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Igneous Rocks Igneous Rocks
Definition of Igneous Rocks Definition of Igneous Rocks
- • Igneous rocks form from cooling andIgneous rocks form from cooling and
crystallization of molten rock-crystallization of molten rock- magmamagma
- – MagmaMagma –– molten rock within the Earthmolten rock within the Earth
- – LavaLava –– molten rock on the Earthmolten rock on the Earth’’s surfaces surface
- • Igneous rocks form the framework for theIgneous rocks form the framework for the
earth’earth’s crusts crust
General Characteristics General Characteristics of Magmaof Magma
- • Igneous rocks form as molten rockIgneous rocks form as molten rock
cools and solidifiescools and solidifies
- • General characteristics of magma:General characteristics of magma:
- • Parent material of igneous rocksParent material of igneous rocks
- • Forms from partial melting of rocksForms from partial melting of rocks
- • Magma at surface is called lavaMagma at surface is called lava
General Characteristics General Characteristics of Magmaof Magma
- • General characteristics of magma:General characteristics of magma:
- • Rocks formed from lava are extrusive,Rocks formed from lava are extrusive, or volcanic rocksor volcanic rocks
- • Rocks formed from magma at depth areRocks formed from magma at depth are intrusive, or plutonic rocksintrusive, or plutonic rocks
Origin of MagmaOrigin of Magma
- • Generating magma from solid rockGenerating magma from solid rock
- • Role of heatRole of heat
- – Temperature increases with depth in the upperTemperature increases with depth in the upper crust (geothermal gradient)crust (geothermal gradient) »» Average between 20Average between 20 oo^ C to 30C to 30 oo^ C per kilometerC per kilometer
- – Rocks in the lower crust and upper mantle areRocks in the lower crust and upper mantle are near their melting pointsnear their melting points
- – Additional heat may induce meltingAdditional heat may induce melting
A Typical Geothermal GradientA Typical Geothermal Gradient
Origin of MagmaOrigin of Magma
- • Role of pressureRole of pressure
- – Increases in confining pressure increases a rockIncreases in confining pressure increases a rock’’ss melting temperaturemelting temperature
- – When confining pressures drop, decompressionWhen confining pressures drop, decompression melting occursmelting occurs
- • Role of volatilesRole of volatiles
- – Volatiles (primarily water) cause melting at lowerVolatiles (primarily water) cause melting at lower temperaturestemperatures
- – Important factor where oceanic lithosphereImportant factor where oceanic lithosphere descends into the mantledescends into the mantle
Decompression Melting Decompression Melting
Igneous rocks Igneous rocks
Formed from the cooling andFormed from the cooling and consolidation of magmaconsolidation of magma
- • Plutonic (intrusive) —Plutonic (intrusive)— cooledcooled below the surfacebelow the surface
- • Volcanic (extrusive) —Volcanic (extrusive)— cooledcooled on the surfaceon the surface
Classification of Igneous Rocks Classification of Igneous Rocks
Defined by texture:Defined by texture:
- • Fine-Fine-grained: extrusive or volcanicgrained: extrusive or volcanic
- •^ Coarse-Coarse-grained: intrusive or plutonicgrained: intrusive or plutonic
General Characteristics General Characteristics of Magmaof Magma
- • Crystallization of magmaCrystallization of magma
- • Cooling of magma results in theCooling of magma results in the systematic arrangement of ions intosystematic arrangement of ions into orderly patternsorderly patterns
- • Silicate minerals result fromSilicate minerals result from crystallization in a predictable ordercrystallization in a predictable order
- • Texture is the size and arrangementTexture is the size and arrangement of mineral grainsof mineral grains
Igneous TexturesIgneous Textures
- • Texture is the overall appearance ofTexture is the overall appearance of
a rock based on the size, shape, anda rock based on the size, shape, and
arrangement of interlocking mineralsarrangement of interlocking minerals
- • Factors affecting crystal size:Factors affecting crystal size:
- • Rate of coolingRate of cooling
- – Slow rate = fewer but larger crystalsSlow rate = fewer but larger crystals
- – Fast rate = many small crystalsFast rate = many small crystals
- – Very fast rate forms glassVery fast rate forms glass
Coarsely Crystalline Granite Coarsely Crystalline Granite
