G100 Lec 16 Earth?s Interior In this lecture ? Seismic waves illuminate mantle, core, and crust structure ? Heat gradient and variations in the mantle ? Magnetic field and fluid geodynamo Laser beams (two) are reflected downward from air-water interface and refracted (bent) as they pass through it. Refraction Reflections P-wave Refraction Refraction & Reflection of seismic waves ? delineate depths of boundaries between major layers Later arrivals (SS & PP) travel through shallow mantle and crust ?Tell us properties of regions shallower than do primary P and S waves ?Refractions and reflections of these waves in sediments can be used to explore for oil and gas Important properties of the Earth?s interior ? Seismic waves illuminate mantle, core, and crust structure ? Heat gradient and variations in the mantle ? Magnetic field and fluid geodynamo ? Hell ? Geothermal Gradient ? Ranges from 10 to 30 o C/km in crust ? high (20-30 o C/km) in tectonically active areas ? low (10-20 o C/km) in cratonic areas ? T = 1300-1400 o C at base of lithosphere ? T = 3000-6000 o C in core ? Quartz fractures for T < 300º C, flows for T > 300º C ? Assuming geotherm of 20º C/km, implies maximum EQ depth of 15 km for faults in continental crust Interior cools via loss of heat through crust ? Conduction ? mechanical transfer of heat through vibrational motion of atoms ? Rocks are POOR conductors ? Convection ? heat transferred by circulation of hot rising and cool sinking material ? must be capable of flow Conduction example: subsidence of seafloor increases as square root of age, precisely as predicted by thermal conduction heat flow models. Earth?s magnetic field and the geodynamo William Gilbert b. 1544 / d. 1603 Physician to Queen Elizabeth I Gilbert (1600) De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth) ?The Earth is a great magnet? Lines of magnetic force point downward in northern hemisphere and upward in southern hemisphere, with axis tilted 11 o from that of rotation ?Lines of force create a Magnetic Dipole Earth?s magnetic field and geodynamo ? Field is more complex than a simple dipole ? Changing location of North magnetic pole 1600?2000 AD ?Secular variation ?Why? Earth?s magnetic field and the dynamo ? Outer core = convecting fluid ? Convection of molten iron generates magnetic field ? 90% of field described by dipole ? 10% of field is non-dipolar at surface ? >10% non-dipolar at core- mantle boundary Paleomagnetism ? Records of magnetization ? Thermoremanent (cooling lava flows or hearth stones) ? Depositional remanent (sediments) Magnetic stratigraphy Supercomputer simulation of magnetic field reversal ? model can tell us about processes in the outer core and conditions in the lower mantle Supercomputer simulation of magnetic field reversal Supercomputer simulation of magnetic field reversal Supercomputer simulation of magnetic field reversal Supercomputer simulation of magnetic field reversal geouser Microsoft PowerPoint - Lec 16 Earths_Interior.ppt
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