS
ACCESSORY MINS.
Felsic
> 65%
Rhyolite
GRANITE
Sanidine (volc.)
Orthoclase (plut.)
Microcline (plut.)
Na-rich plagioclase
Biotite and/or hornblende
Quartz
Zircon
magnetite
muscovite
Intermediate
55-65%
ANDESITE
Diorite
Na-Ca plagioclase
pyroxene and/or
hornblende
NO QUARTZ
biotite
magnetite
ilmenite
(FeTiO3)
Mafic
45-55%
BASALT
Gabbro
Olivine
Pyroxene
Ca-rich plagioclase
magnetite
ilmenite
(FeTiO3)
Ultramafic
< 45%
Komatiite
(rare)
Dunite ---------->
Peridotite ------>
Olivine
Olivine + Pyroxene
Kimberlite
General rules that will help put order in the system:
magnetite
chromite
ilmenite
olivine, pyroxene, biotite, ilmenite,
serpentine, diamonds
(rocks in bold will be seen in the lab!)
Silica content and mineral content:
1. Igneous rocks low in silica will consist mostly of minerals at or near the top of Bowen's Reaction Series.
2. Igneous rocks high in silica will consist mostly of minerals near the bottom of Bowen's Reaction Series.
3. Intermediate igneous rocks will consist of minerals in the middle of Bowen's Reaction Series.
Crystal size (texture) and origins:
1. Igneous rocks with a coarse-grained texture (generally, all or almost all grains at least sand-sized, over 1 mm in diameter) are going to be plutonic
in origin - having crystallized slowly, underground.
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; this occurs in both plutonic and volcanic rocks. The grain size in the groundmass indicates whether the rock as a
whole had a volcanic or plutonic origin.
(See reverse for more details on textures.)
TEXTURES IN IGNEOUS ROCKS
Aphanitic
Fine-Grained
Porphyritic
Pyroclastic
Coarse-Grained
Glassy textures in volcanic rocks 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 rock surface may be smooth, but still 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 0.1-0.5 mm across, the size of fine to medium sand, and sometimes even smaller).
Porphyritic textures may be the most common volcanic texture, especially for volcanic rocks on the continents, and is surprisingly
common in plutonic rocks as well. 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 deep underground, then became entrapped
in the fine-grained groundmass that formed either as the magma was intruded into cooler country rocks, or erupted from a volcano,
and cooled more quickly. Phenocrysts are commonly minerals higher in Bowen's Reaction Series than dominant groundmass
minerals, and in volcanic rocks may even include minerals such as hornblende or even biotite, which may not occur, or be much less
common, in the groundmass. Plagioclase feldspar phenocrysts will commonly be more calcium-rich than the plagioclase feldspars
in the groundmass.
Pyroclastic textures in volcanic rocks 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.
Coarse-Grained textures occur only in plutonic rocks, and result from slow, gradual cooling. Individual crystals are commonly >2mm
in diameter, commonly much larger. Note that all grains are intergrown in the texture.