EARTH MATERIALS V The Rock Cycle: Igneous Rocks Professor Peter Doyle

THE ROCK CYCLE

Basic formation

Relative proportions of rock groups Metamorphic rocks: smaller proportion of total

Rock Cycle I: igneous rocks crystallisation magmasIgneous rocks form by crystallisation from magmas in the crust or at the surface Magmasdifferent compositions different source rocksMagmas of different compositions originate via melting of different source rocks in the lower crust and mantle controlledMelting is controlled by –source rock compositions, water contents –pressure, temperature

Settings control igneous rock chemistry

Crustal depth controls crystal size Intrusives: Intrusives: slow cooling Extrusives: Extrusives: rapid cooling

Extrusive (volcanic) igneous rocks rapid crystallisationForm by rapid crystallisation from magmas at the surface Characterised by volcanic glass (e.g. obsidian) And /or fine grained crystals (e.g basalt). Two major types - LavasLavas - forming flows PyroclasticsPyroclastics - forming deposits of volcanic fragments Both may be found in most volcanic settings

Sites of active volcanoes

PYROCLASTIC ERUPTION Montserrat eruption of Soufriere Hills volcano

Lava flow Lava flow on Kilauea volcano Hawaii: a bit hot!

Volcanic rocks: texture & appearance Lavas show a variety of textures vesiclesEscaping gas creates holes or vesicles phenocrysts porphyriticSome lavas contain large crystals (phenocrysts) formed during slower cooling produce a porphyritic texture PyroclasticsPyroclastics (or tuffs) form from violent explosive eruptions Ash & lava fragments formed compact & often weld together

Lavas, pyroclasics and intrusives Intrusives: Intrusives: coarser grained Lavas & pyroclastics: Lavas & pyroclastics: finer grained

Ropy lava texture Cindery lava texture LAVA

VESICULAR BASALTS Textures produced by escaping gas bubbles amygdales some gas bubbles infilled by minerals - amygdales

phenocrysts Basaltic lava showing small white phenocrysts

Porphyriticphenocrysts Porphyritic igneous rock with white phenocrysts of feldspar

Obsidian Pumice Ash PRODUCTS OF VOLCANIC ERUPTION PYROCLASTICS GLASS

WELDED TUFF Volcanic ash flow showing flattened and deformed volcanic clasts and glass

Intrusive or plutonic igneous rocks Form by slow crystallisation from magmas below the Earth’s surface Composed entirely of interlocking crystals (holocrystalline) - Granite (acidic) - Gabbro (basic)

GABBRO GRANITE

Plutonic rocks Plutonic rocks: intruded into crust Larger the body, coarser the grain Larger the body, deeper the crustal setting Plutons: Plutons: deep, large bodies (e.g. batholiths) Shallower intrusives: Sills - Sills Dykes - Dykes

DYKE A DYKE OF INTRUSIVE IGNEOUS ROCK CROSS-CUTTING SEDIMENTARY ROCK LAYERS

SILL IGNEOUS SILL INTRUDED BETWEEN LAYERS OF SEDIMENTARY ROCKS

Mineralogy of igneous rocks Silicate minerals characterise igneous rocks Most common are: quartz, feldspars, micas, pyroxenes, amphiboles, olivines. ACIDIC >65% SiO 2 BASIC 45-55%SiO 2

COARSE GRAINEDFINE GRAINED MAFIC FELSIC

Mineralogy of igneous rocks

Bowen’s Reaction Series I Tries to explain how basaltic magma cools & evolves Fractional crystallisation creates different mineral phases producing silica rich magmas

Bowen’s Reaction Series II Assumes all magmas start out basaltic (not true) Magmas of varying compositions arise from melting of mixed source rocks in upper mantle and crust, e.g. rocks in upper mantle melt might partially (basaltic magmas) mixtures of sedimentary & basaltic rocks in subduction zones (intermediate magmas) mixtures of sedimentary, igneous & metamorphic rocks (granitic magmas)

Igneous rock fabrics Crystal/grain size: –fine <1mm; medium 1-5mm; –coarse mm; very coarse > 30mm Crystals: holocrystalline –interlocking crystals – holocrystalline hypocrystalline –glass plus crystals hypocrystalline holohyaline –glass only – holohyaline :Crystal shapes: –Euhedral –Euhedral – well developed crystal shapes with preserved faces; usually phenocrysts –Subhedral –Subhedral – some well developed crystal shapes with preserved faces –Anhedral –Anhedral – no well developed crystal shapes and no preserved faces

Phenocrysts Phenocrysts of euhedral euhedral hornblendes anhedral Interlocking anhedral crystals of minerals

Textures GranularGranular – uniform sized medium to coarse crystals AphaniticAphanitic – uniformly fine grained; cannot differentiate individual crystals with the naked eye. GraniticGranitic – irregular granular mixture of euhedral, subhedral and anhedral crystals PorphyriticPorphyritic – large phenocrysts in finer grained groundmass VesicularVesicular – cavities caused by escape of gas bubbles Amygdaloidal –Amygdaloidal – cavites later infilled with crystals PoikiliticPoikilitic – later crystallising mineral encloses smaller earlier formed crystals

PORPHYRITIC VESICULAR GRANULAR GRANITIC