GOLF 1-8-2 Antarctica Expedition 2006/2007
How Rocks get Magnetized

The Golf-182 team is sampling volcanic rocks to study how Earthís magnetic field has changed over the past few million years. The rocks act like a paleo-compass by preserving a record of the direction and strength of Earthís magnetic field at the time they erupted and cooled. This way if we can find volcanic rocks of different ages, we also have a record of the magnetic field at different points in time.

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 Earthís 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 Ė 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 donít 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) to measure them. This instrument will tell us both the direction and intensity of the magnetization.

But the rocks donít 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 donít 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). This is inserted into the drill hole and leveled. You can then measure the angle ("dip") your drill hole makes with the horizontal, and you can measure the direction in which it dips using the compass. However, even away from the South Pole the magnetic compass may not be ideal; if rocks are strongly magnetic, they can influence the direction of the compass needle, giving you an incorrect reading.

2) 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. This turns the Pomeroy into a sun compass Ė something a bit similar to a sundial. The gnomon casts a shadow on the face of the Pomeroy, and we note where that shadow falls, as well as the date and time. Because the position of the sun in the sky is known with remarkable precision, we can then calculate the orientation of our core. It is incredible that such old technology remains the most reliable way for us to orient our samples!

3) Of course, the catch with using a sun compass is that you must have sun! Although weíve 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). This allows us to find the angle between the baseline and our sample, and with a few simple calculations, the orientation of our sample with respect to north.

 

Julie Bowles
13 December 2006

 


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