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

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


Geoph 300 physics of the earth

Source: http://www.mantleplumes.org


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


Geoph 300 physics of the earth

Source: R. McCaffrey

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


Geoph 300 physics of the earth

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


Geoph 300 physics of the earth

The classical model

Geodynamics, Seismology & Plumes

  • Gillian R. Foulger University of Durham, UK

http://www.mantleplumes.org


Geoph 300 physics of the earth

The alternative model


Geoph 300 physics of the earth

Plume

Plate


Geoph 300 physics of the earth

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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