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Magmas & Igneous Rocks
Associate Professor John Worden
DEC
University of Southern Qld
Magma & Igneous Rocks
 An Igneous rock is a “crystalline or glassy”rock that formed directly
from a “MAGMA”.
 Magma Definition:
 “A mixture of molten rock, suspended mineral grains and dissolved gases
that forms in the Crust or Mantle at high T”.
 Characterised by a range of compositions- (45 to ~75 wt% SiO2)
 Compositions determined by elements in the Earth, ie Si, Al, Fe, Ca,
Mg, Na, K, H, and O2.
 Volatiles minor (0.2-5.0 wt%), with H2O dominant.
 H2O + CO2 together make up 98% of volatiles.
 Remaining 2% comprises N2 , Cl2 , S, and Ar.
 As Magma cools and crystallises, it reorganises into
minerals that individually have simpler chemistry
than the parental magma.
Magmas & Igneous Rocks
 Direct evidence of Magmas provided by modern “Lava flows”.
Three Magma Types most common-Basaltic, Andesitic, and Rhyolitic.
 Basaltic:
• Contains 45-50 wt% SiO2 & very little dissolved gas;
• Low Viscosity (100-300 poise) at 1200-1400 ° C; and
• Chills to Basalt or Gabbro.
 Andesitic:
• Contains 55-60 wt% SiO2 & considerable dissolved gas;
• Moderate Viscosity (104 -105 poise) at 1200 ° C; and
• Cools to Andesite or Diorite.
 Rhyolitic:
• Contains 70-75 wt% SiO2 , High content of gas.
• Very high Viscosity (108 -1010 poise) at 800-1000 ° C;
• Cools to Rhyolite or Granite.
Magmas & Igneous Rocks
 Viscosity:
 Controls resistance to flow.
 Both T and Composition dependent; especially on SiO2 content.
 Higher the SiO2 content, the more viscous the magma.
 Temperature:
 Magma T range from ~ 750° C to ~ 1200° C, on eruption.
 The higher the T, the less viscous the magma, & the more it will flow.
 Basaltic magmas T ~ 1200° C. Form Lava flows.
 Rhyolitic magmas T ~ 750° C.  Pyroclastic sheets.
 Abundance of Magma type:
 Basaltic ~ 80% ; Andesitic ~ 10% ;
 Rhyolitic ~ 10%
Magmas & Igneous Rocks
 Volcanoes and Eruption:
 Erupted magma at Earth’s surface is termed “LAVA”.
 Lava erupted from vents termed “Volcanoes”.
 Magmas rise through Crust since they are less dense than solid rock.
 Confining P lessens as magma rises, (P  to depth).
 P controls amount of dissolved gases.
 As magma rises, P decreases, and gases exsolve forming gas bubbles.
 Exsolved gas controls explosive nature of magma.
• Basaltic - low viscosity, little gas, rarely explosive.
• Forms smooth ropy-surfaced lava flows termed
‘PAHOEHOE’ lava.
• As flow chills, lava changes to blocky & rubbly =‘AA’.
• Trapped gas bubbles termed ‘VESICLES’.
Magmas & Igneous Rocks
 Explosive Eruptions:
 Viscous magmas (Andesitic & Rhyolitic) have higher dissolved gases.
 Quantity of gas controls violence of eruption & speed of gas unmixing.
 If magma rise rapid, gas bubbles can shatter magma.
 Magma fragments termed ‘PYROCLASTS’.
 Ejected pyroclasts + glass shards + ash= Deposits of ‘TEPHRA’ ( ie
Tuff, etc)
 Consolidated Tephra = Pyroclastic rocks.
 Hot gas & Pyroclasts lead to explosive eruptions.
 Pyroclastic flows or ‘Nuée Ardente’
• “Glowing clouds” move at speeds up to 700 km/hr.
• Fluidised suspension of rock, hot gases, etc.
Magmas & Igneous Rocks
 Volcano types:
 Volcano type controlled by magma type.
 Three main types:
• Shield volcano
• Tephra Cone volcano
• Stratovolcano
 Shield volcano has gentle slopes, dome shaped & a pile of lava flows.
• Examples- Hawaii, and Tahiti.
• Formed from basaltic lava
• % of pyroclasts & ash is small.
 Tephra Cone volcano is steep- sided around vent.
