Igneous RocksChapter 4

General characteristics of magmaIgneous rocks form as molten rock cools and solidifiesGeneral characteristics of magma:Parent material of igneous rocksForms from partial melting of rocksMagma at surface is called lava

General characteristics of magmaGeneral characteristic of magmaRocks formed from lava = extrusive, or volcanic rocksRocks formed from magma at depth = intrusive, or plutonic rocks

General characteristics of magmaThe nature of magmaConsists of three components:Liquid portion = meltSolids, if any, are silicate mineralsVolatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

General characteristics of magmaCrystallization of magmaCooling of magma results in the systematic arrangement of ions into orderly patternsThe silicate minerals resulting from crystallization form in a predictable orderTexture - size and arrangement of mineral grains

Igneous texturesTexture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking mineralsFactors affecting crystal sizeRate of coolingSlow rate = fewer but larger crystalsFast rate = many small crystalsVery fast rate forms glass

Igneous texturesFactors affecting crystal size % of silica (SiO2) presentDissolved gases

Igneous texturesTypes of igneous texturesAphanitic (fine-grained) textureRapid rate of coolingMicroscopic crystalsMay contain vesicles (holes from gas bubbles)Phaneritic (coarse-grained) textureSlow coolingLarge, visible crystals

Aphanitic texture

Phaneritic texture

Igneous texturesTypes of igneous texturesPorphyritic textureMinerals form at different temperaturesLarge crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)Glassy textureVery rapid cooling of lavaResulting rock is called obsidian

Igneous texturesTypes of igneous texturesPyroclastic textureFragmental appearance produced by violent volcanic eruptionsOften appear more similar to sedimentary rocksPegmatitic textureExceptionally 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) silicatesOlivine, pyroxene, amphibole, and biotite micaLight (or nonferromagnesian) silicatesQuartz, muscovite mica, and feldspars

Igneous compositionsGranitic versus basaltic compositionsGranitic compositionLight-colored silicatesTermed felsic (feldspar and silica) in compositionHigh amounts of silica (SiO2)Major constituent of continental crust

Igneous compositionsGranitic versus basaltic compositionsBasaltic compositionDark silicates and calcium-rich feldsparTermed mafic (magnesium and ferrum, for iron) in compositionHigher dense than granitic rocksComprise the ocean floor and many volcanic islands

Basaltic lava dropping into the ocean along Kilauea Volcano along the southeastern coast of the big island of Hawaii

Igneous compositionsOther compositional groupsIntermediate (or andesitic) compositionContain 25% or more dark silicate mineralsAssociated with explosive volcanic activityUltramafic compositionRare composition that is high in magnesium and ironComposed entirely of ferromagnesian silicates

Igneous compositionsSilica content as an indicator of compositionExhibits a considerable range in the crust45% to 70%Silica content influences magma behaviorGranitic magmas = high silica content and viscousBasaltic magmas = much lower silica content and more fluid-like behavior

Igneous compositionsNaming igneous rocks granitic rocksGranitePhaneriticOver 25% quartz, about 65% or more feldsparVery abundant - often associated with mountain buildingThe term granite includes a wide range of mineral compositions

Granite

Igneous compositionsNaming igneous rocks granitic rocksRhyoliteExtrusive equivalent of graniteMay contain glass fragments and vesiclesAphanitic textureLess common and less voluminous than granite

Rhyolite

Igneous compositionsNaming igneous rocks granitic rocksObsidianDark coloredGlassy texturePumiceVolcanicGlassy textureFrothy appearance with numerous voids

Pumice is very glassy and sharp, with countless vesicles.

Igneous compositionsNaming igneous rocks intermediate rocksAndesiteVolcanic originAphanitic textureDioritePlutonic equivalent of andesiteCoarse grained

Andesite

Diorite

Igneous compositionsNaming igneous rocks basaltic rocksBasaltVolcanic originAphanitic textureComposed mainly of pyroxene and calcium-rich plagioclase feldsparMost common extrusive igneous rock

Basalt

Igneous compositionsNaming igneous rocks mafic rocksGabbroIntrusive equivalent of basaltPhaneritic texture consisting of pyroxene and calcium-rich plagioclaseSignificant % of the oceanic crust

Gabbro

Igneous compositionsNaming igneous rocks pyroclastic rocksComposed of fragments ejected during a volcanic eruptionVarietiesTuff = ash-sized fragmentsVolcanic breccia = particles larger than ash

Origin of magmaHighly debated topicGenerating magma from solid rockRole of heatTemperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer of depthRocks in the lower crust and upper mantle are near their melting pointsAny additional heat may induce melting

Origin of magmaRole of pressureIncreases in confining pressure cause an increase in a rocks melting temperature When confining pressures drop, decompression melting occursRole of volatilesVolatiles (primarily water) cause rocks to melt at lower temperaturesImportant factor where oceanic lithosphere descends into the mantle

Decompression melting

Evolution of magmasA single volcano may extrude lavas exhibiting very different compositionsEach volcanic eruption tends to exhibit a unique geochemical fingerprint, defined by trace element percentagesBowens reaction series Minerals crystallize in a systematic fashion based on their melting pointsDuring crystallization, the composition of the liquid portion of the magma continually changes

Bowens reaction series

Evolution of magmasProcesses responsible for changing a magmas compositionMagmatic differentiationSeparation of a melt from earlier formed crystalsAssimilationChanging a magmas composition by the incorporation of surrounding rock bodies into a magma

Magma evolves as the hotter minerals crystallize and settle to the bottom of the magma chamber

Evolution of magmasProcesses responsible for changing a magmas compositionMagma mixingTwo chemically distinct magmas may produce a composition quite different from either original magma

Assimilation, magma mixing, and magmatic differentiation

Evolution of magmasPartial melting and magma formationIncomplete melting of rocks is known as partial meltingFormation of basaltic magmasMost originate from partial melting of ultramafic rock in the mantle at oceanic ridgesLarge outpourings of basaltic magma are common at Earths surface

Evolution of magmasPartial melting and magma formationFormation of andesitic magmasProduced by interaction of basaltic magmas and more silica-rich rocks in the crust May also evolve by magmatic differentiation

Evolution of magmasPartial melting and magma formationFormation of granitic magmasMost likely form as the end product of crystallization of andesitic magmaGranitic magmas are more viscous than other magmas so they tend to lose their mobility before reaching the surfaceTend to produce large plutonic structures