Things are Chang­ing!

A geo­mag­netic rever­sal is a change in a planet’s mag­netic field such that the posi­tions of mag­netic north and mag­netic south are inter­changed, while geo­graphic north and geo­graphic south remain the same. The Earth’s field has alter­nated between peri­ods of nor­mal polar­ity, in which the pre­dom­i­nant direc­tion of the field was the same as the present direc­tion, and reverse polar­ity, in which it was the oppo­site. These peri­ods are called chrons.

The time spans of chrons are ran­domly dis­trib­uted with most being between 0.1 and 1 mil­lion years[citation needed] with an aver­age of 450,000 years. Most rever­sals are esti­mated to take between 1,000 and 10,000 years. 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. That rever­sal lasted only about 440 years with the actual change of polar­ity last­ing around 250 years. Dur­ing this change the strength of the mag­netic field weak­ened to 5% of its present strength. Brief dis­rup­tions that do not result in rever­sal are called geo­mag­netic excursions.

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