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Accretion and Layering of Terrestrial Planets Following condensation of planetary embryos in solar system disk: EmbryoEmbryo-embryo collisions, giant impacts for 100 m.y. – Redistributed and segregated elements chaotically – Led to incremental development of FeFe-rich core Sinking of dense metallic elements (Fe, Ni) released gravitational energy ShortShort-lived radioisotopes = heating These energy sources combined to heat interior to temperature of 7000 K at earth’s center, 5500 K at corecore-mantle boundary Accretion and Layering of Terrestrial Planets Heat loss at surface due to radiation – thin chilled crust forms – Insulates thick subsurface magma ocean Coalescing blobs of Fe sink through convecting magma ocean Result is a layerd Earth – Thin rocky (silcate) crust » Feldspar+quartz+pyroxene – Thick rocky mantle » Olivine+pyroxene (Fe & Mg – rich silicate minerals) – Metallic (Fe(Fe-rich) core The Earth’s Interior Earth’s interior is largely inaccessible The Earth’s Interior How do we know that the Earth comprises: – crust – mantle – metallic core (solid and liquid portions) ? How deep have we drilled? continental crust Kola, Russia 12.2 km deep hole 15 years to drill ocean crust East Pacific 2.2 km deep hole 10 years to drill The Earth’s Interior Most of what we know is from geophysics Geophysics is basically “remote sensing” of Earth’s interior – seismic waves – magnetic field – gravity – heat Evidence from Seismic Waves Seismic reflection – return of seismic energy to surface – rock layers of different density » boundary reflects energy like a mirror » time since earthquake gives depth to boundary Seismic refraction – bending of seismic waves passing from one rock to another – similar to light rays passing through eyeglasses – generates curved ray paths Evidence from Seismic Waves Seismic reflection – return of seismic energy to surface – rock layers of different density » boundary reflects energy like a mirror » time since earthquake gives depth to boundary Seismic refraction – bending of seismic waves passing from one rock to another – similar to light rays passing through eyeglasses – generates curved ray paths Earth’s Internal Structure Established using seismic reflection, refraction The crust – Continental » » – Oceanic » » Less dense 20-70 km thick more dense 5-10 km thick Mohorovičic 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 » – Asthenosphere » » – Crust & uppermost mantle Low velocity zone lubrication for plate tectonics Lower mantle » » » boundaries at 400 & 670 km Pressure increases with depth more dense mineral structures 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 = faster seismic wave velocities Earth’s Internal Structure The Core – P-wave Shadow Zones – S-wave Shadow Zone – What satisfies these observations? » A solid inner core, molten liquid outer core Earth’s Internal Structure A “static” picture Earth’s Internal Structure Composition of the Core – Overall density of Earth = 5.5 g/cm3 – Core must be dense to offset mantle+crust (<3 g/cm3) – Made of metallic iron (+Nickel, Oxygen, Sulfer?) » » required to generate magnetic field iron meteorites = planetary interior – Molten convecting outer/solid inner parts The Core-Mantle Boundary – Convection in both core and mantle – Ultra Low Velocity Layer – D” layer A Dynamic Mantle! Seismic Tomography – “CAT Scan” of mantle – Seismic waves fastest in: » Dense cold rock (blue) » Old, cold, subducted ocean plates ? – Waves slowest in: » Less dense, warmer rock (red) » Upwelling hot plumes, heated by core? – Upwelling of deepest mantle “plumes” coupled with sinking of subducted plates (“slabs”) » Mantle convection animation