Earth's magnetic field is in a state of change, say researchers who are beginning to understand how it behaves and why.
Right: The movement of Earth's north magnetic pole across the Canadian arctic, 1831-2001. (source: Geological Survey of Canada)
Scientists have long known that the magnetic pole moves. James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. No one returned until the next century. In 1904, Roald Amundsen found the pole again and discovered that it had moved-at least 50 km since the days of Ross.
The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating "to 40 km per year," says scientist Larry Newitt of the Geological Survey of Canada. At this rate it will exit North America and reach Siberia in a few decades.
Keeping track of the north magnetic pole is Newitt's job. "We usually go out and check its location once every few years," he says. "We'll have to make more trips now that it is moving so quickly."
Earth's magnetic field is changing in other ways, too: Compass needles in Africa, for instance, are drifting about 1 degree per decade. And globally the magnetic field has weakened 10% since the 19th century. When this was mentioned by researchers at a recent meeting of the American Geophysical Union, many newspapers carried the story. A typical headline: "Is Earth's magnetic field collapsing?"
As remarkable as these changes sound, "they're mild compared to what Earth's magnetic field has done in the past," says University of California professor Gary Glatzmaier.
Sometimes the north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago.
Above: Magnetic stripes around mid-ocean ridges reveal the history of Earth's magnetic field for millions of years. The study of Earth's past magnetism is called paleomagnetism. (Image source: USGS)
To understand what's happening, says Glatzmaier, we have to take a trip ... to the center of the Earth where the magnetic field is produced.
At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2o of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."
Right: a schematic diagram of Earth's interior. The outer core is the source of the geomagnetic field.
Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"-whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.
What happens when the reversal occurs? Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti. Weird. But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.
Above: Supercomputer models of Earth's magnetic field. On the left is a normal dipolar magnetic field, typical of the long years between polarity reversals. On the right is the sort of complicated magnetic field Earth has during the upheaval of a reversal.
The magnetic field has a major importance in the seismicity on Terra.
The basic concept embodied in plate tectonics was recognized several centuries ago by persons working with world maps who noted that the outline of the west oast of Africa is a good match for the east coast of South America and suggested that the two continents might at one time have been joined together. Although this fact was a matter of great curiosity, it was not until much later that a Frenchman, Antonio SniderPellegrini made the first attempt to develop the concept of substantial continental movement; in a book published in 1858 he hypothesized that the Atlantic Ocean had been formed by the tearing apart and separation of the two continents. The author supported this postulation of "continental drift."(as it came to be known)
A much more comprehensive argument for the concept of continental drift was made in 1915 by a German meteorologist, AlfredWegener, in his book, "On the Origin of Continents and Oceans", in which he claimed that essentially all the world's large land masses at one time had been joined in a single supercontinent.
In the following years numerous other scientists began to accept the idea of continental drift, but most geophysicists rejected it because they could not imagine how major continental units could be driven through the oceanic crust for any signicant distance.
The American geologist H. H. Hess (1962) conceived a general hypothesis for motions of the Earth's crust. This continual movement is thought to be driven by heating of the mantle due to radioactivity, which causes molten rock to well upwards along the ocean ridges. As this material reaches the surface and cools, it forms the crust at the surface of the lithosphere; the entire lithosphere floating on the plastic asthenosphere is caused to spread outwards in both directions by the continued upwelling of additional molten rock. This new crust then sinks beneath the surface of the sea as it cools and spreads outward, and the motion continues until eventually the lithosphere reaches a deep sea trench where it plunges downward into the asthenosphere in a process called subduction. This concept of sea floor spreading is supported by many types of physical evidence, including the presence of seamounts at great depths as was mentioned earlier, but the most striking proof of the theory is given by patterns of magnetic orientation as shown by the zebra stripe patterns that have been observed in maps of sea floor magnetism.
As the Eath's crust is first formed, it is magnetized in accordance with the polarity of the Earth's magnetic field at that time, and it maintains that polarity as it spreads outward from the ridge. However, when the earth's eld changes polarity, as has happened at intervals of one half to one million years during the present Cenozoic era (65 million years), the crust that has just developed on both sides of the ridge shows the newly reversed polarity The rate of sea floor spreading may be evaluated by relating the spacing of these polarity stripes to the chronology of the polarity changes.
A plausible explanation for the intense seismic activity in the recent years could be the change that is taking place in the Earth's magnetic field. This causes the tectonic plates to move, colide more often than they normaly would. See the great earthquakes from all around the world.
