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
Copyright (c) 2005 Pearson Education Canada, Inc.
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Magma: The Parent Material of
Igneous Rock
Igneous rocks form as molten rock cools
and solidifies
General Characteristic of magma
• Parent material of igneous rocks
• Forms from partial melting inside the Earth
• Magma that reaches the surface is called lava
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Magma: The Parent Material of
Igneous Rock
General Characteristic of Magma
• Generally formed by partial melting in Upper
Mantle (~1200° C) or Lower Crust (~850° C)
• Rocks formed from lava at the surface are
classified as extrusive or volcanic rocks
• Rocks formed from magma that crystallizes at
depth are termed intrusive or plutonic rocks
• Magmas are buoyant, gas laden & transport Heat
• 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
• Consists of three components:
– A liquid portion, called melt, that is composed of
mobile ions derived from the partial melting of
minerals
– Solids, if any, are silicate minerals that have
already crystallized from the melt:
– Phenocrysts are large, Microlites are small
– Volatiles, which are gases dissolved in the melt,
including water vapour (H2O), carbon dioxide
(CO2), sulphur dioxide (SO2) & minor HF, HCl,
He
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Magma: The Parent Material of
Igneous Rock
From Magma to Crystalline Rock
• Cooling of magma results in the systematic
arrangement of ions into orderly patterns,
cations + anions = minerals
• The silicate minerals resulting from
crystallization form in a predictable order
• 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
• Texture in igneous rocks is determined by
the size and arrangement of mineral crystals
• Igneous rocks are typically classified by
– Textures
– Mineral compositions & proportions
– Rocks with similar textures can have different
compositions (glasses all appear similar)
– 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:
• Cooling Rates
– Fast cooling forms glass or may tiny crystals (microlites)
– Slow cooling rates promote the growth of fewer larger phenocrysts
• Volatiles are Solvents
– Volatiles lower viscosity & increase diffusion leading to larger
crystals
• Nucleation
– 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
• Cooling Rate for magma
– 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)
• Amount of Silica (SiO2) present
– Mafic (Low silica) magmas like basalts (<50% SiO2 ) flow easily
– Felsic (High silica) magmas are stiff and explosive
• Nucleation of crystals
– Contamination with crustal rocks promotes nucleation
• Amount of Dissolved Gases
– Affects viscosity, diffusion and explosivity
– <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
• Aphanitic (fine-grained) texture  Volcanic!
– Rapid rate of cooling of lava or magma (in air, water)
– Microscopic crystals
– May contain vesicles (holes from gas bubbles) and thus rocks that
contain them have a vesicular texture
• Porphyritic = large phenocrysts & smaller groundmass
– Generally lava flows or sub-volcanic intrusions (dykes/sills)
– Phenocrysts grew slowly then eruption quenched the lava
• Phaneritic (coarse-grained) texture  Plutonic!
– Crystals can be identified without a microscope (>2mm)
– Generally caused by slow cooling (heat loss at depth)
• Pegmatitic (very coarse-grained) texture  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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Igneous Textures: Volcanic
Fe Staining: Weathering
Hand
Specimen
Andesite with twinned Plagioclase laths,
birefringent Augite, Magnetite & Glass
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Thin Section
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Igneous Textures: Plutonic
Granite with:
Pink K-Feldspar,
White Plagioclase,
Grey Quartz &
Black Biotite
Phaneritic
Biotite-birefringent, Qtz-grey/white
Feldspars-Twinned Black/white
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Igneous Textures: Volcanic
Types of igneous textures
• Porphyritic texture: 2 different crystal sizes
– Minerals form at different temperatures as well as
crystallizing at differing rates
– Large crystals, called phenocrysts, are embedded in a
matrix of smaller crystals, called the groundmass
– Sudden loss of volatiles can arrest crystallization
• Glassy texture (Vitreous & Conchoidal fracture)
– Very rapid cooling to volcanic rock in air or water
– This is common in very viscous, Hi-Silica magmas
– 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
• Pyroclastic texture (volcanic)
– Various fragments ejected during a violent volcanic
eruption (rocks, crystals, glass shards, foams)
– Textures often appear more similar to sedimentary
rocks, but usually angular & partly glassy
• Pegmatitic texture (plutonic)
– Exceptionally coarse-grained
– Form in late stages of crystallization of magmas
• Xenolithic / Xenocrystic texture (plutonic)
– Accidental rock fragments from mantle or crust
– 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
• Dark (or ferromagnesian) silicates, ∑= Colour Index
• Depends on Mg + Fe content of magma
• Crystallize in order of falling Temperature
• May react with magma to form a lower T°C phase
– Spinel or Magnetite (oxides not silicates) Fe3O4
– Olivine (Mg,Fe)SiO4 , lone tetrahedra
– Pyroxene Ca(Mg,Fe)Si2O6 , single chains
– Amphibole Ca2(Mg,Fe)5Si8O22(OH)2 , double chains
– Biotite mica K(Mg,Fe)3(Al,Si)3O10(OH,F)2 , sheets
