Saturday, February 11, 2006

classification of igneous rocks

Classification of igneous rocks
Igneous Rocks have a two-dimensional classification scheme based on chemistry, grain size and texture.

The key to chemical classification in igneous rocks is the amount of Silica (SiO2) in the magma. (Of course people who study this make a much bigger deal out of it! If magmas don't have much silica, their minerals are dominated by magnesium and iron (Fe) - hence the term MAFIC (MA- from the magnesium and FIC from the Fe), or even ULTRAMAFIC for the really silica poor varieties. Silica rich magmas have a mineral named feldspar in them (see book) and are called FELSIC as a result. You will also see the words "acidic" and "basic" used for felsic and mafic respectively and you should be aware that this has nothing to do with pH! One can often tell about how much silica is in a rock just by its color. The more silica, the lighter the color.

grain size:
The main control of grain size is how fast the rock cooled from the molten state. Slow cooling allows bigger crystals to form, and fast cooling makes smaller crystals and even glass (no crystals). So the second dimension of igneous rock classification is whether the rock was formed by cooling on the surface as an extrusive rock. or in the crust as an intusive rock. Magma can either be erupted (extruded) as ash to make pyroclastic rock or as lava to make volcanic rocks.

Igneous textures are classified by the presence or absence of crystals, the size of the crystals, and the size and density of vesicles (holes). Check out this page for a nice summary of igneous textures.

Volcanic Rocks:
Volcanic rocks are mainly classified by the amount of silica. There are four main categories with increasing silica: basalt, andesite, dacite and rhyolite.

Intrusive Rocks
Intrusive rocks cool slower and have coarser grain sizes than their extrusive counterparts. The big four of intrusive rocks are with increasing silica: gabbro, diorite, granodiorite, and granite.

Increasing silica not only makes magmas lighter (in color), but makes them more viscous (stiffer), so the beautiful movies of flowing glowing lava flows from Hawaii are of basaltic lavas with little silica.

Melting temperature:
Increasing silica lowers the melting temperature, so that granites melt at about half the temperature that basalts do.

Adding water also lowers melting temperature and decreases viscosity. Also, water and other gases make bubbles in the magma, contributing to the explosive power of some eruptions and also leaving holes in the rocks (vesicles).
Melting: While we're on the subject of melting, it is very important to note that each mineral has a different melting temperature, so rocks do not melt all at once, but bit by bit. Thus most magmas have floating bits of crystals in them (phenocrysts


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