IGNEOUS ROCKS Introduction Igneous Compositions Igneous Textures Rocks and the rock cycle Igneous rocks Igneous Compositions Igneous Textures Classification Bowen’s Reaction Series Origin of Magmas Evolution of Magmas

Igneous Rocks Introduction

Rocks Most are aggregates of mineral crystals GEOL 131: Igneous Rocks – Introduction Rocks Most are aggregates of mineral crystals A few kinds of rocks aren’t made of minerals Granite is made of mineral crystals Obsidian (volcanic glass) has no mineral crystals Images from: geology.com

Rocks Igneous Sedimentary Metamorphic GEOL 131: Igneous Rocks – Introduction Rocks Igneous Sedimentary Metamorphic

The Rock Cycle GEOL 131: Igneous Rocks – Introduction From: mnh.isu.edu

Igneous Rocks Form by cooling and crystallization of magma GEOL 131: Igneous Rocks – Introduction Igneous Rocks Form by cooling and crystallization of magma From: en.wikipedia.org From:meteorites.wustl.edu

Magma’s chemical composition Magma’s cooling rate GEOL 131: Igneous Rocks – Introduction Igneous Rocks What kind of igneous rock will form when a magma crystallizes? Depends on: Magma’s chemical composition Magma’s cooling rate

Igneous Rocks Igneous Compositions

Igneous Rock Compositions GEOL 131: Igneous Rocks – Compositions Igneous Rock Compositions What minerals the rock is made of Orthoclase Quartz Hornblende From: geology.com

Igneous Rock Compositions GEOL 131: Igneous Rocks – Compositions Igneous Rock Compositions Percentage of dark silicate minerals From: ux1.eiu.edu From: geology.com From: nwnature.net

FELSIC (“granitic”) 0-25% dark minerals GEOL 131: Igneous Rocks – Compositions FELSIC (“granitic”) 0-25% dark minerals This GRANITE contains about 10% dark minerals From: geology.com

FELSIC (“granitic”) 0-25% dark minerals GEOL 131: Igneous Rocks – Compositions FELSIC (“granitic”) 0-25% dark minerals RHYOLITE is also felsic in composition From: itc.gsw.edu

INTERMEDIATE (“andesitic”) GEOL 131: Igneous Rocks – Compositions INTERMEDIATE (“andesitic”) 25-45% dark minerals DIORITE: This sample is about 40% dark crystals ANDESITE: Another intermediate rock From: nwnature.net From: geology.com

MAFIC (“basaltic”) 45-85% dark minerals GEOL 131: Igneous Rocks – Compositions MAFIC (“basaltic”) 45-85% dark minerals This GABBRO contains about 70% dark minerals BASALT: Light silicate crystals are present, but are too small to see without magnification From: ux1.eiu.edu From: geology.com

ULTRAMAFIC 85-100% dark minerals GEOL 131: Igneous Rocks – Compositions ULTRAMAFIC 85-100% dark minerals PERIDOTITE: This sample contains olivine and pyroxene, both dark silicate minerals From: newark.osu.edu

Igneous Rocks Igneous Textures

IGNEOUS TEXTURES Visual appearance, not feel GEOL 131: Igneous Rocks – Textures IGNEOUS TEXTURES Visual appearance, not feel Usually refers to size of mineral crystals Determined by magma’s rate of cooling

PHANERITIC “Coarse-grained” Most crystals visible to naked eye GEOL 131: Igneous Rocks – Textures PHANERITIC “Coarse-grained” Most crystals visible to naked eye From: geology.com From: nwnature.net

PHANERITIC Slow cooling rate GEOL 131: Igneous Rocks – Textures PHANERITIC Slow cooling rate Crystals have time to accumulate more atoms & grow large INTRUSIVE cooling From: windows2universe.org

PEGMATITIC Very coarse-grained GEOL 131: Igneous Rocks – Textures From: marlimillerphoto.com

APHANITIC “Fine-grained” Most crystals not visible to naked eye GEOL 131: Igneous Rocks – Textures APHANITIC “Fine-grained” Most crystals not visible to naked eye From: geology.com From: itc.gsw.edu

APHANITIC Rapid cooling rate GEOL 131: Igneous Rocks – Textures APHANITIC Rapid cooling rate Not enough time for crystals to grow large EXTRUSIVE (volcanic) cooling From: bbc.co.uk

PORPHYRITIC Two distinct crystal sizes in same rock GEOL 131: Igneous Rocks – Textures PORPHYRITIC Two distinct crystal sizes in same rock Phenocrysts From: geo1.tcu.edu

PORPHYRITIC Two distinct crystal sizes in same rock GEOL 131: Igneous Rocks – Textures PORPHYRITIC Two distinct crystal sizes in same rock Phenocrysts Aphanitic matrix From: pitt.edu

PORPHYRITIC Two-stage cooling First, slow cooling produces phenocrysts GEOL 131: Igneous Rocks – Textures PORPHYRITIC Two-stage cooling First, slow cooling produces phenocrysts Then, rapid cooling of remaining magma produces smaller crystals

OTHER TEXTURES GLASSY Very fast cooling GEOL 131: Igneous Rocks – Textures OTHER TEXTURES GLASSY Very fast cooling No time for any crystals to form

