Transcript PPT
THE LANGUAGE OF THE EARTH – PART I IGNEOUS ROCKS Average composition of the Earth’s Crust (by weight, elements, and volume) Layers of the Earth OCEANIC CONTINENTAL CRUST CRUST SiO2 47% 56% Al2O3 16% 18% FeO 13% 9% MgO 10% 3% CaO 10% 4% Na2O 2% 5.5% K2 O 0.7% 2.5% TiO2 1.1% 1.3% P2O5 0.2% 0.7% MANTLE SiO2 – 45% MgO – 37% FeO – 8% Al2O3 – 4% CaO – 3% others – 3% CORE Fe – 86% S – 10% Ni – 4% Compositional Layers Structural Layers 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) Igneous Environments Fine-grained = Volcanic Coarse-grained = Plutonic IGNEOUS ROCK TEXTURES f (rate of cooling = crustal environment) Plutonic---Volcanic 1 mm Degree of Undercooling 1 mm 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 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 Cinder Cone Fissure Eruption Basalt Lobe Mt Pinatubo Pahoehoe Crust Mt Fuji Mt St Helens New Zealand 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 • 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/ Water solubility (carrying capacity) in rhyolite as function of pressure; from Yamashita (1999) 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 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. Mt. St. Helens Explosive (pyroclastic) Eruptions Costa Rica Phillipines Mt. St. Helen’s Caldera Formation: The Fate of all Stratovolcanoes Pillowed Basalts Submarine Eruptions