THE LANGUAGE OF THE EARTH – PART I IGNEOUS ROCKS

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

MANTLE SiO 2 – 45% MgO – 37% FeO – 8% Al 2 O 3 – 4% CaO – 3% others – 3% OCEANICCONTINENTAL CRUSTCRUST SiO 2 47%56% Al 2 O 3 16%18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na 2 O 2% 5.5% K 2 O 0.7% 2.5% TiO 2 1.1% 1.3% P 2 O 5 0.2% 0.7% Layers of the Earth Compositional Layers Structural Layers CORE Fe – 86% S – 10% Ni – 4%

THE ROCK CYCLE

Plate Tectonics The 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 = Volcanic Coarse-grained = Plutonic Igneous Environments

IGNEOUS ROCK TEXTURES f (rate of cooling = crustal environment) Plutonic---Volcanic 1 mm Degree of Undercooling

Porphyritic Texture Indicative of two-stage cooling

Magma Composition Related to Partial Melting 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 (Tectonic) Settings

Volcanism and Earth’s Systems Atmosphere – 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 Magmas Mt St HelensNew Zealand Mt Fuji Mt Pinatubo Cinder Cone Fissure Eruption Basalt Lobe 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

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 BASALTS Basaltic Volcanism in Continental Settings

NORTH SHORE Flood Basalts of the Midcontinent Rift

Shield Volcanoes Basaltic Volcanism in Oceanic Settings Mauna Loa

Cinder Cones –sputtering basalt eruptions

Mt. St. Helens Stratovolcano Steep, 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 all Stratovolcanoes

Pillowed Basalts Submarine Eruptions