Magmas & Igneous Rocks Associate Professor John Worden DEC University of Southern Qld

Magmas & Igneous RocksMagmas & Igneous Rocks

Associate Professor John Worden Associate Professor John Worden DECDEC

University of Southern QldUniversity of Southern Qld

Magma & Igneous RocksMagma & Igneous Rocks

An An Igneous rockIgneous rock is a “crystalline or glassy”rock that formed directly is a “crystalline or glassy”rock that formed directly from a “from a “MAGMAMAGMA”.”.

Magma DefinitionMagma Definition:: ““A mA mixture of molten rock, suspended mineral grains and dissolved gases ixture of molten rock, suspended mineral grains and dissolved gases

that forms in the Crust or Mantle at high Tthat forms in the Crust or Mantle at high T”.”. Characterised by a range of compositions- (45Characterised by a range of compositions- (45 to to ~~75 wt% SiO75 wt% SiO22)) Compositions determined by elements inCompositions determined by elements in the the Earth, ie Si, Al, Fe, Ca, Earth, ie Si, Al, Fe, Ca,

Mg, Na, K, H, and OMg, Na, K, H, and O22.. Volatiles minor (0.2-5.0 wt%), with HVolatiles minor (0.2-5.0 wt%), with H22O dominant.O dominant. HH22O + COO + CO22 together make up 98% of volatiles. together make up 98% of volatiles. Remaining 2%Remaining 2% comprises comprises N N22 , , ClCl22 , S, and Ar., S, and Ar. As Magma cools and crystallises, it reorganises intoAs Magma cools and crystallises, it reorganises into

minerals that individually have simpler chemistry minerals that individually have simpler chemistry than the parental magma. than the parental magma.

Magmas & Igneous RocksMagmas & Igneous Rocks Direct evidence of Magmas provided by modern “Lava flows”.Direct evidence of Magmas provided by modern “Lava flows”.

Three Magma Types most common-Basaltic, Andesitic, and Rhyolitic.Three Magma Types most common-Basaltic, Andesitic, and Rhyolitic. Basaltic:Basaltic:

• Contains 45-50 wt% SiOContains 45-50 wt% SiO22 & very little dissolved gas; & very little dissolved gas;• Low Viscosity (100-300 poise) at 1200-1400 Low Viscosity (100-300 poise) at 1200-1400 °° C; andC; and• Chills to Basalt or Gabbro.Chills to Basalt or Gabbro.

Andesitic:Andesitic:• Contains 55-60 wt% SiOContains 55-60 wt% SiO2 2 & considerable dissolved gas; & considerable dissolved gas;• Moderate Viscosity (10Moderate Viscosity (1044 -10 -1055 poise) at 1200 poise) at 1200 °° C; andC; and• Cools to Andesite or Diorite.Cools to Andesite or Diorite.

Rhyolitic:Rhyolitic:• Contains 70-75 wt% SiOContains 70-75 wt% SiO22 , High content of gas. , High content of gas.• Very high Viscosity (10Very high Viscosity (1088 -10 -101010 poise) at 800-1000 poise) at 800-1000 °° C; C;• Cools to Rhyolite or Granite.Cools to Rhyolite or Granite.


Viscosity:Viscosity: Controls resistance to flowControls resistance to flow.. Both T and Composition dependent; especially on SiOBoth T and Composition dependent; especially on SiO22 content. content.

Higher the SiOHigher the SiO22 content, the more viscous the magma. content, the more viscous the magma.

Temperature:Temperature: Magma T range from ~ 750Magma T range from ~ 750°° C to C to ~~ 1200 1200°° C, on C, on eruption.eruption. The higher the T, the less viscous the magma, & the more it will flow.The higher the T, the less viscous the magma, & the more it will flow. Basaltic magmas T ~ 1200Basaltic magmas T ~ 1200°° C. Form Lava flows. C. Form Lava flows. Rhyolitic magmas T ~ 750Rhyolitic magmas T ~ 750°° C. C. Pyroclastic sheets. Pyroclastic sheets.

