Volcanoes and Igneous Activity Earth

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Transcript Volcanoes and Igneous Activity Earth

Chapter 3

Igneous Rocks

PowerPoint Presentation Stan Hatfield .

SW Illinois College

Ken Pinzke .

SW Illinois College

Charles Henderson .

University of Calgary

Tark Hamilton .

Camosun College

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Magma : The Parent Material of Igneous Rock Igneous rocks form as molten rock cools and solidifies General Characteristic of magma

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Parent material of igneous rocks

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Forms from partial melting inside the Earth

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Magma that reaches the surface is called lava

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Magma: The Parent Material of Igneous Rock General Characteristic of Magma

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Generally formed by partial melting in Upper Mantle (~1200° C) or Lower Crust (~850° C)

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Rocks formed from lava at the surface are classified as extrusive or volcanic rocks

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Rocks formed from magma that crystallizes at depth are termed intrusive or plutonic rocks

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Magmas are buoyant, gas laden & transport Heat

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Flow rates vary over many orders of magnitude from cm/yr to supersonic

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Magma : The Parent Material of Igneous Rock The Nature of Magma

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Consists of three components:

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A liquid portion, called melt , that is composed of mobile ions derived from the partial melting of minerals

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Solids , if any, are silicate minerals that have already crystallized from the melt:

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Phenocrysts are large, Microlites are small

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Volatiles , which are gases dissolved in the melt, including water vapour (H 2 O), carbon dioxide (CO 2 ), sulphur dioxide (SO 2 ) & minor HF, HCl, He

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Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock

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Cooling of magma results in the systematic arrangement of ions into orderly patterns, cations + anions = minerals

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The silicate minerals resulting from crystallization form in a predictable order

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Textures & inclusion relations tells order of crystallization (early small crystals get surrounded by later larger phenocrysts)

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Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock

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Texture the in igneous rocks is determined by size and arrangement of mineral crystals

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Igneous rocks are typically classified by

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Textures

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Mineral compositions & proportions

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Rocks with similar textures can have different compositions (glasses all appear similar)

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Rocks with similar compositions can have different textures (rhyolite and granite look different because of different cooling histories)

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Textures + Minerals = Igneous Rock

Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals Factors affecting crystal size :

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Cooling Rates

– –

Fast cooling forms glass or may tiny crystals (microlites) Slow cooling rates promote the growth of fewer larger phenocrysts

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Volatiles are Solvents

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Volatiles lower viscosity & increase diffusion leading to larger crystals

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Nucleation

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More crystal nuclei promote the growth of more, but smaller crystals which impinge on each other

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Igneous Textures

Factors affecting crystal size

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Cooling Rate for magma

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Slow cooling (°C/yr) allows crystals to chemically react with magma

– –

Fast rate (~few °C/hr) forms many small crystals Very fast rate (~hundreds of °C/sec) forms glass (disordered ions)

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Amount of

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Silica (SiO 2 ) present Mafic (Low silica) magmas like basalts (<50% SiO 2 ) flow easily

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Felsic (High silica) magmas are stiff and explosive

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Nucleation of crystals

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Contamination with crustal rocks promotes nucleation

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Amount of Dissolved Gases

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Affects viscosity, diffusion and explosivity

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<5% dissolved volatiles allows flows of km/day More than ~5% volatiles exsolve and form explosive foams

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Igneous Textures

Types of igneous textures

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Aphanitic (fine-grained) texture

 – – –

Rapid rate of cooling of lava or magma (in air, water) Microscopic crystals Volcanic!

May contain vesicles (holes from gas bubbles) and thus rocks that contain them have a vesicular texture

• • •

Porphyritic = large phenocrysts & smaller groundmass

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Generally lava flows Phaneritic or sub-volcanic intrusions ( dykes/sills ) Phenocrysts grew slowly then eruption quenched the lava (coarse-grained) texture

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Plutonic!

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Crystals can be identified without a microscope (>2mm) Generally caused by slow cooling (heat loss at depth) Pegmatitic (very coarse-grained) texture

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Plutonic!

– – – –

Crystals (>2cm) Generally caused by very slow cooling Also caused by abundant volatiles (increases diffusion rates) Rare metals (Au, B, Be, Sn) & unusual minerals can occur

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Hand Specimen

Igneous Textures: Volcanic

Thin Section 3-10

Igneous Textures: Plutonic

Granite with: Pink K-Feldspar, White Plagioclase, Phaneritic Grey Quartz & Black Biotite Biotite-birefringent, Qtz-grey/white Feldspars-Twinned Black/white Copyright (c) 2005 Pearson Education Canada Inc.

