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.
Instruments can measure the magnetization of iron-bearing rocks, such as basalt. Therefore, if a volcano has produced many lava flows, scientists can analyze the magnetic orientations of the rocks from the various flows and from them get an idea on how the direction of the local Earth's magnetic field varied in the when the rock solidified. This procedure has suggested the over geologic time the magnetic polarity of the Earth has sometimes reversed itself. Scientists believe the time between reversals on the Earth falls somewhere between every 100,000 to 25 million years, with the reversal itself taking about 5,000 years. During much of that time, the geomagnetic field strength decreases - which counts as part of the 'reversal' process.
One contributing factor to the decrease and eventual reversal may be intensifying core spots within the Earth's inner core that are magnetized in a sense opposite of the main field; while competing energy sources, such as heat loss at the mantle-core boundary and growth of the inner core may also be contributing factors.
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 used as a geologic relative dating tool when compared with standard polarity-change time scales to estimate geologic ages of formations.