AbundanceAbundance of Magma type: of Magma type: Basaltic ~Basaltic ~ 80% ; Andesitic ~ 10% ;80% ; Andesitic ~ 10% ; Rhyolitic ~ 10%Rhyolitic ~ 10%


Volcanoes and Eruption:Volcanoes and Eruption: Erupted magma at Earth’s surface is termed Erupted magma at Earth’s surface is termed ““LLAVA”AVA”.. Lava erupted from vents termed Lava erupted from vents termed ““VVolcanoolcanoes”es”.. Magmas rise through Crust since they are less dense than solid rock.Magmas rise through Crust since they are less dense than solid rock. Confining P lessens as magma rises, (P Confining P lessens as magma rises, (P to depth). to depth). P controls amount of dissolved gases.P controls amount of dissolved gases. As magma rises, P decreases, and gases As magma rises, P decreases, and gases exsolveexsolve forming gas bubbles. forming gas bubbles. ExsolvedExsolved gas controls explosive nature of magma. gas controls explosive nature of magma.

• Basaltic - low viscosity, little gas, rarely explosive.Basaltic - low viscosity, little gas, rarely explosive.

• Forms smooth ropy-surfaced lava flows termedForms smooth ropy-surfaced lava flows termed ‘PAHOEHOE’‘PAHOEHOE’ lava. lava.

• As flow chills, lava changes to blocky & rubbly =‘As flow chills, lava changes to blocky & rubbly =‘AAAA’.’.

• Trapped gas bubbles termed Trapped gas bubbles termed ‘‘VESICLESVESICLES’’..


Explosive Eruptions:Explosive Eruptions: Viscous magmas (Andesitic &Viscous magmas (Andesitic & Rhyolitic) have higher dissolved gases.Rhyolitic) have higher dissolved gases. Quantity of gas controls violence of eruption & speed of gas unmixing.Quantity of gas controls violence of eruption & speed of gas unmixing. If magma rise rapid, gas bubbles can shatter magma.If magma rise rapid, gas bubbles can shatter magma. Magma fragments termed ‘Magma fragments termed ‘PYROCLASTSPYROCLASTS’.’. Ejected pyroclasts + glass shards + ash= Deposits of ‘Ejected pyroclasts + glass shards + ash= Deposits of ‘TEPHRATEPHRA’ ( ie ’ ( ie

Tuff, etc)Tuff, etc) Consolidated Tephra = Pyroclastic rocks.Consolidated Tephra = Pyroclastic rocks. Hot gas & Pyroclasts lead to explosive eruptions.Hot gas & Pyroclasts lead to explosive eruptions. Pyroclastic flows orPyroclastic flows or ‘‘Nuée ArdenteNuée Ardente’’

• ““Glowing clouds” move at speeds up to 700 km/hr.Glowing clouds” move at speeds up to 700 km/hr.

• Fluidised suspension of rock, hot gases, etc. Fluidised suspension of rock, hot gases, etc.


Volcano types:Volcano types: Volcano type controlled by magma type.Volcano type controlled by magma type. Three main types:Three main types:

• Shield volcanoShield volcano

• Tephra Cone volcanoTephra Cone volcano

• StratovolcanoStratovolcano

Shield volcano has gentle slopes, dome shaped & a pile of lava flows.Shield volcano has gentle slopes, dome shaped & a pile of lava flows.• Examples- Hawaii, and Tahiti.Examples- Hawaii, and Tahiti.

• Formed from basaltic lavaFormed from basaltic lava

• % of pyroclasts & ash is small.% of pyroclasts & ash is small.

Tephra Cone volcano is steep- sided around vent.Tephra Cone volcano is steep- sided around vent.• Consists of tephra depositsConsists of tephra deposits

• Size of tephra controls cone slopesSize of tephra controls cone slopes

• Characteristic of Rhyolitic & Andesitic lavas.Characteristic of Rhyolitic & Andesitic lavas.


