Palaeomagnetism

Palaeomagnetism, magnetization of rocks that preserves indications of the orientation and strength of the Earth’s magnetic field at the time the rock formed. The magnetization is normally quite faint.

Many geological materials are composed of metallic minerals that can become magnetized. In the case of igneous rocks, magnetic mineral crystals (for example, magnetite or haematite) acquire their magnetic properties as the minerals cool. At high temperatures (above 500° C) the atoms that comprise these crystals undergo strong vibrations. These vibrations prevent the atoms from achieving the alignment necessary to produce a noticeable magnetic field. As the mineral crystal cools, however, the vibrations are reduced to the point at which local collections of atoms orient themselves parallel to the Earth’s local magnetic field. By the time a temperature of approximately 450° C has been reached, further atomic reorientations become impossible. If a sufficient number of oriented magnetic crystals are present in the rock, its palaeomagnetism, or “natural remnant magnetism”, may be measured. Sedimentary rocks may acquire a natural remnant magnetism if a sufficient number of small magnetic mineral grains have an opportunity to orient themselves parallel to the Earth’s magnetic field during deposition. Metamorphic rocks may take on the natural remnant magnetism of their pre-metamorphosed materials or, if heated above the temperature at which magnetization is lost (the Curie temperature), they may achieve a new natural remnant magnetism after cooling.

The Earth’s predominantly dipolar magnetic field is thought to be caused by electrical currents in the outer portions of the planet’s liquid, metallic (Fe-Ni) core. These electrical currents are most likely created as a result of physical convection currents within that liquid, and given a consistent orientation as a result of the planet’s rotation. The fact that the Earth’s magnetic poles have always been located close to its rotational poles provides direct evidence for this model. However, beginning in the early 20th century, geologists measuring the palaeomagnetism of ancient rocks and sediments discovered that the polarity of the planet’s magnetic field had been reversed during many intervals in Earth’s past. In other words, at certain times in the Earth’s history the “North” arrow of a compass would have pointed to the “South” magnetic pole. A comprehensive study of the patterns of geomagnetic pole reversals through time was undertaken throughout the mid-20th century and geologists now have a detailed knowledge of this pattern of palaeomagnetic reversals for the past 600 million years. It is thought that polarity reversals of the Earth’s magnetic field arise as a result of relatively short-term instabilities in the arrangement of convection cells within the Earth’s outer core.

Interestingly, the reversal pattern is not regular (as with the Sun), but chaotic. Reversed magnetic polarity intervals can be as short as 100,000 years or as long as tens of millions of years. Moreover, the polarity reversal process itself can take place in as short an interval as a few weeks, or as long an interval as 10,000 years. A few sediment sequences also appear to record relatively long intervals of geographical “excursions” in the location of the magnetic field’s poles.

The Earth’s magnetic field participates in a variety of physical and biotic processes. In the physical realm, the field notably serves as a shield to ionizing radiation emanating from the Sun that otherwise would irradiate the Earth’s surface. Such radiation has long been known to cause genetic mutations. In the biotic realms many types of organisms possess the ability to sense the orientation of local magnetic fields. In some instances (as, for example, in certain birds) this information is known to be used during migration, while in others (in some bacteria, for instance) its purpose is as yet unknown. With these factors in mind, the fossil records of magnetic field reversals and extinctions have been compared, but no consistent association has been demonstrated to date.