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Classification of Igneous Rocks As is the case with most modern scientific classifications the one for igneous rocks is based upon the origin of the things being classified. Because igneous rocks crystallize from a molten state (a magma) the classification depends upon the consequences of that cooling. There are two aspects to the cooling, as we have seen: the temperature at which crystallization is achieved and the rate at which the cooling occurs. The temperature of crystallization dictates which minerals can and cannot be present – that is, the mineral composition of the rock. The cooling rate (along with other factors in some cases) dictates what the texture will be – that is, the sizes of the crystals present in the rock. The compositional side of the classification is derived from Bowen’s reaction series. The various minerals are stable at different temperatures and so certain ones tend to occur together commonly. Other combinations are rare and require special conditions to form. There is a useful shorthand version of the classification that you should familiarize yourself with, given here. Olivine ultramafic -- ~nothing but ferromagnesian minerals Pyroxene Amphibole Biotite Ca Plagioclase Mixed Plag. mafic – ferromagnesian mineral + feldspar intermediate – between mafic and felsic Na Plagioclase Muscovite & K Fsp Quartz felsic – little or no femag -- mostly feldspar The shorthand version of the classification us useful when we only want to consider crystallization temperature (which geologists often do, actually) but tells us nothing about the rate of cooling. The more expansive classification gives a name to each combination of composition and texture that occurs in any igneous rock. Some possible combinations don’t actually occur in nature and so there is no associated name for that possibility. This table includes most of the names of common igneous rocks. The rows include rocks with the same composition – mineralogic and/or chemical. The columns contain rocks with the same igneous texture. XXX indicates those rocks that do not actually occur. Ultramafic rocks are always phaneritic because they cool at too high a temperature to escape the crust as liquid. Glassy rocks only occur with felsic content because all the other magmas are too hot, and therefore too thin, to preclude crystallization. (Note that obsidian is not technically a rock because it is made of glass, not crystalline minerals. Chemically it is the same as the other rocks in its row, but it did not crystallize.) The emphasized rocks are the ones you will be responsible for knowing. These are far and away the most common rocks on Earth, each occurring in a different part of the planet. Comp./Texture: Phaneritic Aphanitic Porphyritic Ultramafic Peridotite XXX Mafic Gabbro Basalt Basalt Porphyry Basalt Tuff Scoria XXX Intermediate Diorite Andesite Andesite Porphyry Andesite Tuff Scoria XXX Felsic Granite Rhyolite Rhyolite Porphyry Rhyolite Tuff Pumice Obsidian XXX Pyroclastic Vesicular XXX XXX Glassy XXX We have only talked about 4 of these textures so those are all we’ll see from here on. Peridotite Gabbro Basalt Basalt Porphyry Scoria Diorite Andesite Andesite Porphyry Scoria Granite Rhyolite Rhyolite Porphyry Pumice Obsidian Basalt is the most common mafic rock Continental crust is felsic/granitic Oceanic crust is mafic/basaltic Granite is the most common felsic rock Moho Peridotite is the only ultramafic rock Mantle is ultramafic/ peridotitic
