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 km per million years).  The rate of separation of continents agrees with the rate of sea- floor spreading.

References/Sources of materials Earth’s geoid: Map and description originally from the National Geodetic Survey. Geoid diagram Earth's gravity field: Gravity Recovery and Climate Experiment (GRACE), joint NASA- German Aerospace Center Earth’s magnetic Earth’s topography and bathymetry: National Oceanic and Atmospheric Satellite images of Pinatubo and Mayon:National Aeronautics Space Administration Smith, D.A., 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