Igneous Sill Salt River Canyon, Arizona the dark band is basalt intruded into horizontal layers of igneous rock 18 Intrusive Structures, Cont. Igneous Dike - intrusion cuts across the rock layers
19 Igneous Dike Thin, pink aplite dikes cut the black basaltic dikes and the gray granite
Photo C.A. Giovanella Location Pender Harbor, Southwest British Columbia 20 Igneous Structure Diagram
21 Aerial View of Intersecting Dikes Intersecting dikes (Tertiary) at Spanish Peaks, Las Animas and Huerfano Counties, CO. View to east. (10Apr66) 22 Spanish Peak Dikes
West Spanish Peak (13,623 ft) and dikes (Tertiary), Las Animas and Huerfano Counties, CO. View to the south. (10Apr66) 23 Intrusive Structures, Cont.
Laccolith 24 Igneous Laccolith 25 Shiprock, New Mexico
The neck of an ancient volcano, which has eroded Structure in the background is a dike 26 Igneous Vein Extensional veins in a thick carbonate turbidite from the Liguride Complex in
the Northern Apennines, Italy Photo David Bice, Carleton College 27 Extrusive Structures Lava Flows Obsidian
28 Lava Flow From the Air 29 Lava Flow From the Ground Lava flows from
Nyiragongo volcano Eruption January 18, 2002 City is Gomo, Congo 30
Newberry Caldera, Oregon Obsidian flow from a vent along the south wall of the caldera Road gives scale
31 Obsidian Domes Photo M.L. Bevier Holocene obsidian domes, Long Valley Caldera California - obsidian is formed from very viscous magma, which is unable to flow long distances 32
Obsidian Upper left: Thin piece of obsidian, showing flow banding Lower left: Thicker piece of obsidian showing
conchoidal fracture 33 Classification of Igneous Rocks
Mineral content Chemical analysis Texture Geologic Association 34 Mineral Content
Igneous rocks may be classified on the basis of what minerals they contain Essential Minerals Accessory Minerals 35
Chemical Content Rocks may be analyzed to see what elements they contain Results are reported as weight percent oxides 36
Texture and Fabric Texture is the general physical appearance or character of a rock Fabric refers to the orientation (or lack of it) in space of the elements of which a rock is composed 37
Aphanitic Texture A very fine grain texture, with crystals invisible to the naked eye Photo M.L. Bevier 38 Pheneritic
Texture Grains are visible and identifiable using the naked eye Photo M.L. Bevier 39 Porphyritic Texture
Large crystals in a fine-grained or aphanitic groundmass Field of view 2 cm Photo: E.J. Tarbuck 40 Pegmatitic
Black Hills, South Dakota Coarse texture, resulting from rapid crystal growth, due to presence of water in melt at time of crystallization Arrows
point to the location of spodumenne crystals, removed during mining Spodumene is a source of lithium 41 Relation of Texture to Cooling History Aphanetic - Rapid cooling leads to very fine crystals or to glass typical of extrusive rocks
Phaneritic - Slower cooling leads to medium or coarse grained rocks, typical of intrusive igneous rocks Pegmatitic - Very slow cooling in a water-rich magma leads to the coarse crystals Porphyritic - Slow cooling while the magma ascends through the earth, followed by rapid cooling after the magma erupts on the surface Crystals are often medium to coarse Groundmass texture will be fine-grained, or glassy, depending on
the rate of cooling 42 Geologic Association Rocks associated by age, position, and by characteristically being found together Constant or regular variation of features within the rock body
43 Bowen's Reaction Series 44 Bowen Tuttle Experiments
Fractional Crystallization 47 Evolution of Magma Over time, fractional crystallization changes the composition of
magma from A to B to C 48 Intrusive Vs. Extrusive Rocks Intrusive Medium to coarse grained
100% Crystalline May contain inclusions of rock which fell into the magma (xenoliths) Extrusive Aphanitic to fine grain size
Often glassy Often contain gas bubbles 49 Granite, Rhyolite Granite has clearly visible crystals
Rhyolite has few visible crystals, and patches of glass Both rocks are felsic 50 Diorite, Andesite Diorite has visible
felsic and mafic crystals Andesite has mainly felsic crystals visible Both rocks are intermediate 51
Gabbro, Basalt Gabbro is composed of dark, visible minerals Basalt is dark, with no visible crystals; this piece has many gas bubbles Both rocks are mafic
52 Obsidian Obsidian is volcanic glass, formed by quick chilling of the magma after eruption This specimen is called Snowflake obsidian - the white crystals are cristobalite, a high-temperature form of silica, which crystallized before eruption
53 Peridotite Ultramafic intrusive rock characteristic of the earths mantle 54 Xenolith
Xenoliths of metamorphic amphibole in dunite, an ultramafic igneous rock 55 Andesite Line Geographic boundary between the basalt/gabbro of the Pacific Ocean basin
and the andesites at the subductive margins of the surrounding continents 56
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