Igneous Processes and Structures GLY 2010 Summer 2013 Lecture 7 1 Definition of Igneous Igneous - Said of a rock or mineral that solidified from molten or partly molten

material, i.e. from a magma Etymology: Latin ignis, ''fire 2 Magma Magma is naturally occurring mobile rock material molten rock Capable of intrusion and extrusion

Igneous rocks are derived from magma through solidification and related processes 3 Viscosity Viscosity is the property of a substance to offer internal resistance to flow; its internal friction

4 Flow Viscosity Initial Position After flow starts Viscosity increases from left to right

5 Viscosity in Magma Video shows a rod being poked in hot, viscous magma on Kilauea, Hawaii 6

Factors Influencing the Viscosity of Magma Temperature Chemical composition Gas content 7 Temperature

The higher the temperature, the lower the viscosity Basaltic magmas at 1200C or higher, are much more fluid (less viscous) than granitic magmas at 800C 8 Low Viscosity Flow Animation

Typical of basaltic magma 9 High Viscosity Flow Animation Typical of Andesitic or Rhyolitic Magmas 10

Chemical Composition The higher the silica content of magma, the higher the viscosity 11 Gas Content As gas content increases, the viscosity decreases

Gases inhibit silica chain formation, and lower overall viscosity 12 Effects of Increasing Viscosity Volcanic violence related to viscosity (magma type) Magma type related to geologic, and

often, plate tectonic setting 13 Partial Melting Different minerals melt at different temperatures As temperature increases, this leads to partial melting

14 Igneous Structures Intrusive magma freezes below the surface Extrusive magma erupts

onto or above the surface 15 Intrusive Structures Plutons are large bodies of magma that solidified well below the surface Magma may be injected under pressure into cracks in the rock

16 Intrusive Structures, Cont. Igneous sill Parallel to existing layers 17

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

45 Magmatic Differentiation Fractional Crystallization (Crystal settling) Partial Melting 46

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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