|Athro, Limited Earth Science Rock Types 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.
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.
|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.