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
Fra
me-
D
ou
ble
S
ing
le
Iso
late
d w
ork
Sh
eet C
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n
C
hai
n
Temperature ofCrystallization
750o
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
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.
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
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