Magnetic polarity reversal

A magnetic polarity reversal is a change of the Earth's magnetic field to the opposite polarity. This has occurred at irregular intervals during geologic time.

Polarity reversals can be preserved in sequences of magnetized rocks. After molten lava emerges from a volcano, it solidifies. In most cases it is a black rock known as basalt, which is faintly magnetic. Its magnetization is in the direction of the local magnetic force at the time when it cools.

Instruments can measure the magnetization of basalt. Therefore, if a volcano has produced many lava flows, scientists can analyze the magnetizations of the various flows and from them get an idea on how the direction of the local Earth's magnetic field varied in the past. This procedure has suggested the over geologic time the magnetic polarity of the Earth has sometimes reversed itself.

Magnetic orientation logs of ocean floor have shown that the magnetization of the beds is linear and orderly, arranged in long strips. The strips on the Atlantic ocean floor in particular are all parallel to the mid-Atlantic ridge. Not only are the magnetic strips lined-up with the central ridge, but their structure and distribution are remarkably symmetric on both sides: if (say) a narrow-wide pair of strips is observed at a certain distance east of the ridge, its mirror image was also found at about the same distance to the west. This pattern can be used to determine the rate of ocean ridge spreading. The reversal patterns recorded in the rocks are termed sea-floor magnetic lineaments.

The magnetic orientation of rocks can be compared with standard polarity-change time scales to estimate geologic ages of formations.