Mid-Ocean Ridges: Magnetics & Polarity
How Fast is the Mid-Ocean Ridge Spreading?
When lava gets erupted at the mid-ocean ridge axis it cools and turns into hard rock. As it cools it becomes permanently magnetized in the direction of the Earth’s magnetic field. Magnetometers, towed near the sea surface behind research ships or mounted on submarines like Alvin, measure the magnetic anomalies or “wiggles” that record the changes in magnetization of the volcanic sea floor.
Less than 60 years ago, scientists discovered that the Earth’s magnetic field has reversed its polarity (direction) hundreds of times during the past several hundred million years. A polarity reversal means that the magnetic North flips to where we know the South Pole is. At the mid-ocean ridge spreading axis, these flips in the direction of the Earth’s magnetic field are recorded in the magnetization of the lava. This creates a symmetrical pattern of magnetic stripes of opposite polarity on either side of mid-ocean ridges.
These patterns of stripes provide the history of seafloor spreading. Geophysicists can read these patterns from the magnetic anomalies they measure with a magnetometer. Where the magnetic wiggles, or anomalies, are broader, the spreading rate has been faster. At slow spreading ridges, the anomalies are squeezed tighter together, but the basic patterns are quite similar so scientists can correlate or relate the magnetic wiggles to different parts of the global mid-ocean ridge.