Lecture'10'Up#from#the#Inferno:#Magma#and#Igneous#Rocks'Igneous'Rocks'are'rocks'that'form'from'a'melt'• Solidified'molten'rock'or'melt'(which'“freezes”'at'high'temp.)'• Earth'is'mostly'Igneous'rock'• Magma'–subsurface'• Lava'0melt'at'the'surface'• Magma'erupts'via'volcanoes'• Melted'rocks'can'cool'above'or'below'ground'• There'are'many'varieties'of'igneous'rocks'Why'does'magma'form?'Earth'is'hot'inside.'Why?'• Planetesimal'and'meteorite'accretion'• Gravitational'compression'• Differentiation'• Radioactive'decay'Magma'Formation'• Partial'melting'in'crust/'upper'mantle'• Melting'is'from:'pressure'release,'Volatile'addition,'heat'transfer'• Geothermal'gradient'–Earth'is'hot'inside'• Geothermal'gradient'varies'form'place'to'place'Pressure'release'• Due'to'high'pressure,'the'rock'does'not'melt'• A'drop'in'pressure'initiates'“decompressional'melting”'Heat'transfer'• Rising'magma'carries'mantle'heat'• This'raises'T'in'crustal'rock'• Crustal'rock'melts'at'lower'T'Addition'of'Volatiles'Volatiles'cause'rocks'to'melt'at'much'lower'Temp.'• Water''• Carbon'dioxide'Adding'volatiles'to'hot,'dry'rocks'initiates'melting'Important'subduction'process'What'is'Magma'made'of?'Magmas'have'3'components'(solid,'liquid,'and'gas)'• Solid'–solidified'are'carried'by'the'liquid'• Liquid'–the'melt'itself'is'comprised'of'mobile'ions'• Different'mixes'of'elements'yields'different'magmas'Magma'Compositions'Composition'controls'density,'Temp.'And'viscosity'• These'characteristics'govern'eruptive'style'Why'are'there'different'magma'compositions?'Magmas'vary'chemically'due'to…'• Initial'source'rock'compositions'• Partial'melting'• Assimilation'• Fractional'crystallization'Magma'variation'Source'rock'dictates'initial'magma'composition'• Mantle'source'–ultramaifc'and'mafic'magmas'• Crustal'source'–Mafic,'intermediate,'and'felsic'magmas'Partial'melting'• Upon'heating,'silica'rich'minerals'melt'first'• Partial'meting,'then'yields'a'silica'rich'magma'• Removing'a'partial'melt'from'its'source'creates:'• Felsic'magma'• Mafic'residue'Assimilation'• Magma'melts'the'country'rock'it'passes'through'• Assimilated'materials'change'magma'composition'Fractional'Crystallization'As'magma'cools,'early'crystals'settle'by'gravity'Melt'composition'changes'as'a'result'Magma'migration'• Magma'is'less'dense'than'rock;'it'moves'upward'o Magma'moves'by:'o Injection'into'cracks'o Melting'overlying'rocks'o Squeezed'by'overburden'• Pressure'decrease'with'upward'migration'releases'volatiles'(bubbles),'thereby'increasing'viscosity'• Viscosity'controls'migration'ease'Viscosity'depends'on'temp,'volatiles'and'silica'Temp:'• Hotter'–lower'viscosity'• Cooler'–higher'viscosity'Volatile'content:'• More'volatiles'–lower'viscosity'• Less'volatiles'–higher'viscosity'Silica'content:'• Less'Silica'–lower'viscosity'• More'silica'–higher'viscosity'Igneous'Environments'• Two'major'categories'–based'on'cooling'site'o Extrusive'settings'cool'at'or'near'the'surface' Cool'rapidly' Chill'too'fast'to'grow'big'crystal s'• Intrusive'settings'–cool'at'depth'o Lose'heat'slowly'o