Igneous Rocks

Physical GeologyTopic – Igneous

Rocks

Igneous Rocks

1. Chemical/mineral composition2. Extrusive versus intrusive3. Classification4. Importance of igneous rocks

1. Chemical/Mineralogic Composition

Main groupings – depending upon silica (SiO2) content:Mafic (Magnesium and iron/Fe)

Generally dark colouredRelatively poor in silica, rich in magnesium

& ironE.g., gabbro/basalt

Also “Intermediate” and “Ultramafic” rocks

1. Chemical/Mineralogic CompositionMain groupings – depending upon

silica (SiO2) content:Felsic (Feldspar & Silica)

Generally light colouredRich in minerals high in silica (quartz,

feldspar, etc)E.g., granite/rhyolite

1200

oC

1000

o

Bowen’s Reaction SeriesTwo series of minerals formed during crystallization

of magmaLow Silica Magma

High Silica Magma

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Iso

late

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ork

Sh

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Temperature ofCrystallization

750o

Mafic

Felsic

Extrusive Intrusive

2. Extrusive vs IntrusiveIntrusive rocks

(plutonic)Formed by slow

crystallization of magma that have intruded rock masses deep in the Earth

Recognized by interlocking large crystals

2. Extrusive vs IntrusiveExtrusive rocks

(volcanic)Formed by rapid

crystallization of magma that erupts at the surface of the Earth

Recognized by glassy or fine-grained nature

2. Extrusive vs Intrusive

Intrusive rocks Igneous Intrusions (Plutons) : large

igneous bodies formed at depth 1 km3 to 100s of km3

Batholiths (large), stocks (small)Discordant – cut across layers of rocks

they intrude

Intrusions and volcanic landforms

2. Extrusive vs IntrusiveIntrusive rocks

Sills & Dikes: small, tabular igneous bodies formed at depthcm to many meters thickDikes are discordant – cut across layers of

rocks they intrudeSills are concordant – parallel to layering

Magma cools for several reasonsRemoval of

volatilesIt rises to a

cooler location and has time to cool Cooling depends

very much on the geometry (surface area) of the intrusion.

Tabular-shape = fast cooling

Spherical shape = slow cooling

Cooling times vary from days to millions of years

3. ClassificationClassification based on

mineralogic composition and textureComposition – origin of magma

Felsic/Intermediate/Mafic/Ultramafic

Texture - conditions at solidificationGrain size (fine, coarse)Grain shape Relationships between grains

(interlocking)

3. Classification

Igneous TexturesPhaneritic

Crystals are evident that one can easily distinguish with the unaided eye

3. Classification

Igneous TexturesAphanitic

Crystals are uniformly fine-grained and interlocking, with individual crystals invisible to the unaided eye

3. Classification

Igneous TexturesPorphyritic

Large, evident crystals called phenocrysts are surrounded by an aphanitic matrix or groundmass.

3. Classification

Igneous TexturesVesicular

Vesicles are simply bubbles produced by gases escaping from lava as it solidifies

3. Classification

Igneous TexturesGlassy

Very rapid cooling may prevent crystal growth altogether, and we get natural volcanic glass, called obsidian.

Igneous rock textures

Fig. 5.4

4. Importance of igneous rocksA high proportion of the world's metal

deposits are associated with igneous rocks porphyry copper; epithermal gold

Igneous activity provides sources of key ingredients: heat / metal / water

Ore deposits are often directly hosted by igneous rocks in veins, disseminated within rock, etc.

Hydrothermal alteration and fracturing of igneous rocks is also involved

4. Importance of rocksStudy rock properties to understand

conditions of formation Sources of fossil fuelsSources of metals and other mineral

resourcesGroundwaterBuilding materialsUnderground storage

Radioactive & other hazardous wastes

Plan Nord

At the moment:• Nearly 160,000 mineral

explorations permits had been issued

• Over 30 hydroelectric projects were either operational or in construction

* 220,000 km2 of forested areas are allocated for forest management and wood harvesting