StratoStrato--volcanoes are large, long-lived, classic-shaped conical volcanoes are large, long-lived, classic-shaped conical mounds:mounds: Examples: Mt Fuji, Japan, & Mt Hood, Washington State, USA.Examples: Mt Fuji, Japan, & Mt Hood, Washington State, USA. Consist of layers of tephra and lava flows with tephra Consist of layers of tephra and lava flows with tephra flows. flows. Particularly Andesitic lava volcanoes.Particularly Andesitic lava volcanoes. Thousands of metres high and slope angles from 6Thousands of metres high and slope angles from 6oo-30-30oo.. Often have aOften have a ‘ ‘CraterCrater’’ or or ‘‘CCalderaaldera’ at summit, formed by settling and ’ at summit, formed by settling and

collapse of partly evacuated underlying magma chamber.collapse of partly evacuated underlying magma chamber. Subsequent eruptions from ring fractures.Subsequent eruptions from ring fractures.

Fissure eruptions:Fissure eruptions: Lava reaches surface by longLava reaches surface by long fractures.fractures. Extremely large volumes of lava extruded.Extremely large volumes of lava extruded. Examples: Deccan TrapsExamples: Deccan Traps India; India; Siberian traps Siberian traps, CIS, CIS, , etc.etc.


Magma Intrusion:Magma Intrusion: Not all magma extruded, much magma is intruded into Crust & Not all magma extruded, much magma is intruded into Crust &

Mantle.Mantle. Texture and grain size of minerals indicate how rapidly & at what Texture and grain size of minerals indicate how rapidly & at what

depth rock formed.depth rock formed. Intrusive bodies of Igneous rock termed ‘Intrusive bodies of Igneous rock termed ‘PlutonsPlutons’.’. Size and shape govern terms applied to particular plutons:Size and shape govern terms applied to particular plutons:

• ‘‘StocksStocks’- Igneous bodies < 10 km in diameter.’- Igneous bodies < 10 km in diameter.

• ‘‘BatholithsBatholiths’- Igneous bodies ’- Igneous bodies >> 1 1000 km in diameter.0 km in diameter.

• ‘‘SillsSills’- Concordant bodies, parallel to rock layers.’- Concordant bodies, parallel to rock layers.

• ‘‘DykesDykes’- Discordant bodies, cutting across rock layers.’- Discordant bodies, cutting across rock layers.

• ‘‘Volcanic pipes’Volcanic pipes’- Cylindrical conduits beneath volcano.- Cylindrical conduits beneath volcano.

Magmas & Igneous RocksMagmas & Igneous Rocks Nature of contacts:Nature of contacts:

• Gradational-Gradational- reflects strong chemical interaction between magma and reflects strong chemical interaction between magma and country rock & little T contrast. Equates to “Slow Cooling” at depth.country rock & little T contrast. Equates to “Slow Cooling” at depth.

• SharpSharp- indicates lack of chemical reaction between magma and country - indicates lack of chemical reaction between magma and country rock. Due either to:rock. Due either to:

– presence of relatively unreactive country rock (ie Quartzite), or

– rapid cooling/ chilling of magma against cool country rock.

• Large T contrast reflected in smaller “grain size” within igneous rock Large T contrast reflected in smaller “grain size” within igneous rock near contact = “ near contact = “ Chilled MarginChilled Margin”.”.

• Concordance or DiscordanceConcordance or Discordance

– Required to differentiate between Sills & Dykes.

Magmas & Igneous Magmas & Igneous RRocksocks Origin of Magmas:Origin of Magmas:

Where do magmas form, and why do the form?Where do magmas form, and why do the form? Virtually all magmas generated within outer 250 km of the Earth by Virtually all magmas generated within outer 250 km of the Earth by

melting solid mineral assemblages.melting solid mineral assemblages. Magmas form in three main regions:Magmas form in three main regions:

• In the Mantle beneath Oceanic Spreading Ridges. Oceanic Crust under tension, In the Mantle beneath Oceanic Spreading Ridges. Oceanic Crust under tension, pulls apart, and magma rises in response to convection cell heating.pulls apart, and magma rises in response to convection cell heating.

