Igneous Rocks Classifications
Igneous Rocks Classifications 20841
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This 3 page Class Notes was uploaded by Katie Thedford on Tuesday September 20, 2016. The Class Notes belongs to 20841 at Fort Lewis College taught by Allie Jackson in Fall 2016. Since its upload, it has received 9 views. For similar materials see Geol 113-2 in Geology at Fort Lewis College.
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Date Created: 09/20/16
Igneous Rocks Classifications Igneous Rocks o It is called magma when below the surface and lava above o The magma has a melting point of 700 degrees but can be melted at a lower temperature if introduced to water or pressure o Composed of different mixtures of silicate minerals Decompression Melting o Magma rises to the surface because it is less dense than the surrounding areas o Can be slow and flowing or exploding depending on gas content Crystallization o As magma cools molecular movements slow down ions arrange into complex structures o Magma does not all cool at once; magma with higher crystallization temperatures solidify first o When magma cools and crystalizes inside earth it’s called intrusive rock (or Plutonic) o When it cools outside earth’s surface it’s called extrusive rock (or Volcanic) Different Crystallization processes lead to different igneous rock textures o Phaneritic- Large enough crystals to see with naked eye Cooled over long period of time (intrusively) o Aphanitic Small crystals that can’t be seen with naked eye Cooled over very short period of time (extrusively) o Glassy Cooled immediately Not enough time to form a crystal lattice o Porphyritic Two different crystal sizes Two different cooling times o Vesicular Bubbles were trapped when it was cooled Causing dips in the surface of the rock o Pegmatite Very large Crystals Usually found in veins with a high fluid content o Pyroclastic rocks Contain fragments from other rocks from explosive eruptions (also called fragmental) 2 Classifications Felsic Rocks o Composed almost entirely of light-colored silicates o high silica content o low iron and magnesium content o (granite and Rhyolite) Mafic Rocks o Contain dark silicate minerals o Low silica content o High iron and magnesium content o (gabbro and basalt) Intermediate Rocks o Contain mixtures of mafic and felsic minerals o (except for olivine or muscovite, never them) o commonly Diorite or Andesite Ultramafic o Rare in near-surface environments o Peridotite or komatiite 3
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