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Igneous Textures: Volcanic

Types of igneous textures

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Porphyritic texture : 2 different crystal sizes

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Minerals form at different temperatures as well as crystallizing at differing rates

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Large crystals, called phenocrysts , are embedded in a matrix of smaller crystals, called the groundmass

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Sudden loss of volatiles can arrest crystallization

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Glassy texture (Vitreous & Conchoidal fracture)

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Very rapid cooling to volcanic rock in air or water

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This is common in very viscous, Hi-Silica magmas

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Resulting rock is called obsidian

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Volcanic Textures

Flattened & fused Glassy shards Indicate flow & Horizontal directions Black Magnetite as dust sized particles darkens the glass Pale green patches are altered to chlorite.

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Igneous Textures

Types of igneous textures

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Pyroclastic texture (volcanic)

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Various fragments ejected during a violent volcanic eruption (rocks, crystals, glass shards, foams)

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Textures often appear more similar to sedimentary rocks, but usually angular & partly glassy

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Pegmatitic texture (plutonic)

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Exceptionally coarse-grained Form in late stages of crystallization of magmas

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Xenolithic / Xenocrystic texture (plutonic)

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Accidental rock fragments from mantle or crust

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Included crystals from other rocks or magmas

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Igneous Textures: Plutonic

Large K-Feldspars, White Plagioclase, Grey Quartz, Brown-Green Hornblende, Black Magnetite Phaneritic Porphyritic

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Igneous Compositions Igneous rocks are composed primarily of silicate minerals

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Dark (or ferro magnesian ) silicates,

∑

= Colour Index

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Depends on Mg + Fe content of magma

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Crystallize in order of falling Temperature

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May react with magma to form a lower T°C phase

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Spinel or Magnetite (oxides not silicates) Fe 3 O 4

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Olivine (Mg,Fe)SiO 4 , lone tetrahedra

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Pyroxene Ca(Mg,Fe)Si 2 O 6 , single chains

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Amphibole Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 , double chains

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Biotite mica K(Mg,Fe) 3 (Al,Si) 3 O 10 (OH,F) 2 , sheets

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Igneous Compositions

Igneous rocks are composed primarily of silicate minerals

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Light (or nonferromagnesian) silicates (with falling Temperature)

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Plagioclase Feldspar: framework silicate

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Anorthite CaAl 2

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K-Feldspar KAlSi 3 O 8 , framework silicate

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Quartz SiO 2 Si 2 O 8 to Albite NaAlSi 3 O 8 , framework silicate

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Muscovite mica K(Mg,Fe) 3 (Al,Si) 3 O 10 (OH,F) (this mineral is only found in plutonic rocks) 2 , sheets,

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Igneous Compositions Felsic versus Mafic Compositions

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Rhyolitic composition

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Common in explosive strato-volcanoes (arcs)

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Granitic composition

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Composed of light-coloured silicates

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Designated as being felsic (feldspar and silica) in composition

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Contains high amounts of silica (SiO 2 ), >68%

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Low temperature melts but high viscosity

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Major rock type in continental crust

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Common in batholiths of continental margin arcs

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Igneous Compositions Mafic versus Felsic Compositions

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Basaltic (or Gabbroic) composition

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Composed of dark silicates and calcium-rich feldspar

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Designated as being mafic (magnesium and ferrum, for iron) in composition

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High Temperature magmas but low viscosity

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More dense than granitic rocks

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Comprise the ocean crust as well as many volcanic islands

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Igneous Compositions

Other compositional groups

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Intermediate (or andesitic ) composition

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Contain at least 25 percent dark silicate minerals

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Associated with explosive volcanic activity

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Present in arc volcanoes and plutons

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Ultramafic composition

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Composition that is high in MgO and FeO > 55%

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Dense, high Temperature, Low viscosity melts

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Composed entirely of >90% ferromagnesian silicates

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Common in mantle (plutonic) but rare in crust

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More common in lower crust and in Precambrian rocks

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Igneous Compositions

Silica Content as an Indicator of Composition

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Silica content in crustal rocks exhibits a considerable range

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A lower than 45% in ultramafic rocks

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Over 75% percent in some felsic rocks

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Tends to increase during fractional crystallization

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Igneous Rock Classification

Rock Names Depend on Mineral %’s & Textures Plutonic/Volcanic

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Igneous Compositions

Silica content influences a magma’s behaviour

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Granitic magma

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High silica content > 68%

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Extremely viscous, flows slowly, explosive

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Liquid exists at temperatures as low as 700 o C

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Forms by differentiation from more Mafic magmas

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Can also form by partial melting of Lower Crust in collisional orogens

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Often high volatile contents: H 2 O, CO 2 etc.

