Orals review session 2004, Janet Baran

1. The Magic of Geophysics: Looking at the interior of the Earth from the surface. Orals review session 2004, Janet Baran Let’s start with the shape of the earth…

2. Shape: Oblate spheroid That mean the earth bulges at the equator, and is shorter at the poles. Why? Balance of forces: angular momentum(outward) and gravity(inward). Earth is slowing down, so angular momentum is lessening. The earth used to be more oblate. How do we know this? Satellite orbits Equatorial radius: 6378.16 km Pole radius: 6356.77 km Flattening of the earth: (Re-Rp)/Re=1/298.247

3. spring What do tides have to do with gravity? neap

4. All the details of Gravity: Sir Isaac Newton • Falls off with 1/r 2 • F=Gmm/r 2 : Universal Law of gravitation • Measure in Gals: cm/sec 2 • Range of earth’s field 978-983 Gals • Geoid: equipotential to sea level. Used to find anomalies. • Two methods of measuring • Absolute: pendulum, simple free fall • Relative measurements: springs • Correct for latitude, elevation, and mass to use what we do know to find out more information about what we don’t know.

5. The geoid in all it’s glory

6. Magnetics: The earth’s magnetic field is like a dipole magnet tipped 11 degrees. A magnetic field exerts forces on moving electrical charges, causing a spiraling like motion. High concentrations of charge particles are found in the Van Allen Radiation belts. The source of these particles is the solar wind.

7. Here’s the problem: The curie temperature is 770 C. This temperature is the locking temperature, meaning as a rock cools below this temperature the magnetic field’s direction is permanently imposed on the rock. The inner earth’s temperature is much great than that.(we’ll get to the temperature distribution of the earth in a little bit). So, how then do we get a magnetic field? The liquid outer core(again, will be coming back to that) has electric currents running through it. The liquid is turbulent. This is called the ‘dynamo effect’. The field is an average of the smaller currents. Magnetic reversals (we think) come from the liquid stopping moving, and then restarting moving in the opposite direction. We’ve seen a decrease in field strength over the last 150 yrs. There’s a range in how long magnetic periods last.

8. A couple quick things about magnetics: Inclination: Declination: The blue line shows the apparent polar wander path for Australia from the mid- to the late Palaeozoic period. (The dots and surrounding ellipses are the "poles" on which the path is based. Note that in the Permian, the poles are on Australia, this indicates that at that time, Australia was at the South Pole. ) Want more info on magnetics? http://istp.gsfc.nasa.gov/earthmag/dmglist.htm http://www.mq.edu.au/scienceresearch/lackie.htm

9. Seismics!! Let’s start with kinds of waves: P wave velocity: [(λ+2μ)/ ρ]^.5 Swave velocity: [μ/ ρ]^.5 .9pwave vel

10. http://www.geo.mtu.edu/UPSeis/waves.html

11. Snell’s Law: sin I/ v= sin I/v No matter what layers you compare, this is always a constant, p, the ray parameter. As you go to a higher velocity material, the ray will get bent closer to the horizontal. This is how we get rays that curve through the earth When i=90, then you get a critical reflection. It travels along the boundary at the faster velocity, and somehow comes back up again. Let’s take a look at how the waves(rays) travel) http://www.phy.ntnu.edu.tw/java/light/flashLight.html

12. Modified Mercalli Intensity Scale from FEMA I. People do not feel any Earth movement. II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move. Richter Earthquake Magnitudes Effects Less than 3.5 Generally not felt, but recorded. 3.5-5.4 Often felt, but rarely causes damage. Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live. 7.0-7.9 Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.

13. Shadow zones

14. Radial structure of the earth • The core is about 55% of the total radius • Average oceanic crust:8km thick • Average continental crust: 45 km thick (30-70 km) • Lithosphere: continental 200 km crust, oceanic variable. • Asthenosphere: from 350 km depth to base of lithosphere.

15. Structure of the crust Mountain belts have deep crustal roots.

16. Dynamics of the Mantle The mantle is a solid, but relaxes stresses by creep on long time scales can be modeled as a viscous fluid. Convective upwelling of the mantle: Plumes Downwelling of the mantle: subduction Surface expression: plate tectonics Layered convection? Tomography indicates not… The relationship between mantle convection and plate tectonics is complex-causality has not been clearly established.

17. An aside: Heat source of the earth: radioactive isotopes decay, original heat Movement of heat: conduction, convection, advection

18. isostasy demo! Post glacial rebound

19. Viscocity of the Mantle Viscosity of the mantle is measureed in two ways: post glacial rebound studies, and laboratory measurements. Viscosity of water: 10-3 Pa s Viscosity of the mantle: 1018 - 1021 Pa s Increasing temperature and volatile content decrease the viscosity of the mantle.

20. Structure of the mantle Crust mantle boundary- Mohorovicic Discontinuity, Moho– seismic boundary at 10 km(oceanic), and at 45 km (avg. continental) Lithosphere-Asthenosphere boundary: rheological boundary at 100-300 Km depth. Mantle phase change boundaries at 400 and 660 km depth: change In crystalline structure to a denser configuration. Above 660km is ‘upper mantle’, below 660 km is the ‘lower mantle’.

21. The core: • Because there is a shadow zone for S waves from 103-180, shows that outer core is a fluid • Inge Lehmann, 1936, proposed the existence of a solid inner core. • Faint arrivals in the shadow zone explained the solid inner core • Tidal deformation and Free oscillations of the solid earth require a core with zero rigidity.

22. Solid inner core is static • Outer core is liquid with viscosity about the same as water and high Rayleigh number • convection in the outer core driven by chemical, crystal setting. • Dynamo: magneto-hydrodynamic process in the outer core that generates the magnetic field • Inner core rotation discovered by Paul Richards w- 0.3 degree/year super rotation • The inner core is coupled with the dynamo which generates the magnetic field

23. How do we know we have an iron core? • Magnetic field of the Earth • Gravity • Moment of Inertia • Iron Meteorites • Seismology How do we know there is an outer liquid core, and a solid inner core? • Seismology • Dynamo Theory • Whole Earth Deformation

26. Other planets • Moon: small or no iron core, no global magnetic field. Surface is 3.8+Ga- no plate tectonics. Orbital radius is growing with time by tidal dissipation. Probably created by giant impact w/Earth/ • Mercury: smallest terrestrial planet 60-70% iron, large iron core with magnetic field, and probably dynamo. Old surface, no plate tectonics. • Venus: Size and density similar to Earth. No dipole magnetic field. Retrograde rotation. • Mars: Largest volcano in solar system: Olympus Mons(26 km high). Large core, and evidence for past magnetic field(none today).