Geomagnetism - 2 of 21
Geomagnetism FAQs - 21 Found
Yes. We know this from an examination of the geological record. When lavas deposited on the Earth's surface subsequently solidify, and when sediments deposited on ocean and lake bottoms solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. This type of magnetization is known as "paleomagnetism." Careful measurements of oriented samples of faintly magnetized rocks taken from many geographical sites allow scientists to work out the geological history of the magnetic field. We can tell, for example, that the Earth has had a magnetic field for at least 3.5 billion years, and that the field has always exhibited a certain amount of time-dependence, part of which is normal secular variation like that which we observe today, and part of which is an occasional reversal of polarity.
Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, because the rotation of the Earth is an important dynamical force in the core, where the main part of the field is generated. Occasionally, however, the secular variation becomes sufficiently large such that the magnetic poles end up being located rather distantly from the geographic poles; we say that the poles have undergone an "excursion" from their preferred state.
We know from physics that the Earth's dynamo is just as capable of generating a magnetic field with a polarity like that which we have today as it is capable of generating a field with the opposite polarity. The dynamo has no preference for a particular polarity. Therefore, after an excursional period of enhanced secular variation, the magnetic field, upon returning to its usual state of rough alignment with the Earth’s rotational axis, could just as easily have one polarity as another.
The consequences of polarity reversals for the compass are dramatic. Nowadays, the compass points roughly north, or, more precisely, the north end of the compass points roughly north at most geographical locations. However, before the last reversal, which was about 780,000 years ago, the polarity was reversed compared to today's, and the compass would have pointed roughly south, and before that reversed state the polarity was like that which we have today, and the compass would have pointed roughly north, and so on.
The timings of reversals forms the "geomagnetic polarity timescale." During a reversal, between polarities the geometry of the magnetic field is much more complicated than it is now, and a compass could point in almost any direction depending on one’s location on the Earth and the exact form of the mid-transitional magnetic field.
One of the things that is interesting about reversals is that there is no apparent periodicity to their occurrence. Reversals are random events. They can happen as often as every 10 thousand years or so and as infrequently as every 50 million years or more.
Magnetic striping used to develop plate tectonic theory
Magnetic reversals in the Juan de Fuca Plate