Magnetic reversals and sea-floor spreading

Magnetic reversals and sea-floor spreading

. Radioactive dating of volcanic rocks on land have been used to determine the time scale of magnetic reversals on the Earth (bottom) They indicate that the Earth's magnetic field has flipped, or changed direction several times during the past five million years. The data describe normal epochs (white) when compasses would have pointed toward the geographic north, as they do now, and reversed epoch (gray) when compasses would have pointed south. The pattern of magnetic reversals on both sides of the volcanic mid-ocean ridge (top) is the same, indicating that sea-floor spreading has carried the solidified lava away from the central ridge. The sea floor is consumed at the other end, when it slides into a deep ocean trench at a subduction zone.

Copyright 2010, Professor Kenneth R. Lang, Tufts University