Igneous Rocks I.G.Kenyon. Definition of Igneous Derived from the latin ‘ignis’ meaning fire...

Igneous Rocks

I.G.Kenyon

Definition of IgneousDerived 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 GroundmassOrthoclase phenocrysts up to 6cm in diameter

Finer groundmass 0.5-1.0 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 0.5-1.0mm

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 diameterRhyolite Basalt

Hypabyssal

Crystal size

0.5-2mm in diameterMicrogranite Dolerite

Plutonic

Crystal size

>2mm in diameterGranite Gabbro

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 augiteEquigranular 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