This presentation at earth.northwestern/people/seth/Lectures/teachinggp

1. Teaching introductory geophysics: what our students don’t know, need to, and what we can do Seth Stein Department of Earth and Planetary Sciences Northwestern University Evanston IL 60208 seth@earth.northwestern.edu This presentation at http://www.earth.northwestern.edu/people/seth/Lectures/teachinggp.ppt

2. Solid earth concepts can be taught purely descriptively at the 100-levelBut this is teaching science as magic, not geophysics Davidson et al AGI: “Geophysics is a branch of experimental physics dealing with the earth”

3. NU introductory geophysics: • First geophysics class (200-level) for geology majors, engineers, or other science folk • Typically sophomores or juniors • Some have descriptive (100-level) intro to earth science • Math, physics, chemistry prerequisite or corequisites • - Global/planetary perspective • Course goes beyond descriptive • Try to challenge students & prepare them for higher level classes

4. Intro geophysics classes integrate topics that we’d like students to know from other classes, but often don’t know well (or at all). These can include GEOPHYSICS Earth structure Earth processes Geologic time GEOLOGY Vectors & vector diff. operators ( ) Spherical geometry Calculus Simple ODE & PDE (wave, heat) ∆ Mechanics E&M Waves Heat & temperature How we measure these PHYSICS MATH Problem solving (not just plug in)

5. In practice, we need to “review” - or teach - much of these This is OK since many students learn them better in context We can’t do detailed derivations, but can give intuition & contexts (often common to different applications) GEOPHYSICS Earth structure Earth processes Geologic time GEOLOGY Vectors & vector diff. operators ( ) Spherical geometry Calculus Simple ODE & PDE (wave, heat) ∆ Mechanics E&M Waves Heat & temperature How we measure these PHYSICS MATH Problem solving (not just plug in)

6. Seek to improve students’ understanding of both underlying concepts & geophysical application GEOPHYSICS Earth structure Earth processes Geologic time GEOLOGY Vectors & vector diff. operators ( ) Spherical geometry Calculus Simple ODE & PDE (wave, heat) ∆ Mechanics E&M Waves Heat & temperature How we measure these PHYSICS MATH Problem solving (not just plug in)

7. Tradeoff between breath of topics covered & depth of each (underlying math & physics) depends on class goals & composition GEOPHYSICS Earth structure Earth processes Geologic time GEOLOGY Vectors & vector diff. operators ( ) Spherical geometry Calculus Simple ODE & PDE (wave, heat) ∆ Mechanics E&M Waves Heat & temperature How we measure these PHYSICS MATH Problem solving (not just plug in)

8. VECTORSPhysics independent of coordinate system (F=ma)Magnitude-angle vs componentsVector operations (sum, gradient, curl, etc.) Stein et al, 2007 Magnetic field Plate motion

9. SPHERICAL GEOMETRY Latitude & longitude Vector components: (r,, ), (N-S, E-W, up-down), (x,y,z) Unit vectors in non-Cartesian system VDOs in spherical coordinates Operations must be done in Cartesian Stein & Wysession, 2003 Plate motion Stein et al, 2007 Magnetic field

10. SYSTEMS OF EQUATIONS/LEAST SQUARES Can motivate w/o math or w/simple algebra Davidson et al, 2002 GPS receiver finds position with radio signals from different satellites Locating earthquake from arrivals at multiple seismometers Tomography

11. MECHANICS F=ma Weight vs mass Gravity on different planets Objects of different mass fall at same speed Torque Magnetic surveys Paleomagnetism Need to understand cross product Magnetic field torque on magnetic dipole causes Compass to work Thermal Remnant Magnetization Stein et al, 2007

12. ROTATION (CIRCULAR MOTION) Torque, angular velocity & momentum, moment of inertia Need to understand cross product & vector nature of acceleration Stein et al, 2007 L=I Precession magnetometer Plate motion Moment of inertia

13. ELECTROMAGNETISM B How fields arise Columb’s law B from moving charges Lorentz force Maxwell’s equations How we measure & use fields V Spinner magnetometer V=-d flux B/dt Stein et al, 2007 Mass spectrometer (age dating) F=q(E + v x B) Circular motion

14. U(x,t) SEISMOLOGY Waves in space & time Observe as either Period, wavelength, speed (c=/T) & dimensions Relation between wave fronts and rays t Wavefront Ray x Stein & Wysession, 2003

15. SEISMOLOGY Idea of geometric optics: when we can use rays, when we need waves Travel time vs amplitude T2 Inquiring minds want to know Stein & Wysession, 2003 Huygens’ principle graphic T1 To Like boat’s wake wavefront -> ray

16. DIFFRACTION Non-geometric optical arrivals Frequency dependent (hear but not see around corners) core D/2d Migration undoes diffraction Single slit Interface end diffraction DEPTH TIME Stein & Wysession, 2003

17. CONSTRUCTIVE & DESTRUCTIVE INTERFERENCE Phase difference between waves Distance & time measurement VLBI CMP stacking earth.leeds.ac.uk/.../ active_tectonics/vlbi.gif Stein & Wysession, 2003 http://comet.nerc.ac.uk/schoolssar_how.html INSAR X-ray diffraction Michelson–Morley experiment (1887) http://www.eserc.stonybrook.edu/ProjectJava/Bragg/

18. RAYS ARE APPROXIMATIONS TO WAVES Stein & Wysession, 2003

19. GEOTHERMICS Conservation of energy (science vs popular usage) Potential, kinetic, chemical, heat, electrical… Most energy available to humans & natural systems is ultimately nuclear from sun or other stars Potential Chemical Nuclear Planets are heat engines Stein et al, 2007

20. HEAT & TEMPERATURE Explain physically (towel) Heat transfer modes Use partial derivative (don’t have to call it grad) Time dependant solutions Stein et al, 2007 Cooling halfspace solution Lava lake Stein & Wysession, 2003

21. THERMAL EVOLUTION OF LITHOSPHERE Simple models of spreading & subduction Tie to observations Density anomalies Plate driving forces Forsyth & Uyeda, 1975 Stein & Wysession, 2003

22. THERMAL CONVECTION Buoyancy force Adiabat Convection cell Solid & fluid earth Stein et al, 2007 Tie to weather & climate change

23. VISCOSITY Define w/o tensor Magma & mantle flow Stokes’ problem Rayleigh number Stein et al, 2007 Tie to glacial geology Stratovolcano GPS Shield volcano Reflects magma chemistry; tie to hazard

24. How & why it occurs Why nuclear energy released Age dating Radioactive heating of earth RADIOACTIVITY Nucleus Meteorites from early solar system 2 E=mc Stein et al, 2007

25. SUGGESTIONS Identify math, physics, chemistry concepts implicitly or explicitly needed in course Assess which students are shaky on “Review” - or teach - these at level suitable for class curriculum & composition Seek to improve students’ understanding of both underlying concepts & geophysical application Develop problems & labs for this goal Accept that some students will benefit more than others “You come in here with a skull full of mush and you leave thinking like a lawyer.”