Geoph 300 physics of the earth
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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

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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

    • 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

  • 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


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)


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


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


Source: http://www.mantleplumes.org


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


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


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”


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


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


Source: R. McCaffrey

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


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

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


The classical model

Geodynamics, Seismology & Plumes

  • Gillian R. Foulger University of Durham, UK

http://www.mantleplumes.org


The alternative model


Plume

Plate


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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