Paleomagnetism is defined as the study of the record of the Earth’s ancient magnetic field using igneous and sedimentary rocks. It has been instrumental for our understanding of how planet Earth operates and is one of the cornerstones of plate tectonic theory. When rocks form, they record the direction, polarity, and absolute or relative intensity of the Earth’s magnetic field, which can be extracted with paleomagnetic methods. Paleomagnetism has several applications, most importantly it is used to reconstruct plate movements and plate tectonic configurations, and in magnetostratigraphy as a dating tool for sedimentary rocks. It can also provide information on the past behavior of Earth's magnetic field in absolute and relative paleointensity studies.
The application of paleomagnetism as a dating tool is widely used by geoscientists (e.g., Langereis, 2010; Tauxe, 1993). Paleomagnetic methods have also been developed to utilize the polarity of the geomagnetic field as the only way to assign absolute (radiometric) ages to sedimentary rocks. Radiometric dating and magnetostratigraphy are the common methods used as absolute dating tools in igneous and sedimentary rocks. Palaeomagnetic studies of igneous rocks provided the first reliable information on magnetic polarity reversals. In 1906, Brunhes observed that lava flows have been magnetized in the opposite direction to the present geomagnetic field (Langereis, 2010; Brunhes, 1906). This was further investigated by Matuyama, (1929), and Hospers, (1951). Additionally, some scientists used polarity of lava flows as a correlation tool for stratigraphy (Irving, 1988; Stern, 2002).
Khramov (1985) was the pioneer who used both volcanic and sedimentary rocks to develop a single geochronogical paleomagnetic time scale. The original paleomagnetic polarity time scale was developed using both radiometric dating and magnetostratigraphy in 1963 (Cox et al., 1963, 1964), and has been improved and completed since, notably by Cande and Kent (1992, 1995), Berggren et al. (1992, 1995), and the most recent compilation by Gradstein et al. (2012). The application of magnetostratigraphy as a dating tool in sedimentary and igneous rocks has been significantly expanded in the past few decades (Figure 1). Back in the 1960s, the stratigraphy was limited only to the last 3 m.yr. of Earth’s age, however it is now documented back to 300 Ma (Opdyke and Channell, 1996; Gradstein et al., 2012).
Figure 1. Development of the geomagnetic polarity time scale (GPTS), (from Langereis, 2010)
Data from orbital tuning of the sediment record have led to the most advanced development of the GPTS, the Astronomically Calibrated Polarity Time Scale (APTS) (Hilgen et al. 1997). In this method, the time scale is tuned to the periodic changes in the Earth’s orbital parameters.
1.2 Magnetization of Sedimentary Magnetic Minerals
Every matter responds to a magnetic field because of the collective alignment of atomic intrinsic magnetic moments. This...