Igneous Rocks
Igneous Rocks
source http://www.columbia.edu/~vjd1/igneous.htmhttps://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/
http://keywordsuggest.org/gallery/1116670.html
https://www.uwgb.edu/dutchs/EarthSC202Notes/igneous.htm
https://www.usgs.gov
Basic features of
Igneous Rocks
Bowens reaction series
Types of intrusive igneous rock
environments
Types of extrusive igneous rock
environments
Some common igneous rocks and
descriptions
Igneous rocks named on textural
criteria
Common igneous textures
see
also E-Learning - Igneous Rocks...
see
also Processes - Volcanic Processes...
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also Processes - Glossary of Volcanic Terms...
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also Petrology - Igneous Rock Classification...
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also Petrology - Origins of Magma and Magmatic Differentiation...
animation of igneous processes
Basic Features of Igneous Rocks
Of the three principal rock types (igneous, sedimentary, and metamorphic), igneous rocks can be thought of as "primary" rocks because they crystallise from a liquid.Sedimentary and metamorphic rocks may be thought of as derivative rocks.
Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into three - categories.
- Intrusive or plutonic rocks crystallise from magma beneath the earth's surface.
- Hypabysal rocks - crystalise from magma at shallow depth below the surface
- Extrusive or volcanic rocks crystallise from lava at the earth's surface.
- slow cooling forms large crystals - magmas and their
resultant plutonic rock bodies cool and crystallise slowly
and are characterized by coarse-grained texture in which the mineral crystals are visible to the unaided eye - fast cooling forms small crystals -lavas cool quickly at the
earth's surface
and are characterized by fine-grained texture, in which the crystals are too small to be seen by the unaided eye. - very quickly cooled - lavas, typically those quenched
in water, will have a glassy texture. They cool too quickly to
form crystals.
Glasses do not have an orderly arrangement of atoms and there are therefore no minerals, in the strict sense, in them. Volcanic glass is called obsidian
In addition to texture, igneous rocks may are classified according to their chemical composition.
The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic( magnesium and ferrum or iron) minerals.
- felsic minerals (quartz, K feldspar, etc) are light colored while - felsic minerals have the lowest melting points (600 to 750 °C)
- mafic minerals (hornblende, pyroxenes) are normally dark colored - mafic minerals have higher melting points (1000 to 1200 °C).
Identification by crystal size, felsic / mafic composition and colour
A more detailed identification chart based on felsic / mafic mineral composition
Bowen's Reaction Series
can be thought of as an idealized order of crystallization of a cooling magma.However, not all of these minerals will be crystallised together in the same rock.
- mafic magmas will begin crystallizing olivine and continue
with pyroxenes and calcium rich plagioclase feldspar.
Some amphiboles may also crystallise before the melt is used up. Mafic melts don’t have enough silica to crystallise potassium feldspar, quartz, etc.
Minerals form in the cooling magama in a "discontinuous series" , that is first one mineral --> then a different mineral --> then a different mineral and so on... - felsic magmas don't have enough iron, magnesium, and
calcium to form olivine, pyroxene, or calcium plagioclase.
The first-formed minerals in a felsic melt are amphiboles (hornblende) or biotite mica, along with some intermediate or sodium plagioclase.
Eventually, as the melt continues to cool and becomes richer in silica (as the metal cations are used up preferentially in the double chain and sheet silicates) potassium feldspar and quartz crystallise.
minerals form in a " continuous series" that is to say ... for example: that the mineral feldspar remains as feldspar but its composition (the percent of potassium vs calcium) increases as the magma cools
This system appears straight forward but as magmas cool their chemical composition changes as higher temperature elements drop out of the solution as minerals.
So for example a mafic magma can turn into a felsic magma as it cools...
Types of Intrusive Igneous Rock Environments
Types of Extrusive Igneous Rock Environments
Some common igneous rocks and descriptions
Intrusive Igneous RocksExtrusive Igneous Rocks
Igneous rocks named on textural criteria:
click on the links below to see an photo and a description of theses rocks...- Scoria: Porous
- Pumice
- Obsidian - Glass
- Tuff - Cemented Ash
- Breccia - Cemented Fragments
- Pegmatite - Extremely Large Crystals
- Aplite - Sugary Texture, Quartz & Feldspar
- Porphyry - Fine Matrix, Large Crystals
Common igneous textures:
Allotriomorphic-granular (gabbroic) texture:
is a feature of phaneritic rocks in which all of the minerals have anhedral shapes; it is characteristic of some mafic and ultramafic rocks, such as dunite and pyroxenite.
Hypidiomorphic-granular (granitic) texture:
igneous texture in which most of the mineral grains are subhedral. Typical of granite, granodiorite, quartz monzonite, etc.
Porphyritic texture:
rocks in which larger grains are contained in a finer grained matrix. May occur in either intrusive or extrusive rocks, but it is most common in extrusive rocks such as basalt, andesite, dacite, and rhyolite.
Pyroclastic texture:
produced by explosive volcanism, broken clasts of mineral, lithic clasts, and glass shards are common. The clasts may be either sorted or unsorted. Common in tuffs.
Intergranular texture:
plagioclase laths with interstitial pyroxene grains that are smaller than the plagioclase; commonly found in basalts.
Intersertal texture:
Small feldspars with glass or altered glass interstitial to the feldspars; common texture of basalts.
Ophitic texture:
Pyroxene grains partially or completely surround plagioclase laths; common texture of gabbros and basalts.
Trachytic texture:
Subparallel feldspars formed during flow in volcanic rocks.
Spinifex texture:
Interlacing olivine or pyroxene, a texture formed by quenching in komatiites.
Other features seen are:
- Exsolution - occurs within grains on certain minerals (pyroxenes and feldspars). This can give an indication of pressure and hence depth.
- Xenoliths - bits of the rock into which the magma intruded
- Cumulate layer - when a mineral grows which is denser than the magma, it will sink to the base of the chamber causing a cumulate layer. Minerals may form from liquid trapped between the grains - interstitial minerals.
