Lan Final 3

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Grain sorting-

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degree of uniformity of grain size

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igneous rocks

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forms as molten rock cools and solidifies
magma is the parent material of igneous rocks, magma at the surface is called lava
Cooling of magma results in the systematic arrangement of ions into orderly patterns.

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texture

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describes the overall appearance of a rock based on the size, shape, and arrangement of the interlocking materials

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Slow cooling-
Fast cooling-
Very fast cooling-

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Slow cooling- fewer and larger crystals
Fast cooling- many small crystals
Very fast cooling- glass

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Factors affecting crystal size-

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% of silica present, dissolved gases

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Ultramifc rocks are dominated by

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olivine and or pyroxene

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Aphanatic-

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fine grained textured, rapid cooling, microscopic crystals, may contain vasicles (holes from gas bubbles)

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phaneric

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coarse grained texture- slow cooling, large, visible crystals

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porphyritic

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- minerals form at different temp, large crystals ae embedded in a matrix of smaller crystals

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glassy texture

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very rapid cooling, results in obsidian

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pyroclastic texture

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fragmental appearance produced by violent volcanic eruptions, often look like sedimentary rocks

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pegmatitic texture

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very coarse grained, form in late stages of crystallization of granitic magmas

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once texture is determined,. igneous rocks are named based on

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mineral assemblage

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dark (ferromagnesian) silicates-

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olivine,
pyroxene,
amphibole,
biotite mica

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light (nonferromagnesian) silicates-

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quartz,
muscovite mice,
feldspars

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intermediate composition-

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contains 25% or more dark silicate materials

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light colored rocks are composed of

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quartz, feldspar, muscovite

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dark colored rocks are composed of

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biotite, amphibole, pyroxene

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mafic rocks are dominated by

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plagioclase and pyroxene (even if you cant see them with the naked eye) and smaller amounts of olivine

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felsic (or granitic or sialic rocks)

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 Dominated by silicon and aluminum, usually lighter in color, forms a stiff lava or magma, 0-15% mafic, minerals commonly in there (k-feldspar, plagioclase, quartz, biotite)

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intermediate rocks (long def)

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• Include andesite (aphanitic and diorite (phanertic), minerals commonly in there (plagioclase, amphibole, pyroxene, biotite, quartz, composed of 16-45% mafic materials

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mafic (or balsaltic)

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contains abundant ferromagnesian materials (magnesium and iron silicates), usually darker in color (dark gray to black), characteristic of earth’s oceanic crust, forms a runny lava, minerals commonly in there- plagioclase, pyroxene, olivine, amphibole, 46-8% mafic minerals

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ultramafic

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 Almost entirely magnesium and iron siliates (ferromagnesian minerals)
 Rarely on earths surface, balsaltic lavas, minerals in there are olivine and pyroxene, 86-100% mafic content