remote sensing of the earth’s interior - richard...
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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 (106 yr scale)• Planetary “embryos” form (108 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
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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
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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
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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
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From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
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
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From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
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From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.8 km
Mid-Ocean Ridge System
From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
Subduction Zones
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From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
Subduction Zones: SeismicTomographic Image
From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.
Plume magmatismFate of subducted slabs
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