GEOPH 300: Physics of the Earth

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14 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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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

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: http://www.mantleplumes.org

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=
ω
Rsin
θ






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/li
ps/data/hotspots.htm

16

The classical
model

Geodynamics, Seismology & Plumes

Gillian R.
Foulger


University of Durham, UK



http://www.mantleplumes.org

17

The alternative
model

18

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