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Why a Magnetometer Can be a Hotspot Scientist’s Best Friend
Submitted by Kevin Kurtz on Sun, 02/06/2011 - 07:45
The scientists’ ability to determine whether the Louisville hotspot has been moving inside the Earth is dependent on our paleomagnetists’ ability to see how the Earth’s magnetic field affects volcanic rock. To find out why, drink a latte and read on, because it’s a little long.
If when you were a kid you ever went to your grandparents’ house and found the magnetic toy Wooly Willy< in the attic and you used it to make Wooly Willy look like Mr. T<, then you might remember that when you used the magnetic “magic wand” to give Wooly Willy a beard and a mohawk, it was pretty easy to make the iron filings look like hair because they all pointed away from the magnetic wand in a way that made them automatically look like the world’s most disgusting moustache, kind of like they look in this photo.
If you look again at the photo, you will also notice that there are only iron filings near the ends with the “N” and “S” and none on the horseshoe part. Since a magnet will stick anywhere on a refrigerator, it is easy to think that the entire surface of a magnet attracts metal, but if you tried to pick up iron filings with the kung fu-grip part of a horseshoe magnet, you might as well be using a regular horseshoe.
The force of attraction from a magnet does not emanate across its entire surface. Instead it projects only out of the poles in a pattern that kind of looks like the splash of water when your Uncle Herb cannonballs in your Grandma’s pond. That is why the iron filings look like two upside-down moustaches in the photo above, because they align themselves in the splash-like pattern of the magnetic field of the horseshoe magnet. (If you want to see a fuller representation of a magnet’s magnetic field and you happen to have a Wooly Willy and a bar magnet handy, place the magnet under the cardboard side of Wooly Willy, and then shake Wooly Willy so the iron fillings spread around until they are captured in the pattern of the bar magnet’s magnetic field on Wooly Willy’s face).
If you want to see the world’s biggest magnet, then look around your feet, because you are standing on it. The Earth acts like a magnet, with a huge magnetic field spewing out near the South Pole and back into the Earth near the North Pole with the force of attraction towards the north (see the simple diagram down below so I do not have to explain this in more detail).
When you pull out a compass (as I know you do regularly) it points north because it aligns itself with the magnetic field that is pointing north. At least that’s what it does now. Every few thousand or million years or so, the Earth has a tendency to reverse its magnetic pole back and forth from around the North Pole to around the South Pole (the magnetic pole and the directional pole are not in the same place), so if you were able to travel million of years back in time with a compass, it might be pointing south. The reversals maintain a very inconsistent, unpredictable schedule, and, as yet, scientists do not know why they happen.
A volcanic rock is actually like a frozen compass from the past. The magnetic particles in lava are free to move and align themselves with the current magnetic field. As soon as that lava solidifies into rock, the magnetic particles lock into place pointing towards wherever the magnetic pole happens to be at the moment it cools and preserves that orientation for as long as the rock remains unaltered rock.
To give you an idea of what I mean, say you found the Wooly Willy in your Grandma’s attic again, but this time somehow all the iron filings had remained frozen in place from the day when your Uncle Herb was a kid and he made Wooly Willy look like Maynard G. Krebs<. Not only would you be able to tell what your Uncle Herb used to watch on TV, but you can also look at the directions of the iron filings to figure out everywhere your Uncle Herb lifted the magic wand off of Wooly Willy’s face, because the iron filings will still be in the pattern of each of those magnetic fields.
The magnetic particles in the volcanic rock point towards the magnetic pole, but they do so by aligning themselves with the Earth’s magnetic field. To understand what I am talking about, take a look at this diagram.
The yellow arrows are the directions the magnetic particles would be pointing if the lava cooled at that latitude of the Earth. The particles are parallel to the magnetic field in that position, so near the South Pole they will look like they are almost pointing the opposite direction of the North Pole, but they actually follow the path of the magnetic field around the Earth to the North Pole.
All of this is important to the science of this expedition because the magnetic particles’ angle of inclination changes depending on what latitude the volcano was at when it erupted (compare the angles of the yellow arrows from south to north in the diagram to see what I mean). Even if the magnetic pole is near the South Pole at the time of eruption, the angles of the magnetic particles at each latitude will be same, because the reversed magnetic field remains the same; it just pulls in the opposite direction. If the Louisville hotspot stayed fixed at the same latitude throughout its existence, then the angles of the magnetic particles in each of the seamounts will all be the same. If the Louisville hotspot has been moving under the lithosphere, the change in latitude can be determined because the angles of inclination will be different on different seamounts. This is how the scientists on the JOIDES Resolution expedition ODP Leg 197 determined the Hawaiian hotspot had moved at least 15 degrees latitude under the lithosphere (see my previous blog< for a little more about that).
So that is what Nicola, Hirohoshi and Jeff, our paleomagnetists, are doing with the help of their trusty magnetometers (like the one in the photo). By measuring and analyzing the magnetic fields of the seamount cores, they are helping us have a better understanding of what is happening deep in the interior of the Earth.
And now I have written what must be the longest blog post in JOIDES Resolution website history, but it was too cool for me not to try to share.