Igneous Rocks I.G.Kenyon

Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling and solidification of molten lava or magma Comprise an interlocking mosaic of crystals

Extrusive Igneous Rocks Molten rock reaching the earth’s surface via volcanoes (lava) is termed extrusive

Intrusive Igneous rocks Molten rock (magma) that solidifies at depth within the lithosphere is intrusive Intrusive rocks may eventually be exposed at the earth’s surface following a long period of uplift and erosion

Crystal size is determined by the rate of cooling of the magma or lava Crystal Size and Cooling Rates Instantaneous cooling of lava erupted under water as pillow lavas results in a glassy texture, devoid of any crystalline form

Crystal Size and Cooling Rates Rapid cooling in lava flows at the earth’s surface over a few months results in crystals of <0.5mm in diameter forming (Volcanic)

Crystal Size and Cooling Rates Slower cooling in dykes and sills over hundreds to thousands of years results in crystals 0.5mm to 2mm in diameter (Hypabyssal)

Crystal Size and Cooling Rates Slow cooling in magma chambers deep underground over millions of years results in larger crystals >2mm in diameter (Plutonic )

Phenocrysts Large well formed (euhedral) crystals in an igneous rock In Shap granite the flesh coloured orthoclase phenocrysts are up to 3cm in diameter

Groundmass The remainder of the igneous rock made up of smaller crystals In the case of Shap granite, the groundmass is mainly crystals of biotite mica and quartz

Phenocrysts and Groundmass Orthoclase phenocrysts up to 6cm in diameter Finer groundmass mm in diameter 1cm Phenocrysts are euhedral and rectangular Implies 2 stage cooling history

Equigranular Texture All the crystals in the rock are roughly the same size Produced by a steady or constant cooling rate

Equigranular Texture 2cm Microgranite – even cooling, all crystals 0.5 – 1.0mm

Porphyritic Texture Large crystals (phenocrysts) set in a finer grained groundmass Produced by two-stage cooling

Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5cm long Groundmass mm Long axes of phenocrysts aligned parallel implies flow of magma

Vesicular Texture Small spherical or ellipsoidal cavities found in lavas Formed by gas bubbles being trapped during solidification of the rock. Eg Pumice

Vesicular Texture Car key for scale Vesicles represent trapped gas bubbles within a lava flow Vesicles range from 2mm to 1.5cm in diameter Vesicles are stretched and curved indicating flow of the lava

Glassy Texture No crystals visible, rocks are often dark green or black in colour and show conchoidal fracture (like glass) Eg Obsidian formed by the instantaneous cooling of acid lava

Glassy Texture - Obsidian 1cm Shows Conchoidal Fracture

Amygdaloidal Texture The vesicles in a lava are later infilled by secondary minerals precipitated from solution Commonly quartz and calcite Amygdale means ‘almond-shaped’

Amygdaloidal Texture Former vesicles infilled by quartz Euro coin for scale Basalt, volcanic, crystals <0.5mm

Mineral Content Igneous rocks are classified chemically as Felsic or Mafic according to the main constituent minerals present

Felsic Igneous Rocks Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite Mica and Muscovite Mica. Rich in silica >66%

Mafic Igneous Rocks Plagioclase Feldspar, Augite and Olivine Contain less silica 45 – 55%

Igneous Rock Classification Felsic Quartz, feldspar and mica Mafic Plagioclase feldspar, augite and olivine Volcanic Crystal size <0.5mm in diameter RhyoliteBasalt Hypabyssal Crystal size 0.5-2mm in diameter MicrograniteDolerite Plutonic Crystal size >2mm in diameter GraniteGabbro

1cm All crystals over 2mm in diameter-Plutonic Glassy, colourless quartz Black biotite mica with pearly lustre White/creamy plagioclase feldspar Subhedral crystal form Cornish Granite

1cm Flesh-coloured orthoclase feldspar phenocrysts up to 3cm long Finer groundmass of quartz and biotite mica 2-3mm in diameter Porphyritic texture, large phenocrysts and finer groundmass Feldspar phenocrysts are euhedral Shap Granite (Ademallite)

Kaolinised Granite Unaltered grey, glassy quartz Iron oxide staining due to release of Fe ions from biotite mica Orthoclase feldspar altered to kaolinite by hydrolysis Biotite mica breaking down to form chlorite Granite is very crumbly and is described as Growan

2 cm Mineralogy: quartz, feldspar and mica Equigranular texture, all crystals 0.5 – 1.5mm in diameter Formed within the crust in a sill or dyke Formed by an even cooling rate over thousands of years Subhedral crystals Micro-Granite

1 cm Fine grained < 1mm, no crystals visible, volcanic Spherical vesicles up to 3mm in diameter Vesicles represent trapped gas bubbles in a lava flow Mineralogy: quartz, feldspar and mica Formed by rapid cooling at the earth’s surface Vesicular Rhyolite

2cm Coarse grained, crystals over 2mm in diameter, suggesting slow cooling Grey/creamy plagioclase feldspar, variety calcium rich anorthite Greenish-black augite Equigranular texture, all crystals roughly similar in size Formed deep underground by very slow cooling over millions of years Gabbro

Hypabyssal, crystal size mainly 1-2mm Mineralogy: plagioclase feldspar, augite and olivine Subhedral phenocrysts of plagioclase feldspar up to 3mm in diameter Groundmass constitutes over 75% of the rock Two-stage cooling, finally forming an intrusive dyke or sill 1 cm Porphyritic Dolerite (Micro-gabbro)

1 cm Crystal size well under 0.25mm, volcanic Mineralogy: plagioclase feldspar, augite and olivine Formed by rapid cooling at the earth’s surface over a few weeks or months Chilled margin, very fine grained almost glassy Basalt

The End