GOLF 1-8-2 Antarctica Expedition 2006/2007
How Rocks get Magnetized
How do the Rocks get Magnetized in the First Place?
To understand how rocks get magnetized, we need to take out our imaginary microscope and zoom in on the individual minerals that make up the rock. A single rock is usually composed of several different kinds of minerals, and most of these minerals are not magnetic. It is really only a few special kinds of minerals (like magnetite) that can become magnetized. To understand how minerals like magnetite get magnetized, we need to zoom in even further and think about individual atoms and their electrons. Minerals like magnetite are special because their electrons are distributed in such a way that they can produce a magnetization. At high temperatures, the electrons can easily change their orientation, and the magnetization will tend to align with the ambient Earth’s field. At low temperatures, however, the electrons cannot change orientation, and the magnetization direction is stuck. So the field recorded in the rock is the field when the rock was last very hot – usually when it erupted.How do we get the Magnetic Field Out of a Rock?
So how do we get the rock to tell us what the field was when it cooled? Unfortunately, rocks don’t usually respond to polite questions. Instead we have to take samples of the rock back home to our laboratory in San Diego, where we can use an instrument called a magnetometer
(see image below)
But the rocks don’t usually give up the ancient field quite that easily. We perform a series of experiments designed to remove any younger magnetic overprint that might partially obscure the ancient magnetization. To take the samples, we drill into the rock, producing small cylindrical cores, one inch in diameter and about two to four inches long (see images below).
But if we take these samples home to measure them, how do we now know what direction the magnetization was in? Was it pointing north? South? Or somewhere in between? Because the direction of magnetization is very important to us, a key component of our fieldwork is properly measuring the orientation of our samples with respect to north. Then, when we measure a magnetization direction in the lab, we know how that corresponds to a direction in geographic space. The orientation of core samples can be accomplished in several ways:
1) Perhaps the most obvious way to orient the core would be with a magnetic compass. In many places this would work fine, but here near the south magnetic pole our
magnetic compasses don’t work very well (see earlier report). However, in other locations, we could use a magnetic compass mounted in something called a Pomeroy orientation device (see image below).
The generally preferred way to orient samples is with the sun. We use the same Pomeroy, but with the addition of a slender metal rod (known as a gnomon) in the center
3) Of course, the catch with using a sun compass is that you must have sun! Although we’ve been fortunate enough to enjoy sunny weather for much of our stay in Antarctica, we need a backup plan for cloudy days or when our rock outcrop is in the shade. In this case we use something
called a differential GPS (see image above). This consists of two GPS receivers mounted on a 1-meter long aluminum base. By knowing the precise position at each end of this baseline we can calculate its azimuth, or the angle it makes with true geographic north. We then use a laser mounted on the baseline
and shoot it at a prism mounted on a modified Pomeroy (see image below)
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