How do sea-floor spreading and magnetic reversal produce bands…?

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1. CupcakeQueen says:

   March 26, 2010 at 11:27 am

   When magma rises to fill in the gap, it contains magnetic elements.

   In 1963, Fred Vine, Drummond Matthews, and others found that the crust surrounding the midocean ridges showed alternating bands — each band magnetized with a polarity opposite the surrounding bands. They suggested that as new sea-floor crust was formed around the rift in the midocean ridge, it magnetized differently, depending upon the polarity of the planet at that time. This supported the theory that Harry Hess had put forth, that the ocean progressivley widens as new sea floor is created along a crack that follows the crest of midocean ridges.

   Molten rock erupts along a mid-ocean ridge, then cools and freezes to become solid rock. The direction of the magnetic field of the Earth at the time the rock cools is "frozen" in place. This happens because magnetic minerals in the molten rock are free to rotate so that they are aligned with the Earth’s magnetic field. After the molten rock cools to a solid, these minerals can no longer rotate freely. At irregular intervals, averaging about 200-thousand years, the Earth’s magnetic field reverses. The end of a compass needle that today points to the north will instead point to the south after the next reversal. The oceanic plates act as a giant tape recorder, preserving in their magnetic minerals the orientation of the magnetic field present at the time of their creation. Geologists call the current orientation "normal" and the opposite orientation "reversed."

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