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Transcript
Chapter 14 Earth’s Interior Earth’s Interior Interior Terms • Crust – Oceanic – Continental • Mantle – Upper – Transition – Lower • Core – Outer – Inner • • • • • • • • • Peridotite Eclogite Spinel Perovskite Xenolith Discontinuity Phase Change Tomography Kimberlite How do we know what is inside the Earth? • Seismology: Earthquake waves Interior Terms • • • • • • • • Basalt • Asthenosphere Chondrule • Lithosphere Multi-anvil press Diamond anvil cell Convection Magnetic reversal Dynamo Magnetic field Earthquakes • Earthquake Monitor • http://www.iris.edu/seismon/bigmap/index.phtml • Cosmochemistry: Meteorites • Experiments: Laboratory synthesis 1 Major Parts • Ocean water 0.025% of mass • Crust (Above the Moho) 0.5% of mass – Oceanic (7 to 10 km of basalt & gabbro) – Continental (30 to 60 km of granite) • Mantle - Moho to 2900 km - Solid Rock – 65% of total mass – Upper (Moho to 410km) (Olivine + pyroxene) – Transition Zone (410 -670 km) (Silicate Spinels) – Lower 670 to 2900 km (Perovskite + periclase) • Core (2900 to 6367km) 35% of mass – Outer Core (Liquid Metal) – Inner Core (Solid Metal) Crust • Oceanic Crust (Basalt and Gabbro) – Thin (7 - 10 km) – Dense (2.9 - 3.1 g/cm3) – Young (< 250 my) • Continental Crust (Granite + Diorite + Seds) – Thick (30 - 60 km) – Light (2.7 - 2.8 g/cm3) – Old (250 - 4000 my) Mantle • Upper Mantle (Moho to 410 km) – Peridotite (Olivine + pyroxene) – Eclogite (Pyroxene + Garnet) • Transition Zone (410 - 670 km) – Spinels and Spinelloids • Lower Mantle (670 - 2900 km) – MgSiO3 - Perovskite – MgO - Periclase How do we know what’s there? • Seismology – S-wave shadow – P and S reflections and conversions • Meteorites: – Earth formed from primitive meteorites • Xenoliths from kimberlites & basalts • Experiments Core • Outer Core (2900 to 5150km) – Molten iron metal (+ ~10% lighter element) – No S-wave transmission • Inner Core (5150 - 6378 km) – Solid Iron Metal – May slowly rotate relative to mantle Seismology • S-wave shadow – No S-waves opposite-side earthquakes – Core (outer) must be molten • P and S reflections – Discontinuities at 410 and 670 km – Phase Changes • P and S travel time anomalies – Tomography – Hot and cold regions 2 Seismology P-waves S-waves Clicker Question • What observation led to the conclusion that the Earth has a liquid outer core? • A) Earth’s gravity field • B) P-wave shadow • C) S-wave shadow • D) Seismic reflection from 660km • E) Polar wandering Clicker Question • Why does the P-wave shadow exist? • A) Seismic refraction into the slower outer core • B) P-waves cannot pass the liquid outer core • C) Seismic reflection at the coremantle boundary • D) Seismic reflection from 660km • E) No earthquakes large enough Velocity Structure and Earthquakes at N. Tonga (VanderHilst, Nature (1995) Red = slow = hot green = fast = cold Clicker Question • What observation led to the conclusion that the Earth has a liquid outer core? • A) Earth’s gravity field • B) P-wave shadow • C) S-wave shadow • D) Seismic reflection from 660km • E) Polar wandering Clicker Question • Why does the P-wave shadow exist? • A) Seismic refraction into the slower outer core • B) P-waves cannot pass the liquid outer core • C) Seismic reflection at the coremantle boundary • D) Seismic reflection from 660km • E) No earthquakes large enough 3 Xenoliths Xenoliths • Xenoliths are ‘strange rocks’ • Xenoliths are inclusions in kimberlites and basalts • Xenoliths provide natural high pressure rocks. – Kimberlites bring diamonds and other rock samples from as deep as 670 km. – Basalts are derived from the lithospheric mantle Xenoliths Breaking Xenoliths at a Diamond Mine in South Africa Xenolith-Peridotite Meteorites • Meteorites tell us of Earth’s bulk composition – Chondrites are primitive meteorites – Earth is “chondritic” • Composition is similar to sun minus H, He and other volatiles. 