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

  2. 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.

  3. 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.

  4. 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, oneruption. • 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%

  5. 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’.

  6. 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.

  7. 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.

  8. 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 longfractures. • Extremely large volumes of lava extruded. • Examples: Deccan Traps India; Siberian traps, CIS, etc.

  9. 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: • ‘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.

  10. 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.

  11. 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 fluidsduring mantle convection over great depth range of 10-100 km.

  12. 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.

  13. 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 .

  14. 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.

  15. 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: • 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”.

  16. 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.

  17. 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.

  18. 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 toname rocks. • ie, Presence/ Absence of Quartz or Olivine • REMEMBER- Olivine and Quartz are incompatible.

  19. Magma & Igneous rocks • Examples: • Assemblage Intrusive Extrusive • Qtz/ no Ol Qtz, Fels, Mi, Hb Granite Rhyolite • No Qtz/ no Ol Fels, Hb, Aug, Mi Diorite Andesite • No Qtz/ Ol Fels, Aug, Ol, Mi Gabbro 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.

  20. 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.

  21. Magmas & Magmatic Rocks

  22. Magmas & Magmatic Rocks