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Igneous Processes and Structures

GLY 2010 – Summer 2012

Lecture 7

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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”

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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

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Viscosity

• Viscosity is the property of a substance to offer internal resistance to flow; its internal friction

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Flow Viscosity

Initial Position After flow starts

Viscosity increases from left to right

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Viscosity in Magma

• Video shows a rod being poked in hot, viscous magma on Kilauea, Hawaii

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Factors Influencing the Viscosity of Magma

• Temperature

• Chemical composition

• Gas content

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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

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Low Viscosity Flow Animation

Typical of basaltic magma

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High Viscosity Flow Animation

• Typical of Andesitic or Rhyolitic Magmas

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Chemical Composition

• The higher the silica content of magma, the higher the viscosity

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Gas Content

• As gas content increases, the viscosity decreases

• Gases inhibit silica chain formation, and lower overall viscosity

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Effects of Increasing Viscosity

• Volcanic violence related to viscosity (magma type)

• Magma type related to geologic, and often, plate tectonic setting

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Partial Melting

• Different minerals melt at different temperatures

• As temperature increases, this leads to partial melting

Igneous Structures

• Intrusive – magma freezes below the surface

• Extrusive – magma erupts onto or above the surface

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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

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Intrusive Structures, Cont.

• Igneous sill – Parallel to existing layers

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Igneous Sill

• Salt River Canyon, Arizona – the dark band is basalt intruded into horizontal layers of igneous rock

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Intrusive Structures, Cont.

• Igneous Dike - intrusion cuts across the rock layers

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Igneous Dike

• Location Pender Harbor, Southwest British Columbia

•Thin, pink aplite dikes cut the black basaltic dikes and the gray granite

•Photo C.A. Giovanella

Igneous Structure Diagram

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Aerial View of Intersecting Dikes

• Intersecting dikes (Tertiary) at Spanish Peaks, Las Animas and Huerfano Counties, CO. View to east. (10Apr66)

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Spanish Peak Dikes

• West Spanish Peak (13,623 ft) and dikes (Tertiary), Las Animas and Huerfano Counties, CO. View to the south. (10Apr66)

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Intrusive Structures, Cont.

• Laccolith

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Igneous Laccolith

Shiprock, New Mexico

• The neck of an ancient volcano, which has eroded• Structure in the background is a dike

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Igneous Vein• Extensional veins in

a thick carbonate turbidite from the Liguride Complex in the Northern Apennines, Italy

• Photo David Bice, Carleton College

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Extrusive Structures

• Lava Flows

• Obsidian

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Lava Flow From the Air

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Lava Flow From the Ground

• Lava flows from Nyiragongo volcano

• Eruption January 18, 2002

• City is Gomo, Congo

Newberry Caldera, Oregon

• Obsidian flow from a vent along the south wall of the caldera

• Road gives scale

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Obsidian Domes

• Holocene obsidian domes, Long Valley Caldera California - obsidian is formed from very viscous magma, which is unable to flow long distances

Photo M.L. Bevier

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Obsidian• Upper left: Thin

piece of obsidian, showing flow banding

• Lower left: Thicker piece of obsidian showing conchoidal fracture

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Classification of Igneous Rocks

• Mineral content

• Chemical analysis

• Texture

• Geologic Association

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Mineral Content

• Igneous rocks may be classified on the basis of what minerals they contain Essential Minerals Accessory Minerals

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Chemical Content

• Rocks may be analyzed to see what elements they contain

• Results are reported as weight percent oxides

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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

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Aphanitic Texture

• A very fine grain texture, with crystals invisible to the naked eye

• Photo M.L. Bevier

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Pheneritic Texture

• Grains are visible and identifiable using the naked eye

• Photo M.L. Bevier

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Porphyritic Texture

• Large crystals in a fine-grained or aphanitic groundmass - Field of view 2 cm

• Photo: E.J. Tarbuck

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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

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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

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Geologic Association

• Rocks associated by age, position, and by characteristically being found together

• Constant or regular variation of features within the rock body

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Bowen's Reaction Series

Bowen – Tuttle Experiments

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Magmatic Differentiation

• Fractional Crystallization (Crystal settling)

• Partial Melting

Fractional Crystallization

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Evolution of Magma

• Over time, fractional crystallization changes the composition of magma from “A” to “B” to “C”

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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

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Granite, Rhyolite

• Granite has clearly visible crystals

• Rhyolite has few visible crystals, and patches of glass

• Both rocks are felsic

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Diorite, Andesite

• Diorite has visible felsic and mafic crystals

• Andesite has mainly felsic crystals visible

• Both rocks are intermediate

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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

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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

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Peridotite

• Ultramafic intrusive rock characteristic of the earth’s mantle

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Xenolith

• Xenoliths of metamorphic amphibole in dunite, an ultramafic igneous rock

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Andesite Line

• Geographic boundary between the basalt/gabbro of the Pacific Ocean basin and the andesites at the subductive margins of the surrounding continents