Igneous Processes and Structures GLY 2010 Summer 2013

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 1200°C or higher, are much more fluid (less viscous) than granitic magmas at 800°C 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. (10 Apr 66) 22

Spanish Peak Dikes • West Spanish Peak (13, 623 ft) and dikes (Tertiary), Las Animas and Huerfano Counties, CO. View to the south. (10 Apr 66) 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 earth’s 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