geology 11 - igneous rocks
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ROCKS
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What is a rock?
What are the types of rocks?
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What is a rock?
a naturally-occurring aggregate of one ormore minerals; may or may not containmineraloids, natural glass and organic
matter.
Types of rocks vary based on composition, color,
texture, structures, etc.
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The Rock Cycle
Classification ofrocks:
Igneous
Sedimentary
Metamorphic
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OBJECTIVES
Definition of magma
Origin of magma
Types of magmas
Classification of igneous rocks
Resources from igneous rocks
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What are igneous rocks?
Ignis= fire
Formed from solidification of magma(intrusive) or lava which flows out fromdepths (extrusive)
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What is magma?
Molten material which may containsuspended crystals and dissolved volatiles(gases e.g. water vapor, CO2, SO2)
Composed of mobile ions of the 8 mostabundant elements in the Earths crust:
Si, O, Al, K, Ca, Na, Fe, Mg
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Magma Molten rock composed of varying amounts of
Liquid
Silicate (sometimes carbonate or sulfide)
Ions of K, Na, Fe, Ca, Mg, Al
Solid
Minerals
Rock fragments
Dissolved gas
H2O, CO2, SO2
Temperature: 600-1200oC
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Magma generation:
role of heat
role of pressure
role of volatiles
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Sources of heat for melting in the crust
original heat of the earth at the time offormation
some elements, e.g. U, produce heat through
radioactive decay heat transfer by conduction from a nearbybody of magma
hot mantle plumes may upwell into the crust
frictional heat caused by rocks grinding pasteach other
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Origin and Formation of Magma
Magma forms at: Mid-Oceanic Ridges (MOR)
Subduction Zones
Hot spots
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MID-OCEANIC RIDGE -basaltic magma
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SUBDUCTION ZONE mostly andesitic magma
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HOT SPOT - basaltic
magma
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Magma is classified according to:
Silica content - amount of SiO2
Viscosity - resistance to flow
Temperature - temperature of melt formation
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Magma
Viscosity property to resist flow
Effects of different factors
temperature, viscosity
SiO2, viscosity
dissolved H2O, viscosity
Density
heavier oceanic crust
basaltic rocks lighter continental crust rhyolitic rocks
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Mafic magma
a. High density
b. Low viscosity
c. Relatively low silica content
d. Crystallize at high temperatures (~1000 - 1200C)
Most common types of magma
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Felsic magmaa. Low density
b. High viscosity
c. Relatively high silica contentd. Crystallize at ~600C)
Most common types of magma
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basaltic (mafic) andesitic (intermediate)
rhyolitic (felsic)
Basaltic magma accounts for about 80 percent ofall magma erupted by volcanoes.
Rhyolitic and andesitic magma accounts for 10percent each.
Most common types of magma
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Two kinds of igneous rocks
Extrusive (volcanic) moltenrock solidified at thesurface.
Ex. Basalt, Andesite,
Rhyolite
Intrusive (plutonic) igneousrocks formed at depth.
Ex. Gabbro, Diorite,Granite
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Classification of igneous rocksBased on texture or crystal size
Aphanitic very fine-grained(5 mm) mineral sizes dueto magma cooling atdepth.
Porphyritic very largecrystals (phenocrysts)embedded in smallercrystals (groundmass).
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Igneous Textures - Crystalline
Coarse Grained Fine Grained
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Igneous Textures - CrystallinePorphyritic
Phenocrysts
Groundmass
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Classification of igneous rocks
Other textures:Vesicular contains tiny holes
called vesicles whichformed due to gas bubblesin the lava or magma.
Glassy
molten rock quenchedquickly as it was ejectedinto the atmosphere.
Pegmatitic interlockingcrystals greater than 1 cm
Pyroclastic formed whenvolcanic materials areextruded violently.
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Igneous Textures - Pyroclastic
Made of rock fragments
rather than crystals
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Igneous Rocks
GraniteDioriteGabbro
RhyoliteAndesiteBasalt Dacite
GranodioriteMafic Intermediate Felsic
Fine
Co
arse
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feldspars, quartz, muscovite
Granite
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Andesite
mostly plagioclase with some amphibole
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No quartz, and about half the rock is made of minerals (pyroxeneand olivine) that are dark-coloured in hand specimen. In this cross-
polars view they show bright colours. The striped grey rectangular
crystals are plagioclase feldspar.
Gabbro
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Ultramafic Rock - Dunite
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Classification of igneous rocks
Volcanic ejecta or pyroclasts or tephra:
Ash 64 mm; block is extruded in a solid statewhile bomb is partially or wholly molten
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Classification of igneous rocks
Based on mineral composition presence orabsence of quartz, composition of feldspars,amount of ferromagnesian minerals
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Classification of igneous rocksBased on mineral composition:
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Summary Table
Magma
Type
Solidified
Volcanic
Rock
Solidified
Plutonic
Rock
Chemical
CompositionTemperature Viscosity Gas Content
Basaltic Basalt Gabbro
45-55 SiO2 %,
high in Fe, Mg,
Ca, low in K, Na
1000 - 1200 C Low Low
Andesitic Andesite Diorite
55-65 SiO2 %,
intermediate in
Fe, Mg, Ca, Na, K
800 - 1000 C Intermediate Intermediate
Rhyolitic Rhyolite Granite
65-75 SiO2 %,
low in Fe, Mg, Ca,
high in K, Na
650 - 800 C High High
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N. L. Bowens Reaction Series
http://wps.prenhall.com/wps/media/tmp/labeling/218686_dyn.gif
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Intrusive igneous rocks
Stock small discordant plutonBatholith more than 100 sq. km. in
outcrop area
Dike tabular body cutting acrossbedding
Sill concordant tabular body
Laccolith blister-shaped sill
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Intrusive igneous rocks
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Intrusive igneous rocks
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Laccolith
Brain Power: What is this?
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Brain Power: What is this?
Batholith
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Brain Power: What is this?
Volcanic neck
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Brain Power: What is this?
Dike
R f I R k
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Resources from Igneous Rocks
Metallic Resources
Produced by igneousprocesses
Hydrothermal solutionscontain metal ions that
eventually precipitate out
Found in:
Veins
Disseminated deposits
Gold, silver, platinum etc
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