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Igneous Compositions
Igneous rocks are composed primarily of silicate
minerals
• Light (or nonferromagnesian) silicates (with falling
Temperature)
– Plagioclase Feldspar: framework silicate
– Anorthite CaAl2 Si2O8 to Albite NaAlSi3O8
– K-Feldspar KAlSi3O8 , framework silicate
– Quartz SiO2 , framework silicate
– Muscovite mica K(Mg,Fe)3(Al,Si)3O10(OH,F)2 , sheets,
(this mineral is only found in plutonic rocks)
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Igneous Compositions
Felsic versus Mafic Compositions
• Rhyolitic composition
– Common in explosive strato-volcanoes (arcs)
• Granitic composition
– Composed of light-coloured silicates
– Designated as being felsic (feldspar and silica) in
composition
– Contains high amounts of silica (SiO2), >68%
– Low temperature melts but high viscosity
– Major rock type in continental crust
– Common in batholiths of continental margin arcs
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Igneous Compositions
Mafic versus Felsic Compositions
• Basaltic (or Gabbroic) composition
– Composed of dark silicates and calcium-rich
feldspar
– Designated as being mafic (magnesium and
ferrum, for iron) in composition
– High Temperature magmas but low viscosity
– More dense than granitic rocks
– Comprise the ocean crust as well as many
volcanic islands
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Igneous Compositions
Other compositional groups
• Intermediate (or andesitic) composition
– Contain at least 25 percent dark silicate minerals
– Associated with explosive volcanic activity
– Present in arc volcanoes and plutons
• Ultramafic composition
– Composition that is high in MgO and FeO > 55%
– Dense, high Temperature, Low viscosity melts
– Composed entirely of >90% ferromagnesian silicates
– Common in mantle (plutonic) but rare in crust
– More common in lower crust and in Precambrian rocks
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Igneous Compositions
Silica Content as an Indicator of Composition
• Silica content in crustal rocks exhibits a
considerable range
– A lower than 45% in ultramafic rocks
– Over 75% percent in some felsic rocks
– Tends to increase during fractional crystallization
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Igneous Rock Classification
Rock Names
Depend on
Mineral %’s
&
Textures
Plutonic/Volcanic
Mineralogy
of common
igneous
rocks
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Canada
Inc. and the magmas from which they form.
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Igneous Compositions
Silica content influences a magma’s behaviour
• Granitic magma
– High silica content > 68%
– Extremely viscous, flows slowly, explosive
– Liquid exists at temperatures as low as 700oC
– Forms by differentiation from more Mafic magmas
– Can also form by partial melting of Lower Crust in
collisional orogens
– Often high volatile contents: H2O, CO2 etc.
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Igneous Compositions
Silica content influences a magma’s behaviour
• Basaltic magma
– Much lower silica content <54%
– Fluid-like behaviour
– Low volatile content < a few %, usually not explosive
– Crystallizes at higher temperatures
– Most common Magma on Earth (or Moon!)
– Partial melt of Peridotite (5% to 25%)
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Igneous Compositions
Naming Igneous Rocks – Felsic (Granitic) Rocks
• Granite
– Phaneritic
– Over 25 percent quartz, about 65 percent or more
feldspar
– May exhibit a porphyritic texture
– Very abundant as it is often associated with mountain
building
– The term granite covers a wide range of mineral
compositions but mostly alkali feldspar & quartz
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Plutonic Igneous Compositions
Granite:
K-spar > Plagioclase
CI < 15
Collisional varieties
have 2 micas
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Igneous Compositions
Naming Igneous Rocks – Felsic (Granitic) Rocks
• Rhyolite
– Extrusive equivalent of granite
– Found in stratovolcanoes & calderas
– May contain glass fragments and vesicles
– Aphanitic texture
– Less common and less voluminous than granite
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Volcanic Igneous Compositions
Rhyolite
Aphanitic
may be any colour
Vesicles & glass
are common
May contain
Quartz phenocrysts
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Felsic Volcanic Compositions
Naming Igneous Rocks – Felsic (Granitic) Rocks
• Obsidian
– Dark-coloured, often flow banded
– Glassy texture, conchoidal fracture
– Gas bubbles (vesicles) are common but flattened
• Pumice
– Low density & Light Coloured Vesicular Volcanic
– Glassy texture with few if any crystals
– Frothy appearance with numerous voids
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Felsic Volcanic Compositions
Obsidian
with
palagonite
(Fe-clays)
in fractures
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Felsic Volcanic Compositions
Pumice
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Intermediate Volcanic Compositions
Naming Igneous Rocks – Intermediate
(Andesitic) Rocks
• Andesite
– Volcanic origin usually in arc stratovolcanoes
– Aphanitic or aphanitic-porphyritic texture
– Often crystal rich with 25% to 40% phenocrysts
– Essential Plagioclase with Pyroxene or Hornblende
– Often resembles rhyolite when pyroclastic (tuffaceous)
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Intermediate
Volcanic
Compositions
Felsic Volcanic
Compositions
Andesite
From Black to White!
