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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.
The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the Mid-Atlantic Ridge at a rate of 1–2 cm (0.4–0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount.
Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in Earth’s mantle, from which it had originated.
Studies conducted with thermal probes, for example, indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the mid-ocean ridges, where at some sites the heat flow measures three to four times the normal value.
Patterns of seafloor spreading in the Pacific (left), Arctic (centre), and Atlantic oceans (right) showing the relative age of oceanic crust.
The youngest regions are coloured red, whereas the oldest regions are coloured blue.magnetic anomalies have further corroborated the seafloor spreading hypothesis.
Magnetic stripes are the result of reversals of the Earth's field and seafloor spreading.
New oceanic crust is magnetized as it forms and then it moves away from the ridge in both directions.