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Unformatted text preview: Igneous Rocks Igneous rock comes from cooled magma and lava. Intrusive Versus Extrusive Igneous Rocks Igneous
• Intrusive igneous rocks form when Intrusive magma cools within existing rocks in Earth’s crust. Earth’s Extrusive igneous rocks form when Extrusive magma cools on Earth’s surface, where they have been “extruded.” they • Textural classification of igneous rocks igneous
Phaneritic: crystals visible with naked eye Phaneritic: crystals Plutonic or intrusive rocks Plutonic or Aphanitic: crystal too small for naked eye Aphanitic: crystal Volcanic or extrusive rocks Volcanic or Porphyritic: two different, dominant crystal sizes two
Large xtals = phenocrysts; small xtals = groundmass Large phenocrysts small groundmass Fragmental: composed of disagregated igneous material Fragmental: composed Pyroclastic rocks Textural classification of igneous rocks igneous
Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled slowly Pegmatitic: Forms veins or layers within plutonic body Glassy: non-crystalline; cools very fast (e.g., obsidian) Glassy: non-crystalline; Volcanic rocks Vesicular: vesicles (holes, pores, cavities) form as gases expand vesicles Volcanic rocks Volcanic Pyroclasts, Tephra, And Tuffs (3) Pyroclasts,
q Pyroclastic rocks are transitional between igneous and Pyroclastic sedimentary rocks. sedimentary • Pyroclastic debris represents hot volcanic fragments blown out of a vent with explosive force. Pyroclastic materials are subdivided on the basis of fragment size:
Ash - Fine (<2 mm) particles that may settle at considerable distance around the Ash vent. Volcanic dust can remain airborne for long periods. Lapilli - 2-64 mm pieces of lava; basaltic fragments are commonly called cinder. Lapilli cinder Blocks - >64 mm angular pieces that were solid when ejected. Blocks Bombs - >64 mm rounded masses that were molten when ejected; the term pumice describes frothy pieces of rhyolite glass that can float on water. pumice Pyroclasts, Tephra, And Tuffs (3) Pyroclasts,
Accumulations of these fragmental materials form several types of deposits that include: types
Tephra - a general term for pyroclastic debris that accumulates through Tephra vertical airfall. vertical Pyroclastic flows - avalanches of incandescent ash and gas (nuees ardentes). Lahars - hot mudflows formed when water mixes with hot pyroclastic debris. When bomb-sized tephra are transformed into a rock they are When called agglomerates. agglomerates They are called tuffs when particles are either lapilli or ash. They tuffs Mineral Assemblage In Igneous Rocks Rocks
Once the texture of an igneous rock is Once determined, its name will depend on its mineral assemblage. All common igneous rocks consist largely of: largely
x x x x x x Quartz Quartz Feldspar (both potassium feldspar and Feldspar plagioclase) plagioclase) Mica (both muscovite and biotite) Amphibole Pyroxene Olivine Color Color
The overall lightness or darkness of a rock is The a valuable indicator of its makeup. valuable
Light-colored (Felsic) minerals are: Quartz. Quartz. Feldspar. Muscovite. Muscovite.
Dark-colored (Mafic) minerals are: minerals Biotite. Biotite. Amphibole. Compositional terms for igneous rocks igneous
Felsic: feldspar + silica Felsic: feldspar silica ~55-70% silica, K-feldspar > 1/3 of feldspars present ~55-70% light-colored silicate minerals — Continental crust light-colored Intermediate: between felsic and mafic ~55-65% silica, plag > 2/3 of feldspars present ~55-65% Na-rich plag predominates over Ca-rich plag Na-rich Mafic: magnesium + ferric iron Mafic: magnesium fe ~45-50% silica; Ca-rich plag dominant feldspar ~45-50% dark silicate minerals — Oceanic crust dark Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars Ultramafic: >90% Mantle-derived Mantle-derived Composition of Igneous Rocks Composition Obisidian Scoria Scoria Figure 4.7 A Granite Rhyolite Gabbro Figure 4.7 C Basalt Pumice Pumice Andesite Andesite END END Lab 2 : Igneous Rocks
# Rock Name Granite Basalt Diorite Gabbro Obsidian Porphyritic Andesite Pumice Texture Phaneritic Aphanitic Phaneritic Phaneritic Glassy Porphyritic Color whitish, light dark salt & pepper dark gray-dark lightmediu m lightmediu m light dark light-gray Compositio n felsic mafic intermediate mafic felsic-mafic intermediate Minerals qtz, k-spar, mica, etc amphibole, olivine, plag, pyroxene qtz, plag, k-spar, amphibole, biotite plag, olivine, pyroxene, amphibole micas, qtz, k-spar, plag, olivine, etc qtz, k-spar, plag, olivine, biotite micas, qtz, k-spar, plag Cooling History slow fast slow slow very fast, "quenched" slow then rapid Extrusive or Intrusive intrusive extrusive intrusive intrusive extrusive intrusive then extrusive extrusive 1 2 3 4 5 6 7 Vesicular felsicinterm ediate felsic mafic felsicinterm ediate rapid, gaseous lava rapid rapid rapid 8 9 Rhyolite Scoria Tuff (volcanic ash) Aphanitic Vesicular Pyroclastic micas, qtz, k-spar, plag plag, olivine, pyroxene, amphibole qz, micas, k-spar, plag extrusive extrusive extrusive 1 ...
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This note was uploaded on 02/02/2011 for the course GEOL 101 taught by Professor Knapp during the Fall '08 term at South Carolina.
- Fall '08