105_04-igneous_part_2 - Igneous Igneous rock classification...

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Unformatted text preview: Igneous Igneous rock classification Names are based on their: Texture - extrusive/volcanic/aphanitic/fine-grained extrusive/volcanic/aphanitic/finevs. intrusive/plutonic/phaneritic/coarse-grained) intrusive/plutonic/phaneritic/coarseComposition (Amount of silica) - Silica tetrahedron (silicon & oxygen) Igneous Textures Texture: Texture: size, shape, and arrangement of mineral grains in a rock Igneous Textures Glassy Glassy - no crystals Cools Cools very, very fast fast Igneous Rocks Classifying Classifying Igneous Rocks – Pegmatites: large minerals (>5 cm) Aphanitic Aphanitic – Crystals barely visible – Extrusive, volcanic Phaneritic Phaneritic – Crystals easily visible – Intrusive, plutonic Pegmatitic Pegmatitic – > 5 cm crystals Cools Cools very slowly slow Mount Rushmore: Harney Peak Granite 1 Pegmatites Tourmaline: Tourmaline: large pegmatite crystals Igneous Textures Porphyritic Porphyritic – Large crystals with very fine matrix – Slow cooling followed by rapid cooling Igneous Textures Vesicular Vesicular – Magmas containing large amounts of gases – Gases trapped, forming small cavities in the rock Amygdaloidal Vesicles Vesicles are filled with secondary minerals 2 Igneous Rock Classification Plutonic Volcanic Igneous Rock Classification Ultramafic Ultramafic – Very dark/often green – less than 45% Silica; 45-100% Olivine 45– Peridotite – intrusive – Komatite – extrusive (rare) Mafic Mafic – Dark – Pyroxene, Feldspar, some Olivine; 45-55% Silica 45– Gabbro - intrusive – Basalt - extrusive Igneous Rocks Intermediate Intermediate – Gray – Pyroxene, Amphibole, Feldspar, Mica, Quartz; 55-65% Silica 55– Diorite - intrusive – Andesite – extrusive (porphyritic) Igneous Igneous Rocks Felsic Felsic – Light – Feldspar, Mica, Quartz; > 65% Silica – Granite - intrusive – Rhyolite - extrusive Intrusive Structures plutons Classification of plutons based on • Size • Shape • Orientation Concordant = parallel to layers in county rock Discordant = cut layers of country rock Intrusive Igneous Bodies Dikes Dikes - discordant, tabular intrustions Sills Sills - concordant, tabular intrusions 3 Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures Dike Dike – Discordant tabular pluton Intrusive Igneous Rock Structures Sill Sill – Concordant tabular pluton Intrusive Igneous Rock Structures Laccolith Laccolith – Lens-shaped pluton Lens- Intrusive Igneous Rock Structures Laccolith Laccolith – Lens-shaped pluton Lens- Laccolith 4 Intrusive Igneous Rock Structures Volcanic Volcanic Neck Church built on top of a volcanic neck, Auvergne region of Central France. Intrusive Igneous Bodies Volcanic Volcanic Pipes and Necks Devil’s Tower, Wyoming Shiprock Peak, ew Mexico Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures Batholith Batholith - Very large pluton (>100 sq. kilometers) Stock Stock – Smaller pluton (<100 sq. kilometers) Xenoliths Batholith 5 Sierra Nevada Batholith Sierra Nevada Batholith Intrusive Igneous Rock Structures Batholiths Batholiths are made up of numerous diapirs, diapirs, or “blobs” of magma Igneous Rocks and Intrusive Igneous Activity Laccolith Chapter 4 6 ...
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This note was uploaded on 05/18/2011 for the course GEOL 105 taught by Professor Jack during the Spring '08 term at University of Louisiana at Lafayette.

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