THE LANGUAGE OF THE EARTH – PART I

IGNEOUS

ROCKS

Average composition of the Earth’s Crust(by weight, elements, and volume)

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%

THE ROCK CYCLE

Plate TectonicsThe Engine Behind the Rock Cycle

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)

Fine-grained Fine-grained = Volcanic= Volcanic

Coarse-grained Coarse-grained = Plutonic= Plutonic

Igneous Environments

IGNEOUS ROCK TEXTURESf (rate of cooling = crustal environment)

Plutonic---Volcanic

1 mm

1 mmDegree of Undercooling

Porphyritic TextureIndicative of

two-stage cooling

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

Magnesium-rich

Iron-rich

Igneous Rock-forming Minerals crystallized from Various Magma Compositions

Generation of Diverse Magma Compositions in Various Geologic Generation of Diverse Magma Compositions in Various Geologic (Tectonic) Settings(Tectonic) Settings

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

Properties of MagmasProperties of Magmas

Mt St Helens New Zealand

Mt FujiMt Pinatubo

Cinder Cone Fissure Eruption

BasaltLobe Pahoehoe Crust

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

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/

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

FLOOD BASALTSBasaltic Volcanism

in Continental Settings

NORTH SHOREFlood Basalts of the Midcontinent Rift

Shield VolcanoesBasaltic Volcanism in Oceanic Settings

Mauna Loa

Cinder Cones –sputtering basalt eruptions

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.

Explosive(pyroclastic)Eruptions

Mt St. Helens

Costa Rica

Phillipines

Mt. St. Helen’s

Caldera Formation:The Fate of allStratovolcanoes

Pillowed BasaltsSubmarine Eruptions