OTHER TEXTURES VESICULAR Vesicles (bubbles) GEOL 131: Igneous Rocks – Textures OTHER TEXTURES VESICULAR Vesicles (bubbles) Gas trapped in rapidly cooling magma From: texasbeyondhistory.net

OTHER TEXTURES PYROCLASTIC Ash and rock fragments GEOL 131: Igneous Rocks – Textures OTHER TEXTURES PYROCLASTIC Ash and rock fragments Violent volcanic eruptions Rock fragments Ash matrix

Igneous Rocks CLASSIFICATION

CLASSIFICATION Why classify? Accurate communication GEOL 131: Igneous Rocks – Classification CLASSIFICATION Why classify? Accurate communication Names contain information about how rocks formed

CLASSIFICATION Based on texture and composition GEOL 131: Igneous Rocks – Classification CLASSIFICATION Based on texture and composition Some rocks can be classified based only on texture

CLASSIFICATION FELSIC INTERM MAFIC PHANERITIC Granite Diorite Gabbro GEOL 131: Igneous Rocks – Classification CLASSIFICATION FELSIC INTERM MAFIC PHANERITIC Granite Diorite Gabbro APHANITIC Rhyolite Andesite Basalt PORPHYRITIC: Used as a modifier for any of the six basic rock names.

CLASSIFICATION (expanded) GEOL 131: Igneous Rocks – Classification CLASSIFICATION (expanded) FELSIC INTERM MAFIC ULTRA MAFIC PHANERITIC Granite Diorite Gabbro Peridotite APHANITIC Rhyolite Andesite Basalt Komatiite (rare) GLASSY OBSIDIAN VESICULAR SCORIA (dark) PUMICE (light) PYROCLASTIC WELDED TUFF PORPHYRITIC: Used as a modifier for any of the six basic rock names.

Igneous Rocks ORIGIN OF MAGMAS

All magmas form in the crust or mantle GEOL 131: Igneous Rocks – Origin of Magmas All magmas form in the crust or mantle From: facweb.bhc.edu

Processes That Create Magmas GEOL 131: Igneous Rocks – Origin of Magmas Processes That Create Magmas “Simple” heating Shallow crust Decompression Upper mantle Hydration From: buzzle.com

Pressure and Melting Point GEOL 131: Igneous Rocks – Origin of Magmas Pressure and Melting Point Higher pressure = higher melting point Rocks under high P must get hotter before they will melt

“Simple” Heating Heating and melting of pre-existing rock GEOL 131: Igneous Rocks – Origin of Magmas “Simple” Heating Heating and melting of pre-existing rock Only occurs in shallow crust Top few kilometers Deeper rock is under too much pressure Only heat source hot enough to melt rock is another magma

GEOL 131: Igneous Rocks – Origin of Magmas From: geochemistry.usask.ca

Decompression Melting GEOL 131: Igneous Rocks – Origin of Magmas Decompression Melting Occurs without an increase in temperature Occurs because of drop in pressure

GEOL 131: Igneous Rocks – Origin of Magmas Hydration Melting

Hydration Melting Occurs without an increase in temperature GEOL 131: Igneous Rocks – Origin of Magmas Hydration Melting Occurs without an increase in temperature Occurs because of addition of water to solid rock above a subducting crustal plate

Bowen’s Reaction Series Igneous Rocks Bowen’s Reaction Series

Bowen’s Reaction Series GEOL 131: Igneous Rocks – Bowen’s Reaction Series Bowen’s Reaction Series Describes sequence in which minerals form as a magma crystallizes Different minerals crystallize at different temperatures First crystals begin to form at about 1200°C

GEOL 131: Igneous Rocks – Bowen’s Reaction Series

Igneous Rocks EVOLUTION OF MAGMAS

GEOL 131: Igneous Rocks – Evolution of Magmas Chemical changes after magma forms, and before it crystallizes Most magmas begin with mafic or ultramafic compositions Lower in silica, rich in Fe and Mg

Processes That Change Magmas GEOL 131: Igneous Rocks – Evolution of Magmas Processes That Change Magmas Crystal settling/partial melting Assimilation Mixing

Crystal Settling MAGMA (liquid) GEOL 131: Igneous Rocks – Evolution of Magmas Crystal Settling Early-formed crystals settle to bottom MAGMA (liquid)

GEOL 131: Igneous Rocks – Evolution of Magmas Crystal Settling Early crystals use up Fe and Mg, but little silica (Si and O) As crystals form, magma becomes depleted in Fe and Mg and enriched in silica Last minerals to form have no Fe/Mg and are silica-rich

Animation of crystal settling GEOL 131: Igneous Rocks – Evolution of Magmas Crystal Settling Animation of crystal settling

Assimilation Magma melts host rock as it migrates through crust GEOL 131: Igneous Rocks – Evolution of Magmas Assimilation Magma melts host rock as it migrates through crust From: wikipedia.org From: jan.ucc.nau.edu

Mixing Magma A mixes into magma B within the crust GEOL 131: Igneous Rocks – Evolution of Magmas Mixing Magma A mixes into magma B within the crust Composition of magma B changes

End of Igneous Rocks chapter