GEOPH 300: Physics of the Earth

1. GEOPH 300: Physics of the Earth • Geodynamics (Physics of plate tectonics) • Spherical geometry, reconstruction of plate motion • Earth’s magnetic field • Seismology • Earthquake seismology • Active source seismology

2. GEOPH 300: Physics of the Earth • Geochronology • Radioactive decay and useful decay systems • Heat flow • The earth as a heat engine – total heat generation and loss from the earth • What geophysics tells about the structure of the deep interior and the continental and oceanic lithospheres • Geophysics to study Earth’s climate • Remote sensing, glacier earthquakes

3. Geodynamics • Lithosphere (lithos – rock) • Strong outermost shell including the crust and uppermost mantle (~100 km) • Thinnest in the oceans, thicker under the continents • Asthenosphere (asthenia – weak or sick) • High temperature and pressure allow for viscous flow at geologic timescales

4. Geodynamics • Lithosphere is divide into a small number of rigid plates moving over the asthenosphere • Most deformation takes place along plate edges • Seismicity outlines the plate boundaries • Seven main plates • Pacific, Antarctic, South American, North American, African, Eurasian, Indian (see IRIS webpage)

5. Geodynamics • Assumptions of plate tectonics • Generation of new plate material occurs by sea floor spreading • The new oceanic lithosphere becomes part of a rigid plate that may or may not include continental material • The Earth’s surface area remains constant • Relative motion between plates is accommodated along plate boundaries

6. Geodynamics • Types of plate boundaries • Divergent (Constructive) • Mid-ocean ridge, material is added to the crust • Convergent (Destructive) • Subduction zone, material is returned to the mantle • Conservative • Transform faults – six basic classes • Most common is ridge-ridge fault

7. Source: http://www.mantleplumes.org

8. Geodynamics • We can think of plate motion as relative • Velocity of plate A with respect to plate B is BvA and B with respect to A is AvB AvB = -BvA

9. Geodynamics • We use vector addition to determine relative motion between multiple plates • Say we know relative motion between A and B, and between A and C, we determine motion between B and C by CvB = CvA+ AvB

10. Geodynamics • Plate motion on a sphere • Euler’s fixed point theorem “The most general displacement of a rigid body with a fixed point is equivalent to a rotation about an axis through that fixed point” or “Every displacement from one position to another on the surface of the Earth can be regarded as a rotation about a suitably chosen axis passing through the center of the Earth”

11. Geodynamics • This suitably chosen axis is call the rotation axis and it intersects the surface at two poles of rotation • Relative velocity between two plates is given by v=ωRsinθ

12. Geodynamics • Determining rotation poles and rotation vectors • Instantaneous poles of rotation and relative angular velocities • Snapshot in geologic time • Strike of active transform faults • Spreading rate along constructive plate boundaries • Focal mechanism analysis • Land displacement surveys • Space based methods • Satellite laser ranging system • Very-long-baseline interferometry • Geodetic grade GPS

13. Source: R. McCaffrey http://ees2.geo.rpi.edu/rob/www/gps/g0026.html#fig3

14. Payne, S.J., R. McCaffrey, and R.W. King, 2008, Strain rates and contemporary deformation in the Snake River Plain and surrounding Basin and Range from GPS and seismicity: Geology, v. 36, 647-650

15. Absolute plate motions – hotspot tracks http://www.ig.utexas.edu/research/projects/lips/data/hotspots.htm

16. The classical model Geodynamics, Seismology & Plumes • Gillian R. Foulger University of Durham, UK http://www.mantleplumes.org

17. The alternative model

18. Plume Plate

19. Christiansen, R.L., Foulger, G.R., and Evans, J.R., 2002, Upper mantle origin of the Yellowstone hotspot: GSA Bulletin, 114 (10), 1245-1256