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These notes go on pp. 7 & 9 of your Igneous rock INB!!! Igneous rocks are formed by the crystallization of molten rock (lava or magma). Classification 1. Mineral Composition 2. Texture of Grain Size Mineral Composition Felsic: high amounts of silica & light in color Felsic Rocks and Minerals Mafic: low amounts of silica & dark in color Mafic Rocks Silica Composition Felsic rocks - more than 65% silica content Intermediate rocks - 50-65% silica content (also called andesitic) Mafic rocks - less than 40-50% silica Pure Silica Texture of Grain Size 1. Igneous rocks formed above ground from lava are called “Extrusive” or “Volcanic” rocks. Obsidian Rhyolite cools rapidly and crystals have a limited time to form; as a result there are small or no mineral crystals Resulting Textures: 1. Glassy 2. Porous/Vesicular 3. Fine-Grained Obsidian-Glassy Felsic Scoria-Vesicular Mafic 2. Igneous rocks formed underground from Magma are called Plutonic or Intrusive rocks. Notice the large crystals cools slowly and large crystals have time to form. Resulting Textures 1. Coarse Grained Granite- Coarse-grained Felsic Gabbro- Coarse-grained Mafic Igneous rocks that begin to cool underground and then are erupted to the surface have both fine and coarse grains. These rocks have undergone two stages of cooling, therefore they have two sizes of crystals. These rocks have porphyritic texture. Basalt Porphyry- Mafic Rhyolite Porphyry-Felsic These notes go on p. 9 of your INB!!! Intrusive Igneous Rock formations: Dikes Dike: Sheet-like Igneous rocks that cut across the structure of pre-existing rock. Dike Sills Sills: Sheet-like Igneous rock that lies parallel to the layering of pre-existing rock. Sill Laccoliths Laccoliths: vertically developed sills. Batholiths Batholiths: a large Plutonic mass of Igneous Rock. Usually makes up the core of volcanic mountains Halfdome- Yosemite, California Enchanted Rock-Texas