Igneous RocksIgneous Rocks

Text ref. Ch5 (pg. 98)Text ref. Ch5 (pg. 98)

ContentsContents

What are igneous rocks?What are igneous rocks?

Classifying igneous rocks.Classifying igneous rocks.

What are igneous rocks?What are igneous rocks?

Text ref. ch.5.1 (pg 99-106)Text ref. ch.5.1 (pg 99-106)

ObjectivesObjectives

Compare and contrast intrusive and Compare and contrast intrusive and extrusive igneous rocksextrusive igneous rocks

Describe the composition of magmaDescribe the composition of magma

Discuss the factors that effect how rocks Discuss the factors that effect how rocks melt and crystallizemelt and crystallize

Igneous RocksIgneous Rocks

Formed from the Formed from the crystallization of magmacrystallization of magma

Lava is magma the flows Lava is magma the flows out onto the Earth’s surface.out onto the Earth’s surface.

Extrusive igneous rocks are Extrusive igneous rocks are fine grained and cool fine grained and cool quickly on Earth’s surface quickly on Earth’s surface (magma)(magma)

Intrusive igneous rocks are Intrusive igneous rocks are coarse grained and cool coarse grained and cool slowly beneath the Earth’s slowly beneath the Earth’s surface. (lava flows)surface. (lava flows)

Composition of MagmaComposition of Magma

Magma is often a Magma is often a slushy mix of molten slushy mix of molten rock, gases, and rock, gases, and mineral crystals.mineral crystals.

Magma is classified Magma is classified on its SiOon its SiO22 content content

GroupGroup SiOSiO22

contentcontent

RhyoliticRhyolitic 70 percent70 percent

AndesiticAndesitic 60 percent60 percent

BasalticBasaltic 50 percent50 percent

Factors that Affect Magma Factors that Affect Magma FormationFormation

TemperatureTemperature

PressurePressure

Water contentWater content

Mineral compositionMineral composition

TemperatureTemperature

Temperature gradient Temperature gradient compares the internal compares the internal temperature of the temperature of the Earth with depth.Earth with depth.

PressurePressure

Pressure also increases with depth.Pressure also increases with depth.

In general, as pressure on a rock In general, as pressure on a rock increases, so does its melting increases, so does its melting temperature.temperature. Ex. Rock sample may melt at 1000Ex. Rock sample may melt at 1000°C at °C at

Earth’s surface, but 1400°C at a depth of Earth’s surface, but 1400°C at a depth of 100m.100m.

Water contentWater content

Also affects magma formation.Also affects magma formation.

As water content increases, melting As water content increases, melting decreasesdecreases

Mineral contentMineral content

Magma containing minerals such as Magma containing minerals such as olivine, calcium feldspar, and pyroxene olivine, calcium feldspar, and pyroxene melt at higher temperaturesmelt at higher temperatures

In general oceanic crust tends to melt at In general oceanic crust tends to melt at higher temperatures than continental higher temperatures than continental crust, due to its Fe content.crust, due to its Fe content.

How Rocks MeltHow Rocks Melt

Rocks melt depending on their Rocks melt depending on their compositioncomposition Partial MeltingPartial Melting Fractional CrystallizationFractional Crystallization

Partial MeltingPartial Melting

The process whereby some minerals melt The process whereby some minerals melt at low temperatures while other minerals at low temperatures while other minerals remain solid is called partial melting.remain solid is called partial melting.

Fractional CrystallizationFractional Crystallization

The process wherein different minerals The process wherein different minerals form at different temperatures is called form at different temperatures is called fractional crystallization.fractional crystallization.

Fe-Mg m

inerals

Iron-rich mineralsIron-rich minerals

Left branch of Bowen’s reaction series Left branch of Bowen’s reaction series represents iron-rich minerals.represents iron-rich minerals.

Undergo abrupt changes during fractional Undergo abrupt changes during fractional crystallization.crystallization.

Layered IntrusionsLayered Intrusions

Minerals sometimes form in layers Minerals sometimes form in layers according to Bowen’s reaction series.according to Bowen’s reaction series.

Known a layered intrusionKnown a layered intrusion

Important formations in mining: largest Important formations in mining: largest sources of platinum in U.S. found in sources of platinum in U.S. found in layered intrusions in Montanalayered intrusions in Montana

Classifying Igneous Classifying Igneous RocksRocks

Text ref. Ch.5.2, 107-113Text ref. Ch.5.2, 107-113

ObjectivesObjectives

Classify different types and textures of Classify different types and textures of igneous rocks.igneous rocks.

Recognize the effects of cooling rates on Recognize the effects of cooling rates on the grain sizes of igneous rocks.the grain sizes of igneous rocks.

Describe some uses of igneous rocks.Describe some uses of igneous rocks.

Mineral CompositionMineral Composition

Felsic rocks such Felsic rocks such as granite are light-as granite are light-colored, have high colored, have high silica contents, and silica contents, and contain quartz and contain quartz and the feldspars the feldspars orthoclase and orthoclase and plagioclase.plagioclase.

Mineral CompositionMineral Composition

Intermediate rocks such as diorite contain Intermediate rocks such as diorite contain moderate amounts of biotite, amphibole, moderate amounts of biotite, amphibole, and pyroxeneand pyroxene

Mineral CompositionMineral Composition

Mafic rocks such as Mafic rocks such as gabbro are dark-gabbro are dark-colored, have lower colored, have lower silica contents, and silica contents, and are rich in iron and are rich in iron and magnesium.magnesium.

Mineral CompositionMineral Composition

Ultramafic rocks Ultramafic rocks are unusual, are unusual, containing containing peridotite, dunite, peridotite, dunite, have low silica have low silica contents and very contents and very high levels of iron high levels of iron and magnesium.and magnesium.

Grain SizeGrain Size

Igneous rocks differ in the size of the Igneous rocks differ in the size of the grains that compose themgrains that compose them

Cooling rates will affect grain sizeCooling rates will affect grain size

Fast cooling rocks will form smaller grainsFast cooling rocks will form smaller grains

Slow cooling rocks will form larger grainsSlow cooling rocks will form larger grains

TextureTexture

Distinct crystal shapes caused by Distinct crystal shapes caused by fractional crystallization can be used to fractional crystallization can be used to indentify igneous rocksindentify igneous rocks

Porphyritic texture is characterized by large Porphyritic texture is characterized by large crystals surrounded by smaller crystals of the crystals surrounded by smaller crystals of the same or other minerals same or other minerals

Igneous Rocks as ResourcesIgneous Rocks as ResourcesUsed as building materials Used as building materials GraniteGranite

Ore DepositsOre Deposits

Veins of gold, platinum, silver, and other Veins of gold, platinum, silver, and other precious metals can be found in igneous precious metals can be found in igneous rock deposits.rock deposits.

Pegmatites are veins Pegmatites are veins that contain extremely that contain extremely large-grained crystalslarge-grained crystals

Pegmatite containing Amethyst

KimberlitesKimberlites

Diamond is found Diamond is found naturally occurring naturally occurring ultramafic rocks ultramafic rocks known as known as kimberlites.kimberlites. Named after Named after

Kimberly, South Kimberly, South Africa where they Africa where they were first discoveredwere first discovered

Form 150-300 km; Form 150-300 km; intense pressures intense pressures required for required for kimberlites to formkimberlites to form