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 Igneous rocks 

Igneous rocks are classified on two axies: Composition and cooling rate. Compositions range from rocks rich in the framework silicates quartz and potasium feldspar, to those rich in minerals containing magnesium and iron (olivines and pyroxines).

In the simplest terms, Granite and Basalt are end members. Granite is a coarse grained (slowly cooled) rock rich in quartz, basalt is a fine grained (rapidly cooled) rock rich in magnesium and iron bearing minerals and poor in quartz.

Quartz/K-Feldspar Pyroxine/Plagioclase
Slow Cooling Granite Gabbro
Fast Cooling Rhyolite Basalt

We can add a little more detail: Some rocks can cool very slowly to produce crystals up to several meters in size - these are peridotites. Other rocks cool very rapidly in volcanic eruptions. The fastest cooling of these form obsidian, volcanic glass. Some rocks are even closer to the composition of the earth's mantle than basalts - these are rocks made up almost entirely of olivine and pyroxines. Peridotites are rare examples of this composition.

Quartz/K-Feldspar Pyroxine/Plagioclase Olivine/Pyroxene
Very Slow Pegmatites
Plutonic Granite Gabbro Peridotite
Volcanic Rhyolite Basalt
Very Fast Pumice, Obsidian

We could keep going further into detail, listing intrusive igneous rocks in a felsic to mafic series as Granite, Granodiorite, Diorite, Gabbro, Peridotite, and Dunite, and go further by specifing the mineral compositions that define each type, and so on. The key point, however, is that basalt (quartz poor) reflects the composition of the earth's mantle, and is the stuff that oceanic plates are made of, while granite (quartz rich) forms the bulk of the continental plates and is, in essence, the result of density fractionated melts rising from subduction zones.

Igneous Rock Builder

Pick: Cooling Rate: Composition:
This produces a:

Hand sample, 4 cm field of view.
Image of granite is of a real rock, others are artistic renditions ilustrating color and grain size.

Go back to the introduction to Sedimentary, Metamorphic, and Igneous Rocks.

Sources: Press and Siever, 1978
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Copyright © 2000 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris
Maintained by Athro Limited
Date Created: 2000 Dec 29
Last Updated: 2000 Dec 31