• At Convergent Plate Margins above sinking subducted Oceanic Crust. Sinking At Convergent Plate Margins above sinking subducted Oceanic Crust. Sinking slab progressively heated as it plunges into the deeper Mantle. Most volcanoes slab progressively heated as it plunges into the deeper Mantle. Most volcanoes near Convergent Plate Margins, ie Andes .near Convergent Plate Margins, ie Andes .

• At Hot Spots. Rising Mantle Plume of thermalised rockAt Hot Spots. Rising Mantle Plume of thermalised rock from Core/ Mantle Boundary. ie Hawaiian Island chain. from Core/ Mantle Boundary. ie Hawaiian Island chain.

Melting due to P release &/ or involvement of fluidsMelting due to P release &/ or involvement of fluidsduring mantle convection over great depth range ofduring mantle convection over great depth range of 10-100 km. 10-100 km.

Magma & Igneous rocksMagma & Igneous rocks

Why and how do Magmas form?Why and how do Magmas form? N.N.L L Bowen discovered with Lab experiments that minerals crystallise Bowen discovered with Lab experiments that minerals crystallise

in a specific sequence, as a magma cools.in a specific sequence, as a magma cools. Furthermore, first formed minerals react with the cooler residual Furthermore, first formed minerals react with the cooler residual

magma, to form different minerals, also in a set sequence.magma, to form different minerals, also in a set sequence. Sequence termed Sequence termed Bowen’s Reaction SeriesBowen’s Reaction Series.. Bowen reasoned that the melting of a single composition ( ie basalt), Bowen reasoned that the melting of a single composition ( ie basalt),

would account for different magma types by fractional crystallisation.would account for different magma types by fractional crystallisation. Minerals settle to bottom of magma by gravity.Minerals settle to bottom of magma by gravity. Residual magma changes composition yielding:Residual magma changes composition yielding:

• Basaltic Basaltic Andesitic Andesitic Rhyolitic Magmas. Rhyolitic Magmas.

• Early-formed crystals removed from magma contact.Early-formed crystals removed from magma contact.


Alternatively, fractional melting explains magma types:Alternatively, fractional melting explains magma types: Reverse of Bowen’s approach.Reverse of Bowen’s approach. Does produce three magma types.Does produce three magma types. Lab experimental evidence, and distribution of different volcanoes Lab experimental evidence, and distribution of different volcanoes

worldwide, confirm close relationship with Plate Tectonic Margin type.worldwide, confirm close relationship with Plate Tectonic Margin type.• Rhyolitic magma only known from Continental Crust. Absent in Oceanic Crust.Rhyolitic magma only known from Continental Crust. Absent in Oceanic Crust.

• Suggests does not form inSuggests does not form in the the Mantle. Mantle.

• Andesitic lavas found on both Crust types. Formed in Mantle, but independent Andesitic lavas found on both Crust types. Formed in Mantle, but independent of overlying Crust?of overlying Crust?

• Andesitic lavas restricted to subducting Oceanic Crust?Andesitic lavas restricted to subducting Oceanic Crust?

• Andesites derived from partial melting of subducted Andesites derived from partial melting of subducted Oceanic Crust . Oceanic Crust .

