GEOPH 300: Physics of the Earth Geodynamics (Physics of plate tectonics) – Spherical geometry, reconstruction of plate motion – Earth’s magnetic field.

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 tectonics1. Generation of new plate material occurs by sea

floor spreading2. The new oceanic lithosphere becomes part of a

rigid plate that may or may not include continental material

3. The Earth’s surface area remains constant4. 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. McCaffreyhttp://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

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The classical model

Geodynamics, Seismology & PlumesGillian R. Foulger

University of Durham, UK

http://www.mantleplumes.org

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The alternative model

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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