Paleomagnetism
Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials.
The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool.
Evidence from paleomagnetism led to the revival of the continental drift hypothesis and its transformation into the modern theory of plate tectonics.
Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading.
Paleomagnetic data continues to extend the history of plate tectonics back in time, constraining the ancient position and movement of continents and continental fragments (terranes).
Early in the 20th century, work by David, Bernard Brunhes and Paul Louis Mercanton showed that many rocks were magnetized antiparallel to the field.
[6][7] Although he produced an abundance of circumstantial evidence, his theory met with little acceptance for two reasons: (1) no mechanism for continental drift was known, and (2) there was no way to reconstruct the movements of the continents over time.
Keith Runcorn[8] and Edward A. Irving[9] constructed apparent polar wander paths for Europe and North America.
Nonetheless, the record has been preserved well enough in basalts of oceanic crust to have been critical in the development of theories of sea floor spreading related to plate tectonics.
[10] Although the Māori people of New Zealand do not make pottery, their 700- to 800-year-old steam ovens, or hāngī, provide adequate archaeomagnetic material.
Lightning-induced remanent magnetization can be distinguished by its high intensity and rapid variation in direction over scales of centimeters.
In order to collect paleomagnetic data dating beyond 200 Ma, scientists turn to magnetite-bearing samples on land to reconstruct Earth's ancient field orientation.
[16] Paleomagnetic evidence of both reversals and polar wandering data was instrumental in verifying the theories of continental drift and plate tectonics in the 1960s and 1970s.
Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved.