Magma & Igneous rocksMagma & Igneous rocks Origin of Magma:Origin of Magma:

Volcanoes erupting basaltic magma occur on both types of Crust.Volcanoes erupting basaltic magma occur on both types of Crust. Must therefore be sourced Must therefore be sourced from thefrom the Mantle. Mantle. Not geographically-restricted like Andesites, suggesting melting of Not geographically-restricted like Andesites, suggesting melting of

Mantle itself.Mantle itself. Therefore, conclude that basaltic magma forms from partial melting of Therefore, conclude that basaltic magma forms from partial melting of

largely anhydrous & gas-poor Mantle. largely anhydrous & gas-poor Mantle. Andesitic magma sourced from partial melting of subducted Oceanic Andesitic magma sourced from partial melting of subducted Oceanic

Crust (including thin, & hydrous sediment drape).Crust (including thin, & hydrous sediment drape). Rhyolitic magma forms by fractional melting of :Rhyolitic magma forms by fractional melting of :

• Continental Crust. Gas rich (HContinental Crust. Gas rich (H22O + COO + CO22).).• Mantle-Derived Basaltic magma under-plates CrustMantle-Derived Basaltic magma under-plates Crust

causing partial melting & generation of Rhyolitic causing partial melting & generation of Rhyolitic Magma. Magma.

Magma & Igneous rocksMagma & Igneous rocks Naming Igneous Rocks:Naming Igneous Rocks:

Igneous rocks form from cooling and solidification of magma.Igneous rocks form from cooling and solidification of magma. Extrusive igneous rocks form from solidification of lavas.Extrusive igneous rocks form from solidification of lavas. Intrusive igneous rocks form when magma solidifies within the Crust Intrusive igneous rocks form when magma solidifies within the Crust

or Mantle.or Mantle. Both extrusive &Both extrusive & intrusive rocks are classified on the basis of rock intrusive rocks are classified on the basis of rock

texture and mineral assemblage.texture and mineral assemblage. Texture:Texture:

• Refers to the size and arrangement of mineral grains.Refers to the size and arrangement of mineral grains.• Grain size- consequence of Cooling History/Grain size- consequence of Cooling History/ Rates.Rates.• Rocks with grain sizes Rocks with grain sizes 1cm termed “ 1cm termed “PegmatitesPegmatites”.”.• Extrusive rocks are fine-grained (< 1mm).Extrusive rocks are fine-grained (< 1mm).• Rapid cooling, too little time to grow large grains.Rapid cooling, too little time to grow large grains.• May even be ‘Glassy’ with no grains, ie “May even be ‘Glassy’ with no grains, ie “ObsidianObsidian”.”.


Texture:Texture: Intrusive rocks are coarse-grained, equigranular as magma cooled slowly Intrusive rocks are coarse-grained, equigranular as magma cooled slowly

in Crust/ Mantle.in Crust/ Mantle. Sufficient time for large grain growth.Sufficient time for large grain growth. Earliest crystallising minerals possess excellent crystal shape, later have Earliest crystallising minerals possess excellent crystal shape, later have

partial crystal shape, and last minerals have no crystal shape. (partial crystal shape, and last minerals have no crystal shape. (DDue to ue to space limitations in cooling magma).space limitations in cooling magma).

Occasionally large + small crystals together= ‘Occasionally large + small crystals together= ‘PorphyriticPorphyritic’’ texture. texture. Large crystals= ‘Large crystals= ‘phenocrystsphenocrysts’ (slow cooling).’ (slow cooling). Small crystals=Small crystals= ‘‘groundmassgroundmass’ (rapid cooling).’ (rapid cooling).

• Large grains represent slow cooling in magma chamber.Large grains represent slow cooling in magma chamber.

• Small grains formed on eruption of crystal-laden magma.Small grains formed on eruption of crystal-laden magma.

• Two stage/step cooling in rock crystallisation.Two stage/step cooling in rock crystallisation.


