Leonard, Jonathan N. The First Farmers: The Emergence of Man. New York: Time-Life Books, 1973. Prothero, Donald R. Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill, 2004.
paleomagnetism Paleomagnetism is the study of natural remnant magnetism in rocks with the goal of understanding the intensity and direction of the Earth's magnetic field in the geologic past and understanding the history of plate motion. The Earth's magnetic field can be divided into two components at any location—the declination and the inclination. The declination measures the angular difference between the Earth's rotational north pole and the magnetic north pole. The inclination measures the angle at which the magnetic field lines plunge into the Earth. The inclination is 90° at the magnetic poles and 0° halfway between the poles.
Studies of paleomagnetism in young rocks have revealed that the Earth's magnetic poles can flip suddenly, over a period of thousands or even hundreds of years. The magnetic poles also wander by about 10-20° around the rotational poles. On average, however, the magnetic poles coincide with the Earth's rotational poles. A researcher can use this coincidence to estimate the north-south directionality in ancient rocks that have drifted or rotated in response to plate tectonics. Determination of the natural remnant magnetism in rock samples can, under special circumstances, reveal the paleo-inclination and paleo-declination, which can be used to estimate the direction and distance to the pole at the time the rock acquired the magnetism. If these parameters can be determined for a number of rocks of different ages on a tectonic plate, then an apparent polar wander path for that plate can be constructed. These show how the magnetic pole has apparently wandered with respect to (artificially) holding the plate fixed—when the reference frame is switched, and the pole is held fixed, the apparent polar wander curve shows how the plate has drifted on the spherical Earth.
Paleomagnetism played an enormous role in the confirmation of seafloor spreading, through the discovery and understanding of seafloor magnetic anomalies. In the 1960s geophysicists surveyed the magnetic properties of the ocean floor and began to discover some amazing properties. The seafloor has a system of linear magnetic anomalies where one "stripe" has its magnetic minerals all oriented the same way as the present magnetic field, and the magnetic minerals in alternate stripes are oriented in the opposite direction. These stripes are oriented parallel to the mid-ocean ridge system; where transform faults offset the ridges, the anomalies are also offset. The anomalies are symmetric on either side of the ridge, and the same symmetry is found across ridges worldwide.
Understanding the origin of seafloor magnetic stripes was paramount in acceptance of the plate tectonic paradigm. The magnetic stripes form in the following way. As oceanic crust is continuously formed at mid-ocean ridges, as if it were being extruded from a conveyor belt, all the magnetic minerals tend to align with the present magnetic field when the new crust forms. The oceanic crust thus contains a record of when and for how long the Earth's magnetic field has been in the "normal" position, and when and for how long it has been "reversed." Terrestrial rock sequences exhibit similar reversals of the Earth's magnetic field, and many of these have been dated. using these data, geologists have established a magnetic polarity reversal time scale. The last reversal was about 700,000 years ago, and the one before that, about 2.2 million years ago. oceanic crust is as old as Jurassic, and documentation of the age of seafloor magnetic stripes has led to the construction
Seafloor magnetic stripes in the northeast Pacific Ocean produced by seafloor spreading on the Juan de Fuca, Gorda, and Explorer ridges
Gilbert Gauss Matuyama Matuyama Brunhes Matuyama Matuyama Gauss Gilbert
Q Infobase Publishing
Symmetric magnetic anomalies produced by conveyor-belt style production of oceanic crust in an alternating magnetic field. The magnetic polarity timescale, above, is constructed by dating rocks with different magnetizations.
of the magnetic polarity time scale back to 170 million years ago.
See also magnetic field, Magnetosphere; plate tectonics.
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