Igneousrockssmall

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


Rapid cooling in lava flows at the earth’s surface over a few months

results in crystals of <0.5mm in diameter forming (Volcanic)


Slower cooling in dykes and sills over hundreds to

thousands of years results in crystals 0.5mm to 2mm in

diameter (Hypabyssal)


Slow cooling in magma chambers deep underground over millions of years results in larger crystals

>2mm in diameter (Plutonic)

Crystal Shape 1- Euhedral

Well formed crystals with a regular and recognisable shape. They form when a crystals can

grow freely in a melt and are not impeded by the presence of any

surrounding pre-existing crystals

Euhedral Olivine

Olivine basalt from Ubekendt Ejland, West Greenland

Six-sided shape3mm

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

Picritic basalt, Ubekendt Ejland, West Greenland

Some faces curved and embayed

Some faces flat and planar

1mm

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

Anhedral Olivine

1mm

Irregular outline with no planar faces evident

Olivine basalt from Mauritius, Indian Ocean

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 Acidic or Basic

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


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

Pyroclastic Rocks

Consist of fragmental volcanic material blown into the atmosphere

by explosive activity

Mainly associated with andesitic and acidic volcanoes

Pyroclastic Rocks – 2 Main Groups

Material ejected from the volcano as liquid globules which solidifies in the air and is deposited as solid particles

Material ejected from the volcano as solid fragments, this solid

material has been fractured by the explosive activity

Materials Ejected in a Liquid State

Pelées Hair

Volcanic Bombs

Pumice

Scoriae

Pelées Hair

A fine mass of hair-like glass

Formed by lava being exuded through a small

orifice and blown about by the wind

Resembles candy floss in

appearance

1cm

Volcanic Bombs

Larger masses of liquid lava thrown into the air

They rotate and take on characteristic shapes

Spindle-bombs and breadcrust bombs are most common-usually vesicular

Vary in size from small droplets to several cubic metres

Volcanic BombsVolcanic bombs are large

fragments of molten lava up to 1m in diameter expelled

during an eruption.

Bombs develop a rounded or almond shape as they are twirled through the air.

Some bombs have a characteristic breadcrust surface, others resemble cauliflowers or cowpats depending on the way they land and solidify.

Section through a Volcanic BombHighly vesicular interior

Breadcrust exterior, finer grained and less vesicular due to more rapid cooling

5cm

Pumice

Highly vesicular material derived from acid lavas

Very high porosity and low density

So light that it may float on water

Pumice

Low density, high porosity, floats on water

Microscopic grain size, very rapid cooling at the earth’s surface


2cm

Volcanic, felsic igneous

rock

Specimen from Mt. Teide,

Tenerife

Vesicles up to 3cm in diameter

Scoriae

Associated with basic lavas

Vesicular but denser than pumice

Globules of lava are ejected and the exterior chills and solidifies

Interior is still hot and molten

Upon landing they are still soft and are flattened into pancake shapes

Section through Strombolian Scoriae Cone

Bedding dips 32°SE

The cone has been half excavated for use in the construction industry

2m

Some layers rich in volcanic bombs

Strombolian refers to the style of pyroclastic eruption in which fragments of incandescent,

vesiculating basaltic magma are ejected to a moderate height, landing

as solid scoria to form a cone

Strombolian Scoriae ConeVolcanic bombs occur up to 50cm in diameter

Scoriae clasts range in size from 3 to 15cm

The structure is very friable and has an unstable surface

Material Ejected in a Solid State

Agglomerate-fragments >64mm in diameter

Lapilli – fragments 64mm - 2mm in diameter

Ash, Tuff & Dust – fragments <2mm in diameter

Agglomerate – Volcanic BrecciaDerived from agglomero

meaning ‘gather into a heap’

Formed of volcanic or country rock in the vent or as part of the cone

Produced by explosive activity which often shatters the top of the cone

Coarse material is ejected a relatively short distance before settling back to earth

Comprises angular fragments >32mm surrounded by finer tuff and lapilli

Agglomerate – Volcanic BrecciaLarge angular fragments

up to 10cm in diameter

Large fragments surrounded by material of ash and lapilli size

Vent Agglomerate

5cm

Agglomerate–Volcanic Breccia, Arico, Tenerife

Chaotic mixture of boulders over 2m to ash <2mm in diameter

Lapilli – Particles 2 – 64mm

Derived from lapillus meaning ‘a little stone’

Most commonly small pea to walnut sized

Tuff – Particles <2mm

The lithified equivalent of volcanic ash

Classified according to the nature of the pyroclastic fragments

Crystal Tuffs – composed of mainly crystals

Lithic Tuffs – composed of fragments of rock

Vitric Tuffs – composed of glassy fragments

Welded Tuffs (Ignimbrites) – hot fragments welded together in Nuées Ardentes eruptions

The Formation of Ignimbrites

Associated with Pyroclastic Flows

Nueés Ardentes style eruptions

Glowing fireclouds 300-1000°C

Particles weld together on settling

Activity on Augustine, Alaska, photograph by M.Krafft

The Chimiche Ignimbrite, Arico, Tenerife

This deposit covers more than 150 km2 of the Chimiche-Arico part of the island. It is thought to represent the collapse of a 10-15 km high plinian eruptive column

It is unwelded as the particles were cool by the

time they had fallen 10-15km through the atmosphere

back to earth

15m

Volcanic Ash – Unconsolidated material <2mm in diameter

Road cutting in the Guimar Valley, Tenerife

The End

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