Igneous Rocks Introduction AS-A level Geology Igneous Rocks Introduction Click anywhere on the screen to move on.

Igneous Rocks

Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling, crystallization 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 and Cooling Rates Crystal size is determined by the rate of cooling of the magma or lava 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 at the Earth’s surface over a few months results in crystals of <1.0 mm in diameter forming lava flows (volcanic)

Crystal Size and Cooling Rates Minor intrusives-dykes and sills cool slower over thousands of years to form crystals 1.0 mm-3.0 mm in diameter

Crystal Size and Cooling Rates Slow cooling in magma chambers deep underground over millions of years results in larger crystals >3.0 mm in diameter (plutonic)

Crystal Shape 1- Euhedral Well formed crystals with a regular and recognisable shape. They form when crystals can grow freely in a melt and are not impeded by the presence of any surrounding pre-existing crystals

Olivine basalt from Ubekendt Ejland, West Greenland Euhedral Olivine Six-sided shape 3mm Olivine basalt from Ubekendt Ejland, West Greenland

Crystal Shape 2 - Subhedral Partially formed crystals with some recognisable shape. They have been partially impeded as they grew by the surrounding pre-existing crystals

Subhedral Olivine Some faces flat and planar 1mm Some faces curved and embayed Picritic basalt, Ubekendt Ejland, West Greenland

Crystal Shape 3 - Anhedral Anhedral – no regular crystalline shape visible. The shape of the growing crystal is controlled by the arrangement and orientation of the surrounding pre-existing crystals

Irregular outline with no planar faces evident Anhedral Olivine Irregular outline with no planar faces evident Olivine basalt from Mauritius, Indian Ocean 1mm

Large well formed crystals in an igneous rock Phenocrysts Large well formed crystals in an igneous rock In Shap Granite the pink coloured orthoclase phenocrysts are up to 3cm in length

The remainder of the igneous rock made up of smaller crystals 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 length Phenocrysts are euhedral and rectangular Implies 2 stage cooling history 1cm Finer groundmass 0.5-1.0 mm in diameter

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

Equicrystalline Texture 2cm Microgranite – even cooling, all crystals 1.0 – 1.5 mm

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

Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5cm long Long axes of phenocrysts aligned parallel implies flow of magma Groundmass 1.0 -1.5 mm

Spherical or ellipsoidal cavities found in lavas Vesicular Texture Spherical or ellipsoidal cavities found in lavas As magma moves up towards the surface and the pressure is reduced, gases are exsolved

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

Glassy Texture e.g. in Obsidian No crystals visible due to very rapid cooling 1cm Shows Conchoidal Fracture like glass

Mineral Content Igneous rocks are classified chemically as Silicic, Intermediate, Mafic or Ultramafic according to the main constituent minerals present and their overall silica content

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

Intermediate Igneous Rocks Up to 10% Quartz, Plagioclase Feldspar, Hornblende, Augite Lower silica content 52%-66%

Contain less silica 45%-52% Mafic Igneous Rocks Plagioclase Feldspar, Augite and Olivine Contain less silica 45%-52%

Ultra-Mafic Igneous Rocks Mainly olivine, small amounts of augite and plagioclase Contain the least silica <45%

The Classification of Igneous Rocks % Silica Content Chemical Group % Dark Minerals % Light Minerals Rock Names   Over 66% Silicic 40% or below 60% or over Rhyolite/Obsidian/Pumice (Fine) Micro-granite (Medium) Granite (Coarse) 52% to 66% Intermediate 40%-70% 30%-60% Andesite (Fine) Micro-diorite (Medium) Diorite (Coarse) 45% to 52% Mafic 70%- 90% 10%-30% Basalt (Fine) Dolerite (Medium) Gabbro (Coarse) Below 45% Ultramafic 90% or over 10% or below - Peridotite (Coarse)

Cornish Granite Glassy, colourless quartz All crystals over 3.0 mm in diameter-Plutonic 1cm Black biotite mica with pearly lustre Subhedral crystal form White/creamy plagioclase feldspar

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

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

Micro-Granite Formed within the crust in a sill or dyke Mineralogy: quartz, feldspar and mica Subhedral crystals Equigranular texture, all crystals 1.0 – 1.5mm in diameter Formed by an even cooling rate over thousands of years 2 cm

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

Gabbro Equicrystalline texture, all crystals roughly similar in size Greenish-black augite Formed deep underground by very slow cooling over millions of years 2cm Coarse grained, crystals over 3.0 mm in diameter, suggesting slow cooling Grey/creamy plagioclase feldspar

Porphyritic Dolerite Medium crystal size mainly 1.0 – 2.0 mm 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

Basalt Chilled margin, very fine grained almost glassy 1 cm Chilled margin, very fine grained almost glassy Formed by rapid cooling at the earth’s surface over a few weeks or months Mineralogy: plagioclase feldspar, augite and olivine Crystal size is fine, well under 1.0 mm, (volcanic)

Predominantly Olivine Tiny Amount of Feldspar Peridotite Coarse Grained >3.0mm Ultra-Mafic Low Silica Content Predominantly Olivine Small Amount of Augite Tiny Amount of Feldspar Mantle Rock 1 cm