Geomagnetic field excursions occurred often during the last million years



Scientists studying western North Atlantic Ocean deep-sea sediments have discovered that the Earth's magnetic field underwent 14 local excursions since the last global magnetic-field polarity reversal 780,000 years ago. These excursions coincide with similar excursions identified elsewhere on the planet—leading to the conclusion that excursions are global in nature, occur a significant portion of the time, and are an integral part of geomagnetic field secular variation between reversals. Excursions are defined [Verosub and Banerjee, 1977] as anomalous magnetic field directions whose equivalent virtual geomagnetic poles (VGPs) are more than 45° away from the North Geographic Pole, while VGPs within 45° of the North Geographic Pole are considered to be typical secular variation. (VGPs are calculated from local magnetic field directions which locate the magneticfield North Pole by assuming that the directions are caused by a simple dipole or bar magnet situated at the center of the Earth.)