4 Meteorites Meteorites Clicker Question Clicker Question • How do we know that the core is made of iron? • How do we know that the core is made of iron? • A) Earth has a permanent magnetic field and iron is ferromagnetic. • B) Iron metal is abundant in meteorites and has about the right density for the core. • C) Iron metal is abundant in volcanic rocks and has about the right density for the core. • D) Iron metal is an electrical conductor • E) Iron is highly compressible. • A) Earth has a permanent magnetic field and iron is ferromagnetic. • B) Iron metal is abundant in meteorites and has about the right density for the core. • C) Iron metal is abundant in volcanic rocks and has about the right density for the core. • D) Iron metal is an electrical conductor • E) Iron is highly compressible. Experiments • Multi-Anvil Press – Synthesis experiments to 1000 km depth – Samples 1-5 mm3 • Diamond Anvil Cell – Spectroscopy – Ultrasound – X-ray diffraction Multi-anvil Press 5000 ton (Bayerisches Geoinstitut) 5 Multi-anvil Press Sample Assembly Carbide cubes retain a lot of strain Multi-Anvil Press Octahedron Multi-anvil Press Sample Assembly Multi-anvil Press Octahedral Assembly: After and Before Diamond-anvil Cell 6 Interior Terms Interior Terms Interior Terms Interior Terms Interior Terms 7 Interior Terms Mica Crystal with Quartz @ 5GPa DAC on Diffractometer Clicker Question • The diamond anvil cell can create pressure greater than the center of the Earth in the laboratory • A) True • B) False Clicker Question • The diamond anvil cell can create pressure greater than the center of the Earth in the laboratory • A) True • B) False 8 410km: Olivine Density 3.2 Olivine 410km:Olivine Wadsleyite Wadsleyite 3.5 g/cm3 525 km Wadsleyite Ringwoodite Wadsleyite Ringwoodite Crystal with Quartz 660 km Ringwoodite Density 3.6 Perovskite + MgO 4.2 g/cm3 9 660 km Ringwoodite Tetrahedral Si Mantle Density vs Depth 4800 Perovskite + MgO Octahedral Si 4600 4400 4200 Lower Mantle 4000 3800 Transition Zone 3600 3400 Upper Mantle 3200 3000 0 Clicker Question 200 400 600 800 1000 1200 Clicker Question • The Earth’s mantle is made of: • The Earth’s mantle is made of: • • • • • • • • • • A) Solid silicate rock B) Molten silicate magma C) Solid iron metal D) Molten iron metal E) Olivine Clicker Question A) Solid silicate rock B) Molten silicate magma C) Solid iron metal D) Molten iron metal E) Olivine Clicker Question • The seismic discontinuity at 660 km is thought to be caused by? • The seismic discontinuity at 660 km is thought to be caused by? • • • • • • A) A change in crystal structure A) A change in crystal structure B) A compositional change to metal C) Melting of silicates. D) Magnetic reversal E) A change to a gas phase • • • • B) A compositional change to metal C) Melting of silicates. D) Magnetic reversal E) A change to a gas phase 10 Convection in solid mantle Convection in solid mantle • Driven by U, Th, K decay • • • • • (internal engine) Moves about 2 cm/yr Causes earthquakes Moves plates Splits and rejoins continents One Cell or two? Clicker Question Clicker Question • The Earth’s outer core is made of: • The Earth’s outer core is made of: • • • • • • • • • • A) Solid silicate rock B) Molten silicate magma C) Solid iron metal D) Molten iron metal E) Olivine A) Solid silicate rock B) Molten silicate magma C) Solid iron metal D) Molten iron metal E) Olivine Magnetic Field • Due to electrical currents from Earth’s dynamo. • Convection in liquid metal outer core. • Convection is coupled to Earth’s rotation. • Field is subject to sporadic reversals. – Last reversal was 30,000 years ago. • Field holds ozone layer (UV shield) in place. 11 Magnetic Field Reversals Clicker Question Clicker Question • The Earth’s magnetic field is caused by: • The Earth’s magnetic field is caused by: • • • • • • A) Permanent magnet in the inner core. • B) Magnetic minerals in the rocks • C) Convection currents in the upper mantle A) Permanent magnet in the inner core. B) Magnetic minerals in the rocks C) Convection currents in the upper mantle D) Convection in the liquid metal outer core E) Solar wind dynamo • D) Convection in the liquid metal outer core • E) Solar wind dynamo Interior Terms • Crust – Oceanic – Continental • Mantle – Upper – Transition – Lower • Core – Outer – Inner • • • • • • • • • Peridotite Eclogite Spinel Perovskite Xenolith Discontinuity Phase Change Tomography Kimberlite Interior Terms • • • • • • • • Basalt Chondrule Multi-anvil press Diamond anvil cell Convection Magnetic reversal Dynamo Magnetic field 12