the language of the earth – part i igneous rocks
TRANSCRIPT
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THE LANGUAGE OF THE EARTH – PART I
IGNEOUS
ROCKS
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Average composition of the Earth’s Crust(by weight, elements, and volume)
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MANTLEMANTLE
SiOSiO22 – 45% – 45%
MgO – 37%MgO – 37%
FeO – 8%FeO – 8%
AlAl22OO33 – 4% – 4%
CaO – 3% CaO – 3%
others – 3%others – 3%
OCEANICOCEANIC CONTINENTALCONTINENTAL
CRUSTCRUST CRUSTCRUST
SiOSiO22 47%47% 56%56%
AlAl22OO33 16%16% 18%18%
FeO FeO 13%13% 9% 9%
MgO MgO 10%10% 3% 3%
CaO CaO 10%10% 4% 4%
NaNa22O O 2% 2% 5.5% 5.5%
KK22O O 0.7% 0.7% 2.5% 2.5%
TiOTiO22 1.1% 1.1% 1.3% 1.3%
PP22OO55 0.2% 0.2% 0.7% 0.7%
Layers of the EarthCompositional Layers
StructuralLayers
CORECOREFe – 86%Fe – 86%S – 10%S – 10%Ni – 4%Ni – 4%
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THE ROCK CYCLE
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Plate TectonicsThe Engine Behind the Rock Cycle
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Definition
IGNEOUS ROCK - An aggregate of minerals crystallized from molten rock (magma). Major distinctions in rock type are based on two criteria:
1) the chemical composition of the magma (mafic to felsic) and
2) the environment of magma emplacement (plutonic or intrusive vs. volcanic or extrusive)
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Fine-grained Fine-grained = Volcanic= Volcanic
Coarse-grained Coarse-grained = Plutonic= Plutonic
Igneous Environments
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IGNEOUS ROCK TEXTURESf (rate of cooling = crustal environment)
Plutonic---Volcanic
1 mm
1 mmDegree of Undercooling
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Porphyritic TextureIndicative of
two-stage cooling
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Magma CompositionRelated to PartialMelting of Source
Melting the mantle makes MAFIC Magma
Melting the crust makes FELSIC Magma
Igneous rocks crystallize over a range of temperatures
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Magnesium-rich
Iron-rich
Igneous Rock-forming Minerals crystallized from Various Magma Compositions
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Generation of Diverse Magma Compositions in Various Geologic Generation of Diverse Magma Compositions in Various Geologic (Tectonic) Settings(Tectonic) Settings
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Volcanism and Earth’s SystemsAtmosphere – originally created from gases released from
volcanic eruption
Hydrosphere – produced by condensation of volcanic water vapor
Biosphere both positively and negatively influenced by volcanism• lava flows and ash weather to produce fertile soils• violent eruptions can destroy nearly all life in their paths• large amounts of ash and volcanic gases in atmosphere can
trigger rapid climate changes and contribute to mass extinctions
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Properties of MagmasProperties of Magmas
Mt St Helens New Zealand
Mt FujiMt Pinatubo
Cinder Cone Fissure Eruption
BasaltLobe Pahoehoe Crust
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Effusive Eruptions• Mafic magma
• Relatively low gas content (<1%)
• Fountaining followed by flow as gas content diminishes
• Creates vesicular to massive lava flows
Photos from USGS
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Explosive Eruptions
Water solubility (carrying capacity) in rhyolite as function of pressure; from Yamashita (1999)
• Mostly involves intermediate to felsic magmas
• Driven by degassing of magma as it rises up the neck of the volcanic vent
• The dramatic increase of volume resulting from degassing causes the magma to be violently thrust out the neck and shattered into fine fragments – VOLCANIC ASH
• Creates pyroclastic deposits
http://www.geology.sdsu.edu/how_volcanoes_work/
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Eruptive Styles and Volcanic Landforms
Effusive - magma fountains or oozes from vents• Fissure eruptions (flood or plateau basalts)• Shield Volcanoes• Submarine Eruptions (pillowed basalts)
Explosive – violent eruptions of gas-rich magmas• Stratovolcanoes (composite)• Pyroclastic eruptions – ash and fragments• Calderas – self-destruction of stratovolcanoes
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FLOOD BASALTSBasaltic Volcanism
in Continental Settings
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NORTH SHOREFlood Basalts of the Midcontinent Rift
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Shield VolcanoesBasaltic Volcanism in Oceanic Settings
Mauna Loa
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Cinder Cones –sputtering basalt eruptions
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Mt. St. Helens
StratovolcanoSteep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows, are called stratovolcanoes. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions.
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Explosive(pyroclastic)Eruptions
Mt St. Helens
Costa Rica
Phillipines
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Mt. St. Helen’s
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Caldera Formation:The Fate of allStratovolcanoes
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Pillowed BasaltsSubmarine Eruptions
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