Pyroclastic rocks:Pyroclastic rocks: Produced byProduced by explosive volcanism. explosive volcanism. Can be formed fromCan be formed from rock fragments of pre-existing volcanic rocks. rock fragments of pre-existing volcanic rocks. Also fromAlso from crystal fragments, and/or combinations of both. crystal fragments, and/or combinations of both. May also have characteristics ofMay also have characteristics of Sedimentary rocks, Sedimentary rocks, ie be layered, etc.ie be layered, etc. Termed ‘Termed ‘AgglomerateAgglomerate’ when tephra ’ when tephra areare large large (ie, bomb-sized).(ie, bomb-sized). When fragments are small, termedWhen fragments are small, termed ‘ ‘TTuffuff’ ’ (i.e, ash).(i.e, ash). If fragments large & angular( If fragments large & angular( 2mm), 2mm), termed atermed a

“ “Volcanic Volcanic BrecciaBreccia””.. Named after predominant fragment/ clast type.Named after predominant fragment/ clast type. Sometimes tuffs areSometimes tuffs are ‘ ‘weldedwelded’’ by by hot volcanic glass hot volcanic glass

fragments. fragments.


Mineral Assemblages:Mineral Assemblages: All common igneous rocks composed of one or more, ofAll common igneous rocks composed of one or more, of the the six six

primary minerals/ mineral groups-primary minerals/ mineral groups- Quartz.Quartz. Felspar (K-felspar/Orthoclase, Felspar (K-felspar/Orthoclase, and/orand/or Plagioclase). Plagioclase). Mica (Muscovite Mica (Muscovite and/orand/or Biotite). Biotite). Amphibole (Hornblende).Amphibole (Hornblende). Pyroxene (Augite).Pyroxene (Augite). OlivineOlivine Together with Texture, these used toTogether with Texture, these used to name rocksname rocks..

• ie, Presence/ Absence of Quartz or Olivineie, Presence/ Absence of Quartz or Olivine• REMEMBER- Olivine and Quartz are incompatibleREMEMBER- Olivine and Quartz are incompatible..


Examples:Examples: Assemblage Intrusive Extrusive Assemblage Intrusive Extrusive

Qtz/ no Ol Qtz, Fels, Mi, Hb Granite RhyoliteQtz/ no Ol Qtz, Fels, Mi, Hb Granite Rhyolite No Qtz/ no Ol Fels, Hb, Aug, Mi Diorite AndesiteNo Qtz/ no Ol Fels, Hb, Aug, Mi Diorite Andesite No Qtz/ Ol Fels, Aug, Ol, Mi Gabbro BasaltNo Qtz/ Ol Fels, Aug, Ol, Mi Gabbro Basalt

Qtz = Quartz; Ol = Olivine; Fels = Felspar; Mi = Mica;Qtz = Quartz; Ol = Olivine; Fels = Felspar; Mi = Mica; Hb = Hornblende; Aug = Augite. Hb = Hornblende; Aug = Augite.

By combining two mineral discriminators & grain By combining two mineral discriminators & grain size parameters, these six igneous rocks can besize parameters, these six igneous rocks can be identified. identified.


Streckeisen Classification:Streckeisen Classification: ““New” 1973 scheme for naming igneous rocks;New” 1973 scheme for naming igneous rocks; Based on vol % Quartz (Q)- Alkali felspar (A)- Plagioclase (P) Based on vol % Quartz (Q)- Alkali felspar (A)- Plagioclase (P)

re-calculated to 100% and plotted on a ‘ternary diagram’;re-calculated to 100% and plotted on a ‘ternary diagram’; New terms for Intrusive rocks- “New terms for Intrusive rocks- “TonaliteTonalite” & “” & “MonzoniteMonzonite”;”; Note the clustering of Basic & Ultrabasic rocks at the ‘P’ apex Note the clustering of Basic & Ultrabasic rocks at the ‘P’ apex

– poorly resolved!– poorly resolved! Additional Ternary diagrams required to Additional Ternary diagrams required to

name these rocks.name these rocks. See last diagrams.See last diagrams.

Magmas & Magmatic RocksMagmas & Magmatic Rocks

Magmas & Magmatic RocksMagmas & Magmatic Rocks

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Magmas & Igneous Rocks Associate Professor John Worden DEC University of Southern Qld