Geomagnetic Reversals

• Iron-rich minerals occur in the lava flows found in the ocean ridges.
• As the lava cools to below about 580°C (the Curie point), the individual mineral grains become magnetized in the direction of the Earth's magnetic field.
• The rocks record the magnetic fields that existed when they cooled.
• Preserving the direction of the ancient magnetic field is called paleomagnetism.
• Examining cooled lava from various places on Earth showed the Earth's magnetic field periodically reverses direction.
• The Earth's magnetic north pole would be at the geographic south pole.
• These reversals happen every few 100,000 years to several million years apart.
• Sensitive instruments showed that the rocks on either side of the Mid-Atlantic ridge showed a pattern of 'normal' and 'reversed' magnetism.
• In 1963, Vine and Matthews suggested that the patterns which ran || (parallel) to the ridge were a recording of the changes of the Earth's magnetic field made as lava was extruded from the ridge and displaced by later lava flow.