Frequently Asked Questions (FAQ's) About Paleomagnetism
These are typcial questions asked by people who have accessed this home page. Feel free to ask more and I will try to answer them here.
Simply put, paleomagnetism is the study of ancient magnetization in rocks. Most rocks have magnetic minerals such as magnetite and hematite that can, under certain conditions form a long-lasting memory of the Earth's magnetic field when the rocks formed. This memory, or remanence, can then be used to reconstruct continental drift patterns through time. Some of the figures shown on the home page show paleoreconstructions.
Most elementary geology textbooks will report a reversal frequency on the order of every 1/2 to 1 million years. These claims are made by simply averaging out the number of reversals over a given time, but the actual magnetic field reversal of the Earth is highly chaotic. The last field reversal took place about 750,000 years ago and the dipole component of the Earth's magnetic field is getting weaker over the past century or so. There are periods in Earth history where the field did not reverse for 10's of millions of years (Cretaceous Long Normal and Kiaman Reverse Intervals for example). One of the outstanding problems in modern geophysics is attempting to determine the cause of reversals and the nature of the Earth's dipole field.
It appears that during a reversal of the magnetic field, the dipole component of the field (the dipole is analagous to a bar magnet) slowly decays and grows in the opposite direction. The time that it takes to complete a reversal is also the subject of ongoing debate, but time scales on the order of 10's to a thousand years are commonly cited. There doesn't seem to be any strong correlation between field reversals and extinctions. This may mean that when the main dipole field reverses, animal and plant life are protected by the higher order components of the Earth's magnetic field.
There are any number of institutions in the USA and abroad that have active paleomagnetic laboratories. Most paleomagnetists are trained as geologists or geophysicists and therefore you need to find a University that has a major program in those fields. A geology major will be trained in many sciences (biology, chemistry and physics), math (algebra, geometry, trigonometry and calculus) and requires good communication skills. A description of the major requirements at Indiana State University can be found here. These are typical of most undergraduate geology degrees in the USA. Programs are a bit different in other countries.
The inner core and outer core of the planet are composed primarily of iron and nickel. The outer core is liquid and the inner core is solid. It is thought that the convective motion of the liquid outer core is responsible for generating the main field. Recent studies indicate that the inner core of the Earth may rotate a bit faster than the outer core and it is possible that this rotation may be associated with field reversals.
We begin by collecting a suite of oriented samples using a portable drill that is cooled by circulating water. The samples are then oriented in the hole and removed and catalogued. Samples are then returned to the laboratory and cut into cylindrical specimens (12.54 cm3). The specimens are then measured in a magnetometer (described here) to determine the declination and inclination of the magnetic minerals in the rock. The declination (N,S,E,W) tells us the amount of rotation of the continent since the rock formed and the inclination (dip-needle) tells us the latitude that the rock formed. The dip needle angle varies depending on the latitude you are at. In a normal magnetic field (north-seeking), the dip needle will point straight down at the north-pole, straight up at the south-pole and will be horizontal at the equator. If both the age of the rocks and the paleomagnetic direction in the rocks are known, then paleomagnetists can use this information to determine the drift history of continents during the past.