Abundant Plagioclase
phenocrysts with small
hornblende microphenocrysts.
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
• Diorite
– Plutonic equivalent of andesite
– Coarse-grained
– Intrusive (like in the roots of the Andes!)
– Composed mainly of intermediate plagioclase feldspar
and amphibole
– K-feldspar is minor if present, < 1/3 of total feldspar
– 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
• Basalt
– Volcanic origin
– Dark green to black in colour
– Aphanitic texture
– Composed mainly of pyroxene and calciumrich plagioclase feldspar
– Most common extrusive igneous rock
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Igneous Compositions
Basalt
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Igneous Compositions
Naming Igneous Rocks – Mafic (Basaltic)
Rocks
• Gabbro
– Intrusive equivalent of basalt
– Phaneritic texture consisting of pyroxene and
calcium-rich plagioclase
– Makes up a significant percentage of the
oceanic crust
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Igneous Compositions
Naming Igneous Rocks – Pyroclastic Rocks
• Composed of fragments ejected during a
volcanic eruption
• Varieties
– Tuff – ash-sized fragments
– Volcanic breccia – particles larger than ash
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Origin of Magma
Highly debated topic
Generating magma from solid rock
• Produced from partial melting of rocks in
the crust and upper mantle
• Role of Temperature
– Temperature increases within Earth’s upper
crust (called the geothermal gradient) average
between 20oC to 30oC per kilometre
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Origin of Magma
Estimated temperatures in the crust and mantle.
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Origin of Magma
• Role of Temperature
– Rocks in the lower crust and upper mantle are
near their melting points
– 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
• Role of Pressure
– An increase in confining pressure causes an
increase in a rock’s melting temperature or
conversely, reducing the pressure lowers the
melting temperature
– When confining pressures drop,
decompression melting occurs
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Origin of Magma
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Decompression melting
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Origin of Magma
• Role of volatiles
– Volatiles (primarily water) cause rocks to melt
at lower temperatures
– 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
• 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
• During crystallization, the composition of
the liquid portion of the magma
continually changes
– Composition changes due to removal of
elements by earlier-forming minerals
– The silica component of the melt becomes
enriched as crystallization proceeds
– Minerals in the melt can chemically react and
change
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How Magmas Evolve
Processes responsible for changing a
magma’s composition
• Magmatic differentiation
– Separation of a melt from earlier formed
crystals to form a different composition of
magma
• Assimilation
– 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
• Magma mixing
– Involves two bodies of magma intruding one
another
– 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
• Incomplete melting of rocks is known as
partial melting
• Formation of a Mafic Magma (basaltic)
– Most originate from partial melting of
ultramafic rock in the mantle
– Basaltic magmas form at mid-ocean ridges by
decompression melting or at subduction zones
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How Magmas Evolve
Partial Melting and Magma Formation
• Formation of Basaltic Magmas
– As basaltic magmas migrate upward,
confining pressure decreases which reduces
the melting temperature
– Large outpourings of basaltic magma are
common at Earth’s surface
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How Magmas Evolve
Partial Melting and Magma Formation
• Formation of Andesitic Magmas
– Interactions between mantle-derived basaltic
magmas and more silica-rich rocks in the crust
generate magma of andesitic composition
– Andesitic magma may also evolve by
magmatic differentiation
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How Magmas Evolve
Partial Melting and Magma Formation
• Formation of Felsic (granitic) Magmas
– Most likely form as the end product of
crystallization of andesitic magma
– Granitic magmas are higher in silica and
therefore more viscous than other magmas
– Because of their viscosity, they lose their
mobility before reaching the surface
– Tend to produce large plutonic structures
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End of
Chapter 3
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