Theory of Plate Tectonics

Mission 1

Optical and infrared photography: photo imagesMagnetometer: magnetic fieldGravimeter: 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: topographyPassive 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 GeoidThe 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 4Seismometers: 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 sea-floor spreading.

References/Sources of materialsEarth’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 NASA-German 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