P6.2 Plate Techtonics

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Some minerals in igneous rocks develop a slight magnetism in alignment with Earth’s magnetic field at the time of their formation. Also, scientists have discovered the polarity of Earth’s magnetic field has periodically reversed and the North Magnetic Pole becomes the South Magnetic Pole, while the South Magnetic Pole becomes the North Magnetic Pole.​

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Scientists looked at the new rock formed at the bottom of the ocean and saw igneous rocks that had recorded the magnetic field of the Earth.

The ancient magnetism, called paleomagnetism, present in rocks on the ocean floor can be used to determine the rate at which the plates are separating and, consequently, the time when they began to separate. Where plates separate along the mid-ocean ridge, magma from the mantle rises to the surface and creates new ocean floor. As the magma cools, the minerals assume magnetism equal to the magnetic field at that time.

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Records of the magnetic polarity reversals have been determined from the oceanic crust across sections of the mid-ocean ridges in the Pacific, South Atlantic, and North Atlantic oceans.​

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This shows spreading from the last 2 million years.​

Hawaiian Islands

Researchers have proposed that a rising plume of mantle material has formed a hot spot below the island of Hawaii. It is believed that the position of this hot spot within the Earth has remained constant during a very long period of time. In the past, as the Pacific plate has moved over the hot spot, the successive volcanic islands of the Hawaiian chain have been built.

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