As anybody who's ever done the compass-and-a-magnet trick knows, there is a pervasive magnetic field generated by the earth. It is huge in extent but generally faint at any given locality, and so does not wreak havoc with computer components or other electrical systems. It is nonetheless useful for navigation, and has a number of subtle but important effects on life on earth.

The earth's magnetic field is roughly aligned with the earth's axis of rotation. However, it is not perfectly aligned. The northern magnetic pole is currently in the Arctic islands of northern Canada. If you look at a USGS 7.5 minute quad topographic map or the topographic maps of most other countries, along the bottom there is usually a set of 3 marks showing the angle between true north, magnetic north, and grid north (a function of the map projection). This is very important for orienteering and surveying. The magnetic pole is also constantly moving, meaning that the correction should be kept up-to-date if you're doing anything serious with it.

It is interesting to note that, as a peculiarity of the development of the compass, the northern magnetic pole is actually what is termed a 'south pole' when speaking of a magnet. If you have two magnets, the north poles of each magnet will repel each other, as will the two south poles. Conversely, the south pole of one magnet will be attracted to the north pole of the other. The designation 'north' was given to the pole of a magnet that was attracted to the north, and 'south' to the pole that tended to point south. For the 'north' pole of a magnet to be attracted to geographic north, that means that magnetically speaking, that the geographic north pole must have 'south' magnetic polarity. If the definition were done otherwise, making the northern magnetic pole of the earth a 'north' pole, then we'd end up with the counter-intuitive situation where the 'south' poles of magnets would always tend to point north.

The Field in Space

The geomagnetic field turns out to be pretty useful. While the ozone layer is important for cutting the sun's UV radiation, far more radiation of various kinds is stopped by the magnetic field. Without the magnetic field, most artificial satellites would be pounded mercilessly by intense radiation, rendering them useless. This would also dramatically increase the load of radiation which would need to be absorbed by the upper atmosphere. By contrast, Mars has a very weak, almost non-existent magnetic field, and a very thin atmosphere, leaving the surface of Mars subject to much higher levels of solar and cosmic radiation.

Of course, the Aurora Borealis and Aurora Australis are created by charged particles of the solar wind being trapped in the magnetic field. They get funneled down to the magnetic poles, where they create the glowing sheets of otherworldly colors which are otherwise only seen by those guys at the Phish concerts.

In the Rocks

The pervasive nature of the geomagnetic field is such that as rocks cool below the Curie temperature (more about that later) they record the orientation of the field at that time. This is called paleomagnetism, and it's a fairly important field in geology. It has been discovered through the study of paleomag that the geomagnetic field has reversed polarity numerous times since it first formed around 3 billion years ago, and has at other times dropped dramatically in strength. Since the orientation of the field relative to the surface varies from zero at the equator (magnetic equator, not geographic) to vertical at the magnetic poles, the paleomag can also give clues as to the latitude where a rock has formed. This gives big clues about the previous locations of various continents and how they've moved through plate tectonics. The discovery of alternating bands of magnetic polarity in the rocks of the ocean floor was instrumental in proving the theory of seafloor spreading which led to modern plate tectonics.

Exploration

Just as the rock is affected by the geomagnetic field, so the local field is affected by the surrounding rocks and other objects. This was put into practice by the military, who used a Magnetic Anomaly Detector (MAD) to detect steel submarines underwater by the local change they caused in the magnetic field.

Geophysicists use the same techniques as a method for determining the structure and composition of the earth. By mapping the magnitude and sometimes the inclination of the geomagnetic field it is possible to see the variations caused by various kinds of rocks in the subsurface. One of the most well-known cases where this technique has been used is the Chicxulub impact site in the Yucatan Peninsula, thought by many to be the cause of the extinction of the dinosaurs. Magnetic methods are also usefull in more mundane studies. Many environmental remediations start with a magnetic survey to quickly locate buried tanks or drums which might be leaking, or metallic objects in a landfill that needs to be cleaned up.

Where does it come from?

Knowing that the earth's core is composed largely of iron and nickel - two highly magnetic and magnetizeable metals - one's initial thought might be that the earth is just like a giant metal magnet, just like the kind we use to keep our fingerpaintings attached to the fridge. Magnets of this kind are created by little magnetic crystals in the material which are all aligned in the same direction, giving the whole object a magnetic field. However, the earth's core is extremely hot. Most of the core is molten, and liquid iron cannot support static magnetism. While the inner core is solid, its temperature is estimated to be between 4500 and 7000 Kelvin. This is above the Curie temperature for either iron or nickel. Therefore the earth's magnetic field cannot be created by ferromagnetism in the core.

The other main form of magnet is the electromagnet. This is effectively a coil of wire with a current running through it, producing a magnetic field. Any moving electrical charge creates a magnetic field (apparently this turns out to be an effect of a moving electrical field under Einstein's theory of relativity). If it moves in a circle, then it creates the bi-polar magnetic field we're familiar with (this is what's happening in a fridge magnet at a molecular/atomic scale). The Earth's outer core is made of a superheated mixture of molten iron and nickel with some light-element impurities (possibly sulfur, with oxygen, hydrogen, nitrogen and/or carbon). The inner/outer core boundary is thought to be about 1000K hotter than the core/mantle boundary, setting up some intense convection. Charged particles carried by this convection are thought to generate what is called the geodynamo, a massive electromagnet in the outer core.

The exact mechanism and geometry of the geodynamo are still not known, although there is extensive computer modeling exploring this subject at places like Los Alamos National Laboratory in New Mexico.

Quo Vadis?

Over the last two centuries, the geomagnetic field has been decreasing in strength. This may be leading up to a flip in orientation of the poles sometime in the next millennium or so. If so, cancer rates could increase dramatically and satellites could malfunction as the earth is bombarded by the solar and cosmic radiation that is normally screened out by the magnetic field.

Return to the volcanic and geologic terms glossary.



For more info on the recent variations in the geomagnetic field, see http://www.observer.co.uk/international/story/0,6903,837058,00.html

Log in or registerto write something here or to contact authors.