Remote Sensing of the Earth s Interior Earth s interior is largely inaccessible Origin and Layering of the Earth: Geochemical Perspectives Composition of Earth cannot be understood in isolation Sun and meteorites are closely linked Solar system formed in Milky Way galaxy @ Big Bang 15 Ma Nucleosynthesis in stars, H+He ejected > rotating gas/dust cloud Material in compressed disk heats, volatilizes, cools Most refractory dust particles cooled first Accretion in several stages: Planetesimals 10 m to 1000 km diameter form (10 kyr time scale) Planetesimals gow by collisions/intersecting orbits (10 6 yr scale) Planetary embryos form (10 8 yr time scale) Embryos collided to form planets Earth-Moon system may reflect such a collision Sun s composition gives best estimate for that of Solar Nebula Mainly H + He Relative abundances of other elements nearly identical to meteorites
Remote Sensing of the Earth s Interior Geophysics: Tools seismic waves (velocity, tomography) gravity heat flow/temperature distribution magnetic field past and present satellite (GPS) geodesy Inferences gross composition of crust, mantle, core boundaries of property-specific regions scale of convection/tectonics structure & dynamics of mantle & crust Remote Sensing of the Earth s Interior Geochemistry Tools: Major, trace & volatile element distribution melts vs. residua Mineralogy Experimental petrology Memory of past events in radioisotopic systems Inferences: composition of crust, mantle, core mechanisms and depth of mantle melting quantitative history from radioisotopic dating signatures of tectonic processes present and past structure & dynamics of mantle & crust
Earth s Internal Structure Established using seismic reflection, refraction Crust Continental Less dense 20-70 km thick Oceanic more dense 5-10 km thick Mohorovicic discontinuity Boundary separating crust from mantle defined by increase in P-wave velocity (to 8 km/sec) Earth s Internal Structure The Mantle Ultramafic Rock Lithosphere Crust & uppermost mantle Asthenosphere Low velocity zone lubrication for plate tectonics Lower mantle boundaries at 400 & 670 km Pressure increases with depth more dense mineral structures
Plate Tectonics Paradigm Consequence of heat loss Convection transfers heat effectively Mantle flows on geologic timescales Lithospheric plates meet along 3 boundaries Divergent Convergent Transform Melting, volcanism coincide with plate boundaries Exception: Hot spot or intraplate magmatism Plate tectonics influences magma generation
Plate Tectonics Paradigm Plate tectonics influences magma generation Decompression melting active upwelling of buoyant mantle plumes passive upwelling associated with removal of lithospheric lid at divergent boundary (MOR) Hydrous (fluxed) melting subduction zones Relative volumes Chemical & isotopic fingerprinting of lavas provides information about mantle that has melted
Mid-Ocean Ridge System 8 km Subduction Zones 7
Subduction Zones: Seismic Tomographic Image Plume magmatism Fate of subducted slabs
Magma erupted at mid-ocean ridges (MORB) plumes (OIB) subduction zones (IAB) Sample mantle from which they come Chemical fingerprinting Trace elements Isotopes Clues to origin & history of mantle From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.