petrologi 5-nomenklature

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