Paleomagnetic dating method
As early as the 18th century, it was noticed that compass needles deviated near strongly magnetized outcrops.In 1797, Von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks).New oceanic crust is magnetized as it forms and then it moves away from the ridge in both directions.The models show a ridge (a) about 5 million years ago (b) about 2 to 3 million years ago and (c) in the present.Paleomagnetism (or palaeomagnetism in the United Kingdom) is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials.Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form.
It is possible to get round this problem if we can find an approximate date of the rocks by other means.
In this article we shall discuss how we can use the paleomagnetism in rocks to attach dates to them (paleomagnetic dating).
The reader may find it useful to go back and read the main article on paleomagnetism before continuing.
In the 19th century studies of the direction of magnetization in rocks showed that some recent lavas were magnetized parallel to the Earth's magnetic field. Blackett provided a major impetus to paleomagnetism by inventing a sensitive astatic magnetometer in 1956.
Early in the 20th century, work by David, Brunhes and Mercanton showed that many rocks were magnetized antiparallel to the field. His intent was to test his theory that the geomagnetic field was related to the Earth's rotation, a theory that he ultimately rejected; but the astatic magnetometer became the basic tool of paleomagnetism and led to a revival of the theory of continental drift.