petrologi 5-nomenklature
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Igneous Rocks
Igneous Rocks“Ignis” means fireRocks that are formed from
cystallization of magmaMagma is molten rock
Lava is magma that is on the Earth’s Surface
Igneous Rocks
Two Types of Igneous RocksExtrusiveExtrusive (Exterior) IntrusiveIntrusive (Interior)
Igneous rocks that solidify into rock beneath Earth’s surface
Coarse GrainedCool Slowly Granite
Extrusive Igneous Rocks
ExtrusiveExtrusive (Exterior) Igneous rocks
that solidify into rock on Earth’s surface
Fine Grained Cool Quickly
Intrusive Igneous Rocks
IntrusiveIntrusive (Interior) Igneous rocks
that solidify into rock beneath Earth’s surface
Coarse Grained Cool Slowly Granite
Igneous Rock Formation
Texture
Igneous rocks:Igneous rocks: Igneous textures
Texture
a. size
b. shape
c. arrangement
of interlocking crystals
crystal sizecrystal size
Slow cooling
cooling ratecooling rate
larger crystals
Fast cooling small or no crystals
Fine-grained Coarse-grained
Large crystals in matrix Glassy
Fine-grained Coarse-grained
Large crystals in matrix Glassy
fast cooling magma/lavafast cooling magma/lava forms at or near surfaceforms at or near surface sometimes holes presentsometimes holes present can’t see individual crystals can’t see individual crystals
forms far forms far below surfacebelow surface slow coolingslow cooling intergrown intergrown crystalscrystals
magma cooled slowly for a magma cooled slowly for a while then erupted while then erupted
minerals crystallize at minerals crystallize at different temperatures anddifferent temperatures and or ratesor rates
very rapidvery rapid coolingcooling ions unable to ions unable to unite in orderlyunite in orderly crystallinecrystalline structurestructure
Magmas with high silica content tend to form long chainlike structures
- impedes ion transport
- increases resistance to flow
viscosity
Pyroclastic rockPyroclastic rock
In violent volcanic eruptions,
rock fragments
ash
molten blobs
large angular blocks
consolidate
Igneous textures
Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals
Factors affecting crystal sizeRate of cooling
Slow rate promotes the growth of fewer but larger crystals
Igneous textures
Factors affecting crystal size Rate of cooling
Fast rate forms many small crystalsVery fast rate forms glass
Amount of silica (SiO2) present
Amount of dissolved gases
Igneous texturesTypes of igneous textures
Aphanitic (fine-grained) textureRapid rate of cooling of lava or magmaMicroscopic crystalsMay contain vesicles (holes from gas bubbles)
Phaneritic (coarse-grained) textureSlow coolingCrystals can be identified without a microscope
Aphanitic texture
Phaneritic texture
Igneous texturesTypes of igneous textures
Porphyritic textureMinerals form at different temperatures as well
as differing ratesLarge crystals, called phenocrysts, are
embedded in a matrix of smaller crystals, called the groundmass
Glassy textureVery rapid cooling of molten rockResulting rock is called obsidian
An obsidian flow in Oregon
Igneous texturesTypes of igneous textures
Pyroclastic textureVarious fragments ejected during a violent
volcanic eruptionTextures often appear to more similar to
sedimentary rocksPegmatitic texture
Exceptionally coarse grainedForm in late stages of crystallization of granitic
magmas
Porphyritic texture
Glassy texture
Igneous CompositionsIgneous rocks are composed primarily of
silicate mineralsDark (or ferromagnesian) silicates
OlivinePyroxeneAmphiboleBiotite mica
Igneous Compositions
Igneous rocks are composed primarily of silicate minerals
Light (or nonferromagnesian) silicatesQuartzMuscovite micaFeldspars
Igneous compositions
Granitic versus basaltic compositionsGranitic composition
Composed of light-colored silicatesDesignated as being felsic (feldspar and
silica) in compositionContains high amounts of silica (SiO2)
Major constituents of continental crust
Igneous compositions
Granitic versus basaltic compositionsBasaltic composition
Composed of dark silicates and calcium-rich feldspar
Designated as being mafic (magnesium and ferrum, for iron) in composition
More dense than granitic rocksComprise the ocean floor as well as many
volcanic islands
Igneous compositions
Other compositional groupsIntermediate (or andesitic) composition
Contain at least 25 percent dark silicate minerals
Associated with explosive volcanic activityUltramafic composition
Rare composition that is high in magnesium and iron
Composed entirely of ferromagnesian silicates
Classifying Igneous Rocks
Igneous rock textures: Crystal size is dictated by the rate of cooling
of the magma body. A slower cooling rate results in larger mineral crystals being formed in the rocks as they cool.
