The magnetized crust shown in the far-field image (click the thumbnail image to download a high resolution 1 Mb image) is in many ways analogous to the crust defined by seismic data, with an important qualification, its temperature. Magnetized crust is cooler than 600 deg C, or more generally, the Curie temperature of the dominant magnetic phase (generally magnetite). Magnetic images of the crust are in some ways superior to those generated by controlled seismic experiments, because they are based on a global, continuous set of observations. High-quality crustal thickness measurements, in contrast, are irregularly distributed over the surface of the globe. Magnetic images will differ from seismic images in locations where excess heat is being generated, such as in the Himalayas where India and Asia are colliding. They will also differ in areas where subduction is occuring, as in the Sumatran region, because the subducting plate is descending so quickly that its temperature does not reach 600 deg C until it is deep within the earth's mantle. The far-field image uses the 3SMAC compositional and thermal model of the crust and mantle as a starting point, and it is then modified in an iterative fashion with the satellite data until the magnetic field predicted by the model matches the observed magnetic field. A unique solution is obtained by assuming that induced magnetizations dominate in continental crust, and that vertical thickness variations dominate over lateral susceptibility variations (Purucker et al., 2002). Other large scale structures evident in the magnetic thickness map are associated with the 90 East ridge (center left), a putative hot spot trail and site of intraplate volcanism. The magnetic thickness of the ocean crust at several localities along the ridge is consistent with underplating of mafic material at the base of the crust (Tiwari et al., 2003).
The magnetic map made using shipborne data, shown at the bottom left, is most sensitive to magnetic materials in the uppermost crust, in contrast to the far-field image which has equal sensitivity to the upper and lower crusts. This may explain the absence of a signature of the putative fossil subduction zone in the near-field map. Such maps have been used in the past in a wide variety of studies, from deciphering the past history of ocean basins, to mineral and oil exploration. Besides the subduction zones, other large scale structures evident in the marine magnetic map include a negative anomaly arch extending westward from south of the Andaman islands, then southwestward to the east of Sri Lanka. There is also a prominent negative anomaly connecting to the subduction zone on the southern end of Sumatra, and trending southward and then southwestward.
The marine magnetic map was made using all data available from the National Geophysical Data Center (NGDC), and encompases the time period from 1960 to 1988. The 330,000 records, representing a total line length of 170,000 nautical miles, were reduced using a combined main and external field model (Sabaka et al., 2004), using techniques described in Ishihara (2004).
The European Space Agency (ESA), with assistance from NASA, is planning a new satellite mission called Swarm (Friis-Christensen et al., 2004) which will significantly improve the resolution of magnetic maps of the earth's crust like the crustal thickness map shown here, and allow long- and intermediate-wavelength satellite magnetic anomaly maps to be joined with detailed magnetic maps of the crust made from aircraft and ships to create full-spectrum magnetic images. Scheduled for launch in 2009, Swarm will consist of three satellites, the lower two of which will fly side-by-side to capture high-resolution magnetic gradient information on the crust. These gradient data, complemented by magnetic field, electric field, and accelerometer data, will provide the necessary observations that are required to separate and model the myriad static and dynamic sources of the geomagnetic field, and increase our understanding of present and past subduction in this dynamic region.
References
Friis-Christensen, E.,et al. 2004, Swarm: The Earth's magnetic field and environment explorers, ESA Special Publication 1279(6)
Ishihara, T., 2004, Application of CM3 model in compilation of marine magnetic anomaly data of North Pacific, EOS Trans. AGU, 85(47), Fall Meet. Suppl., Abstract GP11D-0882.
Purucker et al., 2002, The southern edge of cratonic North America: Evidence from new satellite magnetometer observations, GRL, 29(15), 8000, doi:10.1029/2001GL013645.
Sabaka et al., 2004, Geophy.J.Int., Extending comprehensive models of the Earth's magnetic field with Orsted and CHAMP data, 159(2), 521-547, doi:10.1111/j.1365-246X.2004.02421.x
Tiwari et al., 2003, Analysis of satellite gravity and bathymetry data over Ninety-East Ridge: Variation in the compensation mechanism and implication for emplacement process, JGR, 108, B2, 2109, doi:10.1029/2000JB000047.
We would like to thank Stefan Maus and the CHAMP team at GFZ-Potsdam for their magnetic field model. Purucker is supported by NASA's Earth Surface and Interior Program. For further information contact Michael Purucker (purucker@geomag.gsfc.nasa.gov) of Raytheon ITSS, located within the Planetary Geodynamics Lab-GSFC/NASA or Takemi Ishihara (t-ishihara@aist.go.jp) of the Geological Survey of Japan.
The marine and satellite data that went into making these maps, GMT scripts to produce these maps, and further details on the maps, can be found here.
Last updated 8 Mar 2005