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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. GEOPH 300: Physics of the Earth . Geochronology

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geoph 300 physics of the earth
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
geoph 300 physics of the earth1
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
geodynamics
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
geodynamics1
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)

geodynamics2
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
geodynamics3
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
geodynamics4
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

geodynamics5
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

geodynamics6
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”

geodynamics7
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

geodynamics8
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
slide13

Source: R. McCaffrey

http://ees2.geo.rpi.edu/rob/www/gps/g0026.html#fig3

slide14

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

absolute plate motions hotspot tracks
Absolute plate motions – hotspot tracks

http://www.ig.utexas.edu/research/projects/lips/data/hotspots.htm

slide16
The classical model

Geodynamics, Seismology & Plumes

  • Gillian R. Foulger University of Durham, UK

http://www.mantleplumes.org

slide18

Plume

Plate

slide19

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

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