Porphyritic texture - Large crystals surrounded by fine grained rock. The rock initially cools slowly to form some large crystals and then cools quickly to form the fine grained rock surrounding them.
Mineralogy of common igneous rocks
Igneous compositionsSilica content as an indicator of
compositionSilica content in crustal rocks exhibits a
considerable rangeA low of 45 percent in ultramafic rocksOver 70 percent in felsic rocks
Igneous compositionsSilica content influences a magma’s
behaviorGranitic magma
High silica contentExtremely viscousLiquid exists at temperatures as low as 700oC
Igneous compositionsSilica content influences a magma’s
behaviorBasaltic magma
Much lower silica contentFluid-like behaviorCrystallizes at higher temperatures
Igneous compositions Naming igneous rocks – granitic (felsic) rocks
Granite Phaneritic Over 25 percent quartz, about 65 percent or more
feldspar May exhibit a porphyritic texture Very abundant as it is often associated with mountain
building The term granite covers a wide range of mineral
compositions
Granite
Igneous compositions
Naming igneous rocks – granitic (felsic) rocks
RhyoliteExtrusive equivalent of graniteMay contain glass fragments and vesiclesAphanitic textureLess common and less voluminous than
granite
Rhyolite
Igneous compositionsNaming igneous rocks – granitic (felsic)
rocksObsidian
Dark coloredGlassy texture
PumiceVolcanicGlassy textureFrothy appearance with numerous voids
Obsidian
Pumice
Igneous compositions
Naming igneous rocks – intermediate rocks
AndesiteVolcanic originAphanitic textureOften resembles rhyolite
Andesite porphyry
Igneous compositions
Naming igneous rocks – intermediate rocks
DioritePlutonic equivalent of andesiteCoarse grainedIntrusiveComposed mainly of intermediate feldspar
and amphibole
Diorite
Igneous compositions
Naming igneous rocks – basaltic (mafic) rocks
BasaltVolcanic originAphanitic textureComposed mainly of pyroxene and calcium-
rich plagioclase feldsparMost common extrusive igneous rock
Basalt
Igneous compositions
Naming igneous rocks – basaltic (mafic) rocks
GabbroIntrusive equivalent of basaltPhaneritic texture consisting of pyroxene and
calcium-rich plagioclaseMakes up a significant percentage of the
oceanic crust
Gabbro
Igneous compositions
Naming igneous rocks – pyroclastic rocksComposed of fragments ejected during a
volcanic eruptionVarieties
Tuff – ash-sized fragmentsVolcanic breccia – particles larger than ash
The Scheme for Igneous Rock Identification
Mineralogy of common igneous rocks
A Brief Tour
Crystal size DescriptionGrain Size
Where it was formedWhere it was formed Outside the volcano: ExtrusiveInside the Earth: Intrusive
Bubbles?Bubbles? Yes= Vesicular
BubblesBubbles??
No=Non-vesicular
ColorColorVery LightLightNotVery LightNot Very DarkDarkVery DarkNeither Light nor Dark
Light
est
Darke
st
DensityDensityVery LightLight DenseVery DenseMedium Density
Finding The Finding The MineralsMineralsIdentify the rockUnless you have other information,
work in the middle of the rock’s box.
This is the amount of Potassium Feldspar in the rock.
This is the amount of Quartz in the rock.
This is the amount of Plagioclase Feldspar in the rock.
Use tick marks on a scrap paper to measure the percentage.Potassium Feldspar 25%Quartz 40%
PracticePractice Name a light-colored, fine-grainedrock with no bubbles.
Rhyolite
Name a coarse-grained, dense rock.
Gabbro
suwun
Igneous rocks have “Intergrown Crystals”
Intergrown
Intergrown
Intergrown
Not Intergrown
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