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Origin of Earth Nancy Van Wagoner, Acadia University Perspective in Space We live on planet Earth solar system Milky way galaxy universe Nancy Van Wagoner, Acadia University Origin of Universe Age? perhaps 8-10 billion years Big Bang Theory At that time all matter that formed the sun, formed the earth and us. Remember matter changes form but is not destroyed. Nancy Van Wagoner, Acadia University The universe was mostly light elements Hydrogen 75% atomic weight 1 Helium 25% atomic weight 2 Nancy Van Wagoner, Acadia University Where did heavier elements come from? thermonuclear reactions in exploding stars called supernova. For more information Supernova--Death of a Star, National Geographic, v.173, n.5, p.618, 1988. Nancy Van Wagoner, Acadia University It is postulated that in this expanding universe about 6 billion years ago, a supernova exploded. Gas and dust began to spin which was caused by rotation of the galaxy. Gravity concentrated most of the mass at the centre which would become the sun and producing a solar nebula. sun dust and gas fig. 1.8, Thompson and Turk Planets condensed from material in the nebula but, because the nebula was stratified with respect to temperature and composition the planets are different in composition. Cold Hot Fe, Al, Ca sun Ice (H,C,N) Mars Earth MercuryVenus Jupiter Asteroid Belt Earth-like Planets Jovian Planets Look more closely at Earth. Earth at one time was a homogeneous spinning and contractng body Fe Si K Ca Al Mg Si Al Si Nancy Van Wagoner, Acadia University Ca Na The protoplanet grew hotter until it reached the melting temperature of iron. – Melting event – The sources of heat were from - meteor impact - radioactive decay - gravitational contraction Nancy Van Wagoner, Acadia University Result of Melting event - lighter materials rose to the surface - heavier ones sank to the core The result: compositional and density stratified the Earth. Nancy Van Wagoner, Acadia University Composition of Earth and Crust Crust Fe O Mg Si S Ni Ca Al Co Na Mn K Ti P Cr Nancy Van Wagoner, Acadia University 5.6 45 2 28 0.03 0.007 4.2 8. 2 0.002 2.4 0.09 2.1 0.57 0.10 0.01 Total Earth 35 28 17 13 2.7 2.7 0.61 0.44 0.20 0.14 0.09 0.07 0.04 0.03 0.01 Periodic Table Exercise notice order of elements Elements expected near centre: Elements expected near surface: The Ur/Th surprise – Ur and Th became concentrated in the crust because they are about the same size and charge as some of the lighter elements (K,Na) and therefore can replace these elements in the crystal structure of minerals. Nancy Van Wagoner, Acadia University Layers of Earth Nancy Van Wagoner, Acadia University Inner Core radius = 1216 km composition = Iron, Nickel +/silicon, carbon 1216 km state = solid Nancy Van Wagoner, Acadia University Outer Core thickness 2270 km Composition = same as inner core state = liquid density = 9-15 gm/cc Nancy Van Wagoner, Acadia University Mantle thickness = 2885 km comp = iron and magnesium silicates and oxides density = 4-7 gm/cc Nancy Van Wagoner, Acadia University Layers of the Mantle 100km lithosphere = crust + upper mantle – cool, rigid, brittle asthenosphere = partially molten 350 km – hot, weak, plastic mesophere = solid but hot – due to high pressure to base of mantle Nancy Van Wagoner, Acadia University crust continental lithosphere oceanic lithosphere oceanic crust 5-10 km rigid mantle continental crust 30-70 km Oceanic Crust dense rocks = 3.2 gm/cc basalt (iron and magnesium silicates) relatively thin Nancy Van Wagoner, Acadia University Continental Crust less dense = 2.8 gm/cc granitic Si, Al thicker Nancy Van Wagoner, Acadia University Why are there Continents and Ocean Basins ? Nancy Van Wagoner, Acadia University Plate Tectonics Lithosphere broken into lithospheric plates Move with respect to each other rate = 1- 12 cm/yr. Nancy Van Wagoner, Acadia University Plate Margins - Divergent: spreading centre; new crust created - Convergent: subduction; collision - Transform: earthquakes - San Andres fault Nancy Van Wagoner, Acadia University