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Igneous Compositions

Silica content influences a magma’s behaviour

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Basaltic magma

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Much lower silica content <54%

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Fluid-like behaviour

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Low volatile content < a few %, usually not explosive

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Crystallizes at higher temperatures

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Most common Magma on Earth (or Moon!)

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Partial melt of Peridotite (5% to 25%)

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Igneous Compositions

Naming Igneous Rocks – Felsic (Granitic) Rocks

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Granite

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Phaneritic

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Over 25 percent quartz, about 65 percent or more feldspar

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May exhibit a porphyritic texture

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Very abundant as it is often associated with mountain building

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The term granite covers a wide range of mineral compositions but mostly alkali feldspar & quartz

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Plutonic Igneous Compositions

Granite:

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Igneous Compositions

Naming Igneous Rocks – Felsic (Granitic) Rocks

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Rhyolite

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Extrusive equivalent of granite

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Found in stratovolcanoes & calderas

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May contain glass fragments and vesicles

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Aphanitic texture

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Less common and less voluminous than granite

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Volcanic Igneous Compositions

Rhyolite

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Felsic Volcanic Compositions

Naming Igneous Rocks – Felsic (Granitic) Rocks

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Obsidian

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Dark-coloured, often flow banded

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Glassy texture, conchoidal fracture

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Gas bubbles (vesicles) are common but flattened

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Pumice

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Low density & Light Coloured Vesicular Volcanic

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Glassy texture with few if any crystals

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Frothy appearance with numerous voids

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Felsic Volcanic Compositions

Obsidian

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Felsic Volcanic Compositions

Pumice

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Intermediate Volcanic Compositions

Naming Igneous Rocks – Intermediate (Andesitic) Rocks

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Andesite

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Volcanic origin usually in arc stratovolcanoes

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Aphanitic or aphanitic-porphyritic texture

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Often crystal rich with 25% to 40% phenocrysts

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Essential Plagioclase with Pyroxene or Hornblende

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Often resembles rhyolite when pyroclastic (tuffaceous)

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Andesite

From Black to White!

Abundant Plagioclase phenocrysts with small hornblende micro phenocrysts.

Magnetite is opaque.

Groundmass is smaller by 50X.

High phenocryst content makes these viscous & explosive.

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Intermediate Plutonic Compositions

Naming Igneous Rocks – Intermediate (Andesitic) Rocks

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Diorite

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Plutonic equivalent of andesite

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Coarse-grained

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Intrusive (like in the roots of the Andes!)

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Composed mainly of intermediate plagioclase feldspar and amphibole

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K-feldspar is minor if present, < 1/3 of total feldspar

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16 < CI < 45 is intermediate

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Intermediate Plutonic Compositions

Diorite Phaneritic Grey Rocks ½ Plagioclase & ½ Pyroxene or Hornblende

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Igneous Compositions

Naming Igneous Rocks – Mafic (Basaltic) Rocks

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Basalt

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Volcanic origin

– – –

Dark green to black in colour Aphanitic texture Composed mainly of pyroxene and calcium rich plagioclase feldspar

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Most common extrusive igneous rock

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Igneous Compositions

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Igneous Compositions

Naming Igneous Rocks – Mafic (Basaltic) Rocks

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Gabbro

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Intrusive equivalent of basalt

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Phaneritic texture consisting of pyroxene and calcium-rich plagioclase

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Makes up a significant percentage of the oceanic crust

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Igneous Compositions

Naming Igneous Rocks – Pyroclastic Rocks

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Composed of fragments ejected during a volcanic eruption

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Varieties

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Tuff – ash-sized fragments

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Volcanic breccia – particles larger than ash

3-39

Origin of Magma

Highly debated topic Generating magma from solid rock

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Produced from partial melting of rocks in the crust and upper mantle

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Role of Temperature

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Temperature increases within Earth’s upper crust (called the geothermal gradient ) average between 20 o C to 30 o C per kilometre

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Origin of Magma

Estimated temperatures in the crust and mantle.

