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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.
These curves diverged, but could be reconciled if it was assumed that the continents had been in contact up to 200 million years ago.
This provided the first clear geophysical evidence for continental drift.
Patterns of seafloor spreading in the Pacific (left), Arctic (centre), and Atlantic oceans (right) showing the relative age of oceanic crust.
This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates.
These include biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and environmental magnetism.
As early as the 18th century, it was noticed that compass needles deviated near strongly magnetized outcrops.
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.
Japanese geophysicist Motonori Matuyama showed that the Earth's magnetic field reversed in the mid-Quaternary, a reversal now known as the Brunhes-Matuyama reversal. Alfred Wegener first proposed in 1915 that continents had once been joined together and had since moved apart.
In 1797, Von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks).