Radiometric dating simulation
The same magnetic anomalies were found over most of the world's oceans, which permitted estimates for when most of the oceanic crust had developed. Dark areas denote periods where the polarity matches today's polarity, while light areas denote periods where that polarity is reversed.
The Cretaceous Normal superchron is visible as the broad, uninterrupted black band near the middle of the image.
The relatively constant rate at which the sea floor spreads results in substrate "stripes" from which past magnetic field polarity can be inferred from data gathered from towing a magnetometer along the sea floor.
Because no existing unsubducted sea floor (or sea floor thrust onto continental plates) is more than about (Ma) old, other methods are necessary for detecting older reversals.
Geomagnetic polarity during the last 5 million years (Pliocene and Quaternary, late Cenozoic Era).
The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime.But the complex routes of ocean cruises rendered the association of navigational data with magnetometer readings difficult.Only when data were plotted on a map did it become apparent that remarkably regular and continuous magnetic stripes appeared on the ocean floors.Most sedimentary rocks incorporate tiny amounts of iron rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed.These rocks can preserve a record of the field if it is not later erased by chemical, physical or biological change.
A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period.