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 1.Generation of new plate material occurs by sea floor spreading 2.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 constant 4.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:

Geodynamics We can think of plate motion as relative – Velocity of plate A with respect to plate B is B v A and B with respect to A is A v B A v B = - B v A

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 C v B = C v A + A v B

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

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,

Absolute plate motions – hotspot tracks ps/data/hotspots.htm

16 The classical model Geodynamics, Seismology & Plumes Gillian R. Foulger University of Durham, UK

17 The alternative model

18 PlumePlate

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