• Consists of tephra deposits
• Size of tephra controls cone slopes
• Characteristic of Rhyolitic & Andesitic lavas.
Magmas & Igneous Rocks
 Strato-volcanoes are large, long-lived, classic-shaped conical
mounds:
 Examples: Mt Fuji, Japan, & Mt Hood, Washington State, USA.
 Consist of layers of tephra and lava flows with tephra  flows.
 Particularly Andesitic lava volcanoes.
 Thousands of metres high and slope angles from 6o-30o.
 Often have a ‘Crater’ or ‘Caldera’ at summit, formed by settling and
collapse of partly evacuated underlying magma chamber.
 Subsequent eruptions from ring fractures.
 Fissure eruptions:
 Lava reaches surface by long fractures.
 Extremely large volumes of lava extruded.
 Examples: Deccan Traps India; Siberian traps, CIS, etc.
Magmas & Igneous Rocks
 Magma Intrusion:
 Not all magma extruded, much magma is intruded into Crust &
Mantle.
 Texture and grain size of minerals indicate how rapidly & at what
depth rock formed.
 Intrusive bodies of Igneous rock termed ‘Plutons’.
 Size and shape govern terms applied to particular plutons:
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‘Stocks’- Igneous bodies < 10 km in diameter.
‘Batholiths’- Igneous bodies > 100 km in diameter.
‘Sills’- Concordant bodies, parallel to rock layers.
‘Dykes’- Discordant bodies, cutting across rock layers.
‘Volcanic pipes’- Cylindrical conduits beneath volcano.
Magmas & Igneous Rocks
Nature of contacts:
• Gradational- reflects strong chemical interaction between magma and
country rock & little T contrast. Equates to “Slow Cooling” at depth.
• Sharp- indicates lack of chemical reaction between magma and country
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
near contact = “ Chilled Margin”.
• Concordance or Discordance
– Required to differentiate between Sills &
Dykes.
Magmas & Igneous Rocks
 Origin of Magmas:
 Where do magmas form, and why do the form?
 Virtually all magmas generated within outer 250 km of the Earth by
melting solid mineral assemblages.
 Magmas form in three main regions:
• In the Mantle beneath Oceanic Spreading Ridges. Oceanic Crust under tension,
pulls apart, and magma rises in response to convection cell heating.
• At Convergent Plate Margins above sinking subducted Oceanic Crust. Sinking
slab progressively heated as it plunges into the deeper Mantle. Most volcanoes
near Convergent Plate Margins, ie Andes .
• At Hot Spots. Rising Mantle Plume of thermalised rock
from Core/ Mantle Boundary. ie Hawaiian Island chain.
 Melting due to P release &/ or involvement of fluids
during mantle convection over great depth range of
10-100 km.
Magma & Igneous rocks
 Why and how do Magmas form?
 N.L Bowen discovered with Lab experiments that minerals crystallise
in a specific sequence, as a magma cools.
 Furthermore, first formed minerals react with the cooler residual
magma, to form different minerals, also in a set sequence.
 Sequence termed Bowen’s Reaction Series.
 Bowen reasoned that the melting of a single composition ( ie basalt),
would account for different magma types by fractional crystallisation.
 Minerals settle to bottom of magma by gravity.
 Residual magma changes composition yielding:
• Basaltic  Andesitic  Rhyolitic Magmas.
• Early-formed crystals removed from magma contact.
Magma & Igneous rocks
 Alternatively, fractional melting explains magma types:
 Reverse of Bowen’s approach.
 Does produce three magma types.
 Lab experimental evidence, and distribution of different volcanoes
worldwide, confirm close relationship with Plate Tectonic Margin type.
• Rhyolitic magma only known from Continental Crust. Absent in Oceanic Crust.
• Suggests does not form in the Mantle.
• Andesitic lavas found on both Crust types. Formed in Mantle, but independent
of overlying Crust?
• Andesitic lavas restricted to subducting Oceanic Crust?
• Andesites derived from partial melting of subducted
Oceanic Crust .
Magma & Igneous rocks
 Origin of Magma:
 Volcanoes erupting basaltic magma occur on both types of Crust.
 Must therefore be sourced from the Mantle.
 Not geographically-restricted like Andesites, suggesting melting of
Mantle itself.
