Visualization
ES0803  Observe how alternating magnetic polarity is recorded in rocks at mid-ocean ridges.
  !   Click the image to see the animation. Use the movie controls to examine the changes.
Jennifer Loomis, TERC

The position of Earth's magnetic field changes over geologic time. Currently, the north magnetic pole is located near Earth's North Pole, a condition called normal polarity. Periods when the north magnetic pole is located near Earth's South Pole are called reversed polarity.

When molten lava solidifies at mid-ocean ridges, iron-containing minerals in the lava align themselves with Earth's active magnetic field. These minerals record the orientation of the magnetic field at the time they solidified.