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Upper Mantle Viscous Drag on the Lithosphere

Upper Mantle Viscous Drag on the Lithosphere . David Terrell Warner Pacific College March 2006. Historical Background. Continental Drift. Ocean Floor Spreading Plate Tectonics. 1957/8 international Geophysical year. 1961-8 papers on world seismology and paleo-magnetism

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Upper Mantle Viscous Drag on the Lithosphere

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  1. Upper Mantle Viscous Drag on the Lithosphere David Terrell Warner Pacific College March 2006

  2. Historical Background • Continental Drift. • Ocean Floor Spreading • Plate Tectonics. 1957/8 international Geophysical year. 1961-8 papers on world seismology and paleo-magnetism Vine 1966 Science “Spreading of the Ocean Floor” Isaks, Oliver, and Sykes 1968 J. G. R. “Seismology and the New Global Tectonics.”

  3. Seismic Analysis/Geomorphology • Isostacy • (explains mountain gravity anomalies) • Global earthquake distribution • Follows definite patterns • Earthquake Depth • Relative to some continental margins

  4. Subduction • Locality/Depth of Earthquakes.

  5. Volcanism • Friction/Phase change

  6. Ocean Floor Spreading • Ocean Floor Magnetic Anomalies

  7. Earth’s configuration • Lithosphere (0 – 100 km) • Crust • Asthenosphere (Soft) • Mantle (100 – 2890 km) • Upper • Lower • Nucleus/Core (2890 – 5378 km) • Outer • Inner ... **

  8. Layer boundaries • Mohorovicic discontinuity • Physical – chemical changes • About 5 km under mid-oceanic ridges • About 75 km under continents • Gutenberg discontinuity • Between silicate mantle/iron nickel core • Lehman discontinuity • Between “liquid” outer and “solid” inner core

  9. Mantle Convection

  10. Mantle Convection • Internally Generated Heat • Asymmetric • Equations of Fluid-Dynamics • Mass conservation • Continuity

  11. Normal Convection

  12. Thermal Convection • Symmetric • Asymmetric FOR MORE INFO... www.gps.caltech.edu/~gurnis/Movies/movies-more.html Butler and Peltier 2002 J.G.R. Thermal Evolution of The Earth: Models with time-dependent layering of mantle convection which satisfy the Urey ratio constraint.

  13. Internally Heated Convection

  14. Upper mantle convection • Heat generated during accretion • Heat generated by Radioactivity • 40K • U • Th • Others now in smaller amounts • (Rb, Sm, etc.)

  15. Stress-Strain • Mantle deformation • Fluid dynamics • Elastic modulus FOR MORE INFO... Non-linear rheology: http://www.geo.ucalgary.ca/~wu/Goph681/Rheology.pdf

  16. Equations of Motion Fluid Dynamics Equations that govern motion Equation of mass conservation Equation of continuity Mass conservation • is density; i is normal unitary vector defining integration surface; and vi is 1st order tensor defining velocity http://www.navier-stokes.net/nsfield.htm

  17. Heat Equation Cp is the heat capacity and  is the expansion coefficient

  18. Rayleigh’s number Convection occurs if R > 1100 - 1700

  19. Stress • A first approximation: Density ρ is about 3.4x103 kg/m3 and viscosity ν is about 1024 poises (1 poise = 10-1 Pa·s)

  20. Accumulated stress • Using estimated values for heat generated and the current estimates for the movement of some plates • Say speeds of ~2-3 cm/y • Values for accumulated stress in about 200 Ma of about 300-400 bars can be calculated • These values are well below values calculated for isostacy in some places but big enough to break a thin (<10 km) crust.

  21. Current Status • Even though this is an old problem new computing (modeling) technologies as well as experimental data have opened this area to new research.

  22. Thanks so much for listening! • Some useful web sites: • www.warnerpacific.edu/personal/dterrell • http://anquetil.colorado.edu/VE/convection2.shtml • http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html • http://www.mantleplumes.org/Convection.html

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