Wednesday, November 7, 2007
Paleomagnetism Studies on the ANDRILL Core
Paleomagnetism, one of the many different geologic studies being done on the ANDRILL core, is the study of the record of Earth’s magnetic field through time. This record is captured in magnetic minerals which align themselves with Earth's magnetic field, (the geomagnetic field), as they are deposited in sedimenatary rocks.
Paleomagnetists have discovered that the earth’s magnetic field changes in two ways. First, the geomagnetic field is varying a little bit all the time. As a result, the magnetic north is constantly changing. Mainly it varies about the true north pole, which is the location about which Earth spins. These variations are small compared to the second type of change in the geomagnetic field: geomagnetic polarity reversals.
During a geomagnetic polarity reversals, the geomagnetic north moves to the geographic south pole. This happens over a
relatively short period of time as a result of changing electric currents and thermal convection in the molten outer core of the earth. The precise timing of these reversals is well documented around the earth back to 350 million years.
Paleomagnetists use the magnetic information in rocks to help date the rocks. Sometimes the paleomagnetic information is also used to reconstruct the location of a rock at the time of formation, prior to any movement by plate tectonics. In order to obtain useful data, paleomagnetists precisely identify the orientation of rock samples at the time of collection. They then drill a small oriented core on which they later perform a series of measurements. The samples are placed in a magnetometer which tells two critical pieces of information: 1) the magnetic declination, a measurement similar to what a compass needle tells us, and 2) the inclination, whether Normal or Reverse. When the paleomagnetic information from a sample is compared with known records of polar wandering and magnetic reversals, the timing and original location of a rock (i.e. where it formed) can be determined. The paleomagnetic information, in combination with other dating techniques, provides valuable information about the age of the core samples.
When the complete core is laid out, the paleomagnetists select the best sections for collecting paleomagnetism samples.
After carefully orienting and marking the sample, it is brought to a special instrument for drilling paleomag samples.
Drilling the core is done with great care, to keep the sample in its original orientation.
After the sample is removed, it is sent to a laboratory for analysis in a cryogenic magnetometer. This is what the section of core looks like after the sample has been removed.
Here is the ANDRILL Paleomagmetism Group under the leadership of Gary Acton, Chronostratigraphy Team Leader for ANDRILL SMS. Left to right: Luigi Jovane, Eloenara Strada, Gary Acton, and Robin Frisch-Gleason (temporary team member, until I leave for the field!).