increments. This leads to an effect known as polar wander. This continuing process is still active today and forces charts of the earth’s magnetic field, known as isomagnetic maps, to be redrawn every 5 years.
The phenomenon of polar wander has been used extensively in geology as an indicator of global position and the geologic age of iron-bearing rocks known as ferromagnetic rocks. Studying paleomagnetics has, in part, established the position and intensity of the Earth’s magnetic pole over geologic time. Geologists use this new sagaciousness to determine aspects of global tectonic history. Using paleomagnetic techniques it now becomes possible to determine the past relative positions of two separate continents, provided they were previously conjoined to the same plate. A paleogeographical reconstruction of the earth can then coalesce from global paleomagnetic data endowing geologists with valuable insight into Earth’s past.
The ability of geologists to determine past magnetic fields is locked in the chemistry of ferromagnetic rocks. These substances have more electrons spinning in one direction than the other; thus the individual magnetic fields of the atoms in a given region tend to align in the same direction. When an igneous rock is crystallized from lava or magma in the presence of a magnetic field, the magnetic elements leave a magnetic signature frozen in the rock. During the cooling of molten rock, iron-bearing minerals become magnetized in alignment with earth’s magnetic field as they descend through a critical temperature known as the Curie Temperature, which occurs at approximately 500°C-600°C. The rock’s magnetic properties are retained (thermal remanent magnetism) for geologically long periods unless the rock is again heated to near the Curie temperature. Rock samples have their thermal remanent magnetism determined by magnetometers in a laboratory. Around the world, thousands of formations and outcrops have been paleomagnetically analyzed and documented making accurate paleocontentinal maps of Earth a reality and furthermore giving geologists an almost clairvoyant gaze into the past.
Dramatic alterations to the polarity of the Earth’s magnetic field were first noticed on the seafloor of the Atlantic Ocean. Known as the mid-Atlantic spreading ridge, this narrow rift continuously deposits new basalt separating the east coast of North America from the west coast of Europe and Northern Africa. The seafloor in the Atlantic is striped with bands of rocks magnetically trending northward and alternating parallel bands trending southward. Presuming the seafloor at the Mid-Atlantic rift spread similarly in the past as it does today, then as time progressed, the Earth’s magnetic field regularly underwent a full reversal in dominant direction. Thus, magnetically, the seafloor in the Atlantic appears as linear anomalous bands. These reversals were recorded in the basalt going back approximately 100 million years. In that time, the rates of reversal have varied considerably from one reversal to the next. These magnetic reversals are recorded not only in basalt, but also in other igneous rocks and sediments. If the reversals themselves represent simultaneous cosmopolitan phenomenon, they then act as unique stratigraphic markers wherever and whenever they occur. These polarity events, therefore, provide a precise tool for chronostratigraphic correlation of marine and terrestrial sediments. Based on seafloor spreading and sediment depositional rates, the reversal process is thought to occur over a period of 1,000 to 10,000 years which includes a 60% to 80% decrease in intensity of the field some 10,000 years preceding the reversal, whereas the actual reversal itself only takes 1,000 to 2,000 years. The field then builds up in intensity returning to normal intensity. The last polar reversal occurred approximately 700,000 years ago. Polar reversals have various durations and are divided into two main groups: chrons (epochs) occur when the polarity is the same for a long period of time, and subchrons (events) occur when the polarity only maintains a single direction for a relatively short period of time.
The use of paleomagnetism as a global chronometer has been extended in many other sciences. In archeology, the pottery and bricks of ancient peoples are aged using carbon 14 dating showing the date of their last cooling. These samples can also give a relative position of the magnetic pole at that time. This has allowed archeomagnetic investigations to reconstruct the recent course of the magnetic pole. The ability to define boundaries between time periods makes paleomagnetism very appealing. Two outstanding examples are the Peking-man site of Zhoukoudian in northern China, and the Trinchera Dolina site, in Atapuerca, Spain. These sites are now considered the oldest unequivocal human occupation sites in Europe thanks to paleomagnetic studies. Neither Zhoukoudian nor the Trinchera Dolina provided material sufficient for isotropic age determination; however, Zhoukoudian paleomagnetic determinations from fine-grain sediment fill have allowed the Brunhes/Matuyama boundary to be isolated at a level beneath the sediment containing the earliest Peking man fossils. Paleomagnetic analysis of the Trinchera Dolina has placed the Brunhes/Matuyama boundary in sediments that were deposited after any sediments with included artifacts and human fossils. These remains were, therefore, estimated to be approximately 800,000 years old. Nowhere else in Europe have fossils been found that demonstrate human occupation or arrival before 500,000 to 600,000 years ago, implying that only the Trinchera Dolina people were present before then.
The properties of the earth’s magnetic field offer scientists inexhaustible opportunities to explore the inner complexities of the earth and the activities of its more recent inhabitants. Using the reversal punctuations decoded from the rocks and sediments of long ago and correlating these to geological and paleontological insights, new and enlightening aspects of ancient and modern history are being brought to life as well as new means to measure that history. Understanding geomagnetism and how it has affected the life history of the earth is in the forefront of geologic research and will continue to be paramount in this discipline.
References:
- Jacobs, J. A. (1984). Reversals of the Earth’s magnetic field. Bristol: Adam Hilger Ltd.
- McElhinny, M. W., & McFadden, P. L. (2000). Paleomagnetism: Continents and oceans. Academic Press.
- Tarling, D. H. (1983). Palaeomagnetism: Principles and applications in geology, geophysics and archaeology. London: Chapman and Hall.
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