Brief disruptions that do not result in reversal are called geomagnetic excursions.In the early 20th century geologists first noticed that some volcanic rocks were magnetized opposite to the direction of the local Earth's field.

geomagnetic reversal dating-75

Geomagnetic polarity during the last 5 million years (Pliocene and Quaternary, late Cenozoic Era).

Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed.

"Breakup and Early Seafloor Spreading between India and Antarctica." Geophysical Journal International 170, no. References Antretter, M., Steinberger, B., Heider, F., Soffel, H. Paleolatitudes of the Kerguelen hotspot: new paleomagnetic results and dynamic modelling, Earth Planet.

Dietmar Mller, Belinda Brown, Takemi Ishihara, and Sergey Ivanov.

The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago; and may have happened very quickly, within a human lifetime.

A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period.

Double flood basalts and plume head separation at the 660 kilometer discontinuity, Science, 266, 1367-1369.

Underlying fracture zone nature of Astrid Ridge off Antarctica, Queen Maud Land, J. Refined spreading history of the Southwest Indian Ridge for the last 96 Ma, with the aid of satellite data, Geophys.

A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged.

The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was in the opposite direction. The time spans of chrons are randomly distributed with most being between 0.1 and 1million years.

That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years.