 Therefore, conclude that basaltic magma forms from partial melting of
largely anhydrous & gas-poor Mantle.
 Andesitic magma sourced from partial melting of subducted Oceanic
Crust (including thin, & hydrous sediment drape).
 Rhyolitic magma forms by fractional melting of :
• Continental Crust. Gas rich (H2O + CO2).
• Mantle-Derived Basaltic magma under-plates Crust
causing partial melting & generation of Rhyolitic
Magma.
Magma & Igneous rocks
 Naming Igneous Rocks:
 Igneous rocks form from cooling and solidification of magma.
 Extrusive igneous rocks form from solidification of lavas.
 Intrusive igneous rocks form when magma solidifies within the Crust
or Mantle.
 Both extrusive & intrusive rocks are classified on the basis of rock
texture and mineral assemblage.
 Texture:
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Refers to the size and arrangement of mineral grains.
Grain size- consequence of Cooling History/ Rates.
Rocks with grain sizes  1cm termed “Pegmatites”.
Extrusive rocks are fine-grained (< 1mm).
Rapid cooling, too little time to grow large grains.
May even be ‘Glassy’ with no grains, ie “Obsidian”.
Magma & Igneous rocks
 Texture:
 Intrusive rocks are coarse-grained, equigranular as magma cooled slowly
in Crust/ Mantle.
 Sufficient time for large grain growth.
 Earliest crystallising minerals possess excellent crystal shape, later have
partial crystal shape, and last minerals have no crystal shape. (Due to
space limitations in cooling magma).
 Occasionally large + small crystals together= ‘Porphyritic’ texture.
 Large crystals= ‘phenocrysts’ (slow cooling).
 Small crystals= ‘groundmass’ (rapid cooling).
• Large grains represent slow cooling in magma chamber.
• Small grains formed on eruption of crystal-laden magma.
• Two stage/step cooling in rock crystallisation.
Magma & Igneous rocks
 Pyroclastic rocks:
 Produced by explosive volcanism.
 Can be formed from rock fragments of pre-existing volcanic rocks.
 Also from crystal fragments, and/or combinations of both.
 May also have characteristics of Sedimentary rocks, ie be layered, etc.
 Termed ‘Agglomerate’ when tephra are large (ie, bomb-sized).
 When fragments are small, termed ‘Tuff’ (i.e, ash).
 If fragments large & angular(  2mm), termed a
“Volcanic Breccia”.
 Named after predominant fragment/ clast type.
 Sometimes tuffs are ‘welded’ by hot volcanic glass
fragments.
Magma & Igneous rocks
 Mineral Assemblages:
All common igneous rocks composed of one or more, of the six
primary minerals/ mineral groupsQuartz.
Felspar (K-felspar/Orthoclase, and/or Plagioclase).
Mica (Muscovite and/or Biotite).
Amphibole (Hornblende).
Pyroxene (Augite).
Olivine
 Together with Texture, these used to name rocks.
• ie, Presence/ Absence of Quartz or Olivine
• REMEMBER- Olivine and Quartz are incompatible.
Magma & Igneous rocks
 Examples:
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 Qtz/ no Ol
 No Qtz/ no Ol
 No Qtz/ Ol
Assemblage
Qtz, Fels, Mi, Hb
Fels, Hb, Aug, Mi
Fels, Aug, Ol, Mi
Intrusive
Extrusive
Granite
Diorite
Gabbro
Rhyolite
Andesite
Basalt
 Qtz = Quartz; Ol = Olivine; Fels = Felspar; Mi = Mica;
Hb = Hornblende; Aug = Augite.
 By combining two mineral discriminators & grain
size parameters, these six igneous rocks can be
identified.
Magmas & Igneous Rocks
 Streckeisen Classification:
 “New” 1973 scheme for naming igneous rocks;
 Based on vol % Quartz (Q)- Alkali felspar (A)- Plagioclase (P)
re-calculated to 100% and plotted on a ‘ternary diagram’;
 New terms for Intrusive rocks- “Tonalite” & “Monzonite”;
 Note the clustering of Basic & Ultrabasic rocks at the ‘P’ apex
– poorly resolved!
 Additional Ternary diagrams required to
name these rocks.
 See last diagrams.
Magmas & Magmatic Rocks
Magmas & Magmatic Rocks