Paleomagnetics

The first ever treatise on experimental science by thirteenth century scholar Petrus Peregrinus of Marincourt dealt with magnetism ("Epistola de Magnete"). However, direct observations of the geomagnetic field were not recorded until the late sixteenth century, when the magnetic compass became a widespread tool for navigation. In order to understand nature and origin of Earth's magnetic field , however, much longer records are necessary. Paleomagnetic research draws this information from rocks that acquire a remanent magnetization upon formation.

The natural magnetization of a rock is parallel to the ambient magnetic field. It is carried by minute amounts of ferrimagnetic minerals and can be stable over geological time scales. Precise snap-shots of the past geomagnetic field are recorded by volcanic rocks, while sedimentary rocks retain smoothed records acquired over discrete intervals of time. Sequences of rocks can thus act like a magnetic tape, which records a piece of music. Unfortunately, the original record is usually altered secondarily through time and various weathering processes. Paleomagnetic methods have to be employed to remove this magnetic noise and extract a true primary magnetization.

Paleomagnetic research has shown that Earth's magnetic field has been a dipole field for more than 99.9% of Earth's history. Its shape resembles that of the field of a barmagnet. The field lines emerge at one pole and re-enter at the other pole. The earth's magnetic field however is not caused by a huge mass of iron with a remanent magnetization, but its origin lies in the outer fluid core where convective motion generates the magnetic field in a self-sustaining dynamo action. This dynamic origin of the geomagnetic field is the main reason why its shape and orientation are not constant but subject to temporal variations on time scales that range from millions of years to days. Recently, for example, the dipole axis is inclined by about 11 (against the spin axis). Averaged over time spans greater than 100,000 years, the dipole axis is parallel with the earth's spin axis.

The earth's magnetic field can characteristically reverse its polarity, meaning that the magnetic poles can switch position. Other second order phenomena are termed the secular variation of Earth's magnetic field.

The temporal variations of Earth's magnetic field are widely used in geosciences. The understanding that Earth's magnetic field can truly reverse its polarity had a huge impact on our view of Earth, because the idea was crucial for the development and break-through of plate tectonics . This view of Earth as a dynamic system which was put forward in 1915 by German geophysicist Alfred Wegener (1880–1930), but was not commonly accepted until the 1960s. Only then was the cause for the characteristic pattern of the oceanic magnetic anomalies understood. They are characterized by alternating stripes of normally and inversely magnetized rocks parallel to the mid-ocean ridges and are caused by the continuous addition of newly formed rocks, adding new layers and pushing the rims away from the ridge, while the geomagnetic field frequently reversed.

The fact that Earth's magnetic field never fundamentally changed its shape through its history allowed paleomagnetists to investigate the movement of plates by calculating the position of the North magnetic pole from the magnetization of rocks. Assuming the earth's field is a dipole over large intervals of time, one can calculate the geographic latitude of a rock at the time when its remanent magnetization was acquired. By investigating rocks from subsequent time intervals, it is then possible to reconstruct the path of a plate relative to the magnetic pole, or vice versa. It allows tracking of the former distribution of plates and continents through time.

Another important application of paleomagnetism to geoscience is the opportunity to use a sequence of reversals for dating and correlating sedimentary sequences on a global scale. Magnetostratigraphy uses the globally simultaneous occurrence of dated polarity changes. This dating method can at best resolve an average of 100,000 years. However, for rocks younger than approximately 10,000 years it is possible to use calibration curves of the paleosecular variation for dating with accuracy better than a few hundred years.

Recently, another branch of paleomagnetism has become a method in its own right. Rock magnetism was developed as a tool to judge the reliability of the paleomagnetic record. Today it is widely used in environmental and paleoclimatic research.

See also Earth (planet); Ferromagnetic; Plate tectonics; Polar axis and tilt