Crystals'grow'large'• Most'mafic'magmas'extrude'• Most'felsic'magmas'do'not'Extrusive'characteristic'• Lava'flows'–sheets'of'cooled'lava'• Lava'flows'exit'volcanic'vents'and'flow'outward'• Lava'cools'as'it'flows,'eventually'sol idifyin g'• Low\viscosity'lava'(basalt)'can'flow'long'distances'Intrusive'Characteristics'• Intrusive'rocks'cool'at'depth,'they'don’t'surface'• Magma'invading'colder'country'rock'initiates'o Thermal'(heat)'metamorphism'and'melting'o Inflation'of'fractures'pushing'the'rock'aside'o Incorporation'of'country'rock'fragments'(xenoliths)'o Hydrothermal'(hot'water)'alteration'• Intrusive'contacts'preserve'evidence'of'high'heat'o Baked'zone'–rim'of'heat'altered'country'rock'o Chill'margin'–magma'at'contact'that'cooled'rapidly'• Xenolith'–country'rock'fragment'in'magma'o Thermally'altered'o Magma'cooled'before'xenolith'could'be'assimilated''Intrusive'activity'o Magma'intrudes'into'other'rocks'in'2'ways'o As'planar,'tabular'bodies'(dikes'and'sills)'and''o As'balloon\shaped'blobs'(plutons)'Tabular'intrusions'o Tend'to'have'a'uniform'thickness'o Can'be'traced'laterally'o Two'major'subdivisions'(sill'and'dike)'o Dikes'and'sills…'o Cause'the'rock'to'expand'and'inflat e'o Thermally'alter'the'country'rock'o Dikes…'o Spread'rocks'sideways'o Dominate'in'extensional'settings'(e.g.'MORs)'o Sills'o Lift'entire'landscapes'skyward'o Usually'intruded'near'the'surface'o Both'dikes'and'sills'exhibit'wide'variability'o Size''o Thickness'(or'width)'o Lateral'continuity''Plutonic'activity''• Most'magma'is'emplaced'at'depth'in'the'Earth'o A'large,'deep'igneous'body'is'called'a'pluton'• Plutonic'intrusions'modify'the'crust'• Plutons'may'coalesce'to'form'a'larger'batholith'o Plutons'are'created'at'subduction'zones'o Magma'generations'may'occur'of'over'10s'of'Ma.'o Long'subduction'history'linked'to'large' b atholiths''Intrusive'and'Extrusive'o Intrusive'and'extrusive'rocks'commonly'co\occur'o Magma'chambers'feed'overlying'volcanoes'o Magma'chambers'may'cool'to'become'plutons'o Many'igneous'geometries'are'possible'Cooling'Rates'o Haw'fast'is'heat'lost?'o Depth:'deep'is'hot,'shallow'is'cool.' Deep'plutons'cool'slowly'' Shallow'flows'cool'rapidly'o Shape:'surface'to'volume'ratio' Spherical'bodies'cool'slowly' Tabular'bodies'cool'faster'o Ground'water' Ground'water'removes'heat'What'you'should'know'o Definition'of'an'igneous'rock'o Igneous'rock'classification'o Basic'igneous'minerals'o Bowen’s'reaction'series'o Basic'textural'properties'of'igneous'rocks'Igneous'classification'o Igneous'rock'classification'is'based'on'composition'and'texture'o Composition'reveals'magma'source'o Texture'reveals'cooling'history'o Fast'cooling'–smaller'crystals'o Slow'cooling'–larger'crystals'Composition'Felsic'\66'to'76%'Silica'(SiO2')'dominated''by'quartz'and'feldspar'Intermediate'52'to'66%'silica'between'felisc'and'mafic'composition''Mafic'\45'to52%'silica'contains'Fe'and'Mg'minerals''Ultramafic'\38'to'45%'silica'dominated'by'Olivine'Minerals'composition'from'a'cooling'melt'o Bowen’s'reaction'series:'the'sequence'of'minerals'that'crystallize'from'silicate'melt''o
View Full Document