Same old magnetism

摘要

Latitudes at which ancient salt deposits occur show that Earth's magnetic field has always aligned along its rotation axis. One possible implication is that ancient global glaciations were not caused by a realignment of this axis. In a paper of admirable scope and thoroughness that appears on page 51 of this issue1, David Evans analyses the magnetization