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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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From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.

•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