3-41

Origin of Magma

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Role of Temperature

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Rocks in the lower crust and upper mantle are near their melting points

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Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting

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Origin of Magma

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Role of Pressure

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An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature

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When confining pressures drop, decompression melting occurs

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Origin of Magma

Decompression melting 3-44

Origin of Magma

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Role of volatiles

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Volatiles (primarily water) cause rocks to melt at lower temperatures

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This is particularly important where oceanic lithosphere descends into the mantle

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How Magmas Evolve

A single volcano may extrude lavas exhibiting very different compositions Bowen’s reaction series and the composition of igneous rocks

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N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points

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How Magmas Evolve

Bowen’s Reaction Series shows the sequence in which minerals crystallize from a magma.

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How Magmas Evolve

Bowen’s reaction series

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During crystallization, the composition of the liquid portion of the magma continually changes

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Composition changes due to removal of elements by earlier-forming minerals

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The silica component of the melt becomes enriched as crystallization proceeds

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Minerals in the melt can chemically react and change

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How Magmas Evolve

Processes responsible for changing a magma’s composition

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Magmatic differentiation

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Separation of a melt from earlier formed crystals to form a different composition of magma

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Assimilation

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Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma

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Processes responsible for changing a magma’s composition

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Magma mixing

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Involves two bodies of magma intruding one another

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Two chemically distinct magmas may produce a composition quite different from either original magma

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How Magmas Evolve

Magma mixing, assimilation and magmatic differentiation.

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How Magmas Evolve

Partial Melting and Magma Formation

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Incomplete melting of rocks is known as partial melting

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Formation of a Mafic Magma (basaltic)

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Most originate from partial melting of ultramafic rock in the mantle

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Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones

3-52

How Magmas Evolve

Partial Melting and Magma Formation

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Formation of Basaltic Magmas

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As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature

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Large outpourings of basaltic magma are common at Earth’s surface

3-53

How Magmas Evolve

Partial Melting and Magma Formation

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Formation of Andesitic Magmas

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Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition

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Andesitic magma may also evolve by magmatic differentiation

3-54

How Magmas Evolve

Partial Melting and Magma Formation

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Formation of Felsic (granitic) Magmas

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Most likely form as the end product of crystallization of andesitic magma

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Granitic magmas are higher in silica and therefore more viscous than other magmas

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Because of their viscosity, they lose their mobility before reaching the surface

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Tend to produce large plutonic structures

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End of Chapter 3

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Volcanoes and Igneous Activity Earth

Transcript Volcanoes and Igneous Activity Earth

Igneous Rocks

Magma : The Parent Material of Igneous Rock Igneous rocks form as molten rock cools and solidifies General Characteristic of magma

Magma: The Parent Material of Igneous Rock General Characteristic of Magma

Magma : The Parent Material of Igneous Rock The Nature of Magma

Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock

Magma: The Parent Material of Igneous Rock From Magma to Crystalline Rock

Textures + Minerals = Igneous Rock

Igneous Textures

Igneous Textures

Igneous Textures: Volcanic

Igneous Textures: Plutonic

Igneous Textures: Volcanic

Volcanic Textures

Igneous Textures

Igneous Textures: Plutonic

Igneous Compositions Igneous rocks are composed primarily of silicate minerals

Igneous Compositions

Igneous Compositions Felsic versus Mafic Compositions

Igneous Compositions Mafic versus Felsic Compositions

Igneous Compositions

Igneous Compositions

Igneous Rock Classification

Igneous Compositions

Igneous Compositions

Igneous Compositions

Plutonic Igneous Compositions

Igneous Compositions

Volcanic Igneous Compositions

Felsic Volcanic Compositions

Felsic Volcanic Compositions

Felsic Volcanic Compositions

Intermediate Volcanic Compositions

Intermediate Plutonic Compositions

Intermediate Plutonic Compositions

Igneous Compositions

Igneous Compositions

Igneous Compositions

Igneous Compositions

Origin of Magma

Origin of Magma

Origin of Magma

Origin of Magma

Origin of Magma

Origin of Magma

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

How Magmas Evolve

End of Chapter 3

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