geology 11 - igneous rocks

7/31/2019 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)



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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.



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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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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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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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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

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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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Laccolith

Brain Power: What is this?


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Batholith


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Volcanic neck


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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

geology 11 - igneous rocks

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