Atlantic Excursions

New Insight Into Earth's Magnetic Quirks
Royal Astronomical Society Press Notice
Ref. PN 99/08, March 29, 1999

... Certain rocks preserve a record of the state of Earth's magnetism at the time they formed. From studying and dating them, geophysicists have known for some time that Earth's magnetic poles have often flipped ompletely in the remote past. The last such reversal took place 700 thousand years ago. But there is also evidence for more frequent episodes when the magnetic poles have moved a large distance - 45 degrees or more away from the geographical pole - then returned. These events, known as 'excursions', are rather like failed attempts at reversal. When they occur, the strength of the magnetic field falls dramatically as well, by a factor of 5 or 10.

Professor Gubbins has drawn on recent experimental results, particularly those from a research group in Utrecht headed by Dr Cor Langereis, which clearly identify six relatively recent magnetic excursions as true global phenomena. All the excursions lasted roughly the same length of time - about 5,000 years. Furthermore, preliminary results from the recent Ocean Drilling Program Leg 172 have revealed more than twenty excursions recorded in sediments of the North Atlantic in the same time period. While these events have not been correlated world-wide, the sediments indicate very clearly that excursions are quite frequent events. Professor Gubbins noted that there are about ten excursions between each full reversal. Every 20 - 50 thousand years, the Earth's magnetic field collapses in a failed attempt to reverse, but then re-establishes itself quickly over a timescale of just a couple of thousand years. ...