Photomicrograph of Granite Photomicrograph of Granite
Classification of Igneous Rocks Classification of Igneous Rocks
Based on Mineral Composition and TextureBased on Mineral Composition and Texture
TexturesTextures--^ reflect rate of coolingreflect rate of cooling
- •^ Phaneritic-Phaneritic-^ mineral crystals are visiblemineral crystals are visible^ e.g. Granite & Gabbroe.g. Granite & Gabbro
- •^ Pegmatite-Pegmatite-^ exceptionally large crystals e.g. Pegmatiteexceptionally large crystals e.g. Pegmatite
- •^ Aphanitic-Aphanitic-^ crystals not visible e.g. Rhyolitescrystals not visible e.g. Rhyolites^ & Basalt& Basalt
- •^ Porphyritic-Porphyritic-^ large crystals surrounded by small crystalslarge crystals surrounded by small crystals-- (indicate slow & abrupt rapid cooling) e.g. Porphyritic Granite(indicate slow & abrupt rapid cooling) e.g. Porphyritic Granite
- • Volcanic glass-Volcanic glass- very rapid coolingvery rapid cooling
- – Pumice (high gaseous silica rich lava) & obsidianPumice (high gaseous silica rich lava) & obsidian
Finely Crystalline Basalt Finely Crystalline Basalt
Photomicrograph of Basalt Photomicrograph of Basalt
Classification of Igneous Rocks Classification of Igneous Rocks
Based on Mineral Composition and TextureBased on Mineral Composition and Texture
TexturesTextures--^ reflect rate of coolingreflect rate of cooling
- •^ Phaneritic-Phaneritic-^ mineral crystals are visiblemineral crystals are visible^ e.g. Granite & Gabbroe.g. Granite & Gabbro
- •^ Pegmatite-Pegmatite-^ exceptionally large crystals e.g. Pegmatiteexceptionally large crystals e.g. Pegmatite
- •^ Aphanitic-Aphanitic-^ crystals not visible e.g. Rhyolitescrystals not visible e.g. Rhyolites^ & Basalt& Basalt
- •^ Porphyritic-Porphyritic-^ large crystals surrounded by small crystalslarge crystals surrounded by small crystals-- (indicate slow & abrupt rapid cooling) e.g. Porphyritic Granite(indicate slow & abrupt rapid cooling) e.g. Porphyritic Granite
- • Volcanic glass-Volcanic glass- very rapid coolingvery rapid cooling
- – Pumice (high gaseous silica rich lava) & obsidianPumice (high gaseous silica rich lava) & obsidian
Igneous textures Igneous textures
GlassyGlassy – no minerals present–no minerals present
CrystallineCrystalline – rocks made of–rocks made of
mineral grainsmineral grains
PorphyriticPorphyritic (^) - - mixture of coarsemixture of coarse
and fineand fine
VesicularVesicular - with bubble holes- with bubble holes
Pyroclastic Igneous Rocks Pyroclastic Igneous Rocks
Obsidian Obsidian PumicePumice
AshAsh
Types of Igneous Rocks Types of Igneous Rocks
Based on Silica ContentBased on Silica Content
- •^ UltramaficUltramafic^ (low silica content < 40%)(low silica content < 40%)
- • MaficMafic (low 45-(low 45- 55%)55%)
- – GabbroGabbro (plutonic)(plutonic)
- –^ Basalt (volcanic)Basalt (volcanic)
- • IntermediateIntermediate (55 -(55- 65%)65%)
- – Diorite (plutonic)Diorite (plutonic)
- –^ AndesiteAndesite (volcanic)(volcanic)
- • FelsicFelsic (high silica content > 65%)(high silica content > 65%)
- – Granite (plutonic)Granite (plutonic)
- –^ RhyoliteRhyolite (volcanic)(volcanic)
Classification of Igneous Rocks Classification of Igneous Rocks
Determined by composition (bothDetermined by composition (both chemical and mineralogical):chemical and mineralogical):
- • magnesium (Mg)magnesium (Mg) + iron (Fe)+ iron (Fe) == maficmafic
- • feldspar + quartz (Si)feldspar + quartz (Si) == felsicfelsic
Types of Igneous Rocks Types of Igneous Rocks
Based on Silica ContentBased on Silica Content
- • UltramaficUltramafic (low silica content < 40%; Mg(low silica content < 40%; Mg--
and Fe-and Fe-rich)rich)
- – PeridotitePeridotite
- • MaficMafic^ (low 45-(low 45- 55%; Mg55%; Mg-- and Feand Fe--rich)rich)
- – GabbroGabbro (plutonic)(plutonic)
- – Basalt (volcanic)Basalt (volcanic)
- • IntermediateIntermediate^ (55(55 -- 65%; also rich in65%; also rich in
feldspar)feldspar)
- – Diorite (plutonic)Diorite (plutonic)
Igneous CompositionsIgneous Compositions
- • Granitic versus basaltic compositionsGranitic versus basaltic compositions
- • Granitic compositionGranitic composition
- – LightLight--colored silicatescolored silicates
- • TermedTermed felsicfelsic (feldspar and silica) in(feldspar and silica) in compositioncomposition - – High silica (SiOHigh silica (SiO 22 ) content) content - – Major constituent of continental crustMajor constituent of continental crust
