Transcript Plate Tectonics 1

Theory of
Plate Tectonics
Mission 1
Optical and infrared photography: photo images
Magnetometer: magnetic field
Gravimeter: gravity field
Important discoveries:
 Earth is volcanically active.
 A prominent magnetic field that is roughly aligned with the planet’s
spin axis.
 A very dense core (probably) consisting of iron and nickel.
Closer, baby, closer…
Mission 2
Radar altimeter: topography
Passive microwave
radiometer: temperature
Important discoveries:
 Linear topographic features on land.
 Variations in the height of the ocean above and below the ideal
ellipsoidal shape.
 Prominent lows offshore of arcuate island chains.
Note: an ellipse Earth = Equatorial radius about 20 km > polar
radius; actual ocean surface (geoid) bulges outward and inward by
up to 100 m (highest north of Australia; lowest south of India)
The Geoid
The major trench systems have obvious impacts on
the geoid, as well as the topography/ocean
boundaries.
 The Hawaiian Island chain may be followed up
through its transition into the Emperor Seamounts and
toward the western end of the Aleutian Islands;
Seamount patterns east of the Marianas Trench.
 The geoid low near the southern tip of India and the
geoid high over New Guinea (north of Australia) stand
out.
Modified after Smith, D.A. (2000)
So close…
Mission 3
Improved equipment:
radar altimeter,
camera and magnetometer
Important discoveries:
 Coastline match across the
Atlantic Ocean.
 Linear anomalies extends from
both sides of the Atlantic.
 Center of the Atlantic shows
gravity ridges and troughs that are perpendicular to fracture zones.
Almost there…
Mission 4
Seismometers:
earthquake foci
Important discoveries:
 Earthquakes are distributed along discrete zones.
 Earthquakes occur down to 650 km beneath the surface.
 Shallow earthquakes occur along the oceanic ridges and virtually
no earthquakes occur off the ridge.
Contact…
Mission 5
Robot survey ship carrying a
sonar, magnetometer, and
equipment to measure the
property and chemistry
of the ocean
Important discoveries:
 A narrow axial ridge (up to 500 m tall) is superimposed on a broad
rise where the average depth is 2.5 km.
 There is a symmetric deepening of the ridge axis as a function of
distance from the the axial high.
 Long stripes of magnetic anomalies parallel to the ridge axis.
 Stripes are symmetrically spaced on both sides of the ridge axis.
Geopoetry by Harry Hess
Curious coincidences in the
paleomagnetic patterns
 The temperature of the crust recording the magnetic stripes must
be cold enough (which precludes Venus’ surface temperature) and
the stripes must be locked within the upper 2 km (too thick and
signature superimposition occurs).
 The depth to the ocean floor is ideal for recording magnetic stripes
that have widths (2 * pi * depth) similar to those observed in the
world’s oceans (boats towing magnetometers would record the
stripes but orbiting satellites wouldn’t).
 To attain this width, the reversal rate must be between 2.5 – 0.3
million years (taking into account the known half-spreading rates of
10-80 km per million years).
 The rate of separation of continents agrees with the rate of seafloor spreading.
References/Sources of materials
Earth’s geoid: Map and description originally from the National Geodetic Survey. Geoid
diagram currently @
http://www.usna.edu/Users/oceano/pguth/website/so432web/GeoidMap.htm
Earth's gravity field: Gravity Recovery and Climate Experiment (GRACE), joint NASAGerman Aerospace Center mission, @ http://www.csr.utexas.edu/grace/
Earth’s magnetic field @ http://www.windows.ucar.edu/windows.html
Earth’s topography and bathymetry: National Oceanic and Atmospheric Administration
@ http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
Satellite images of Pinatubo and Mayon:National Aeronautics Space Administration
(NASA) @ http://visibleearth.nasa.gov/
Smith, D.A., 2000. Gravity and the Geoid at NGS. Presented at the 2000 Geodetic
Advisor Convocation, Maryland, U.S.A.
University of Leeds, United Kingdom
United States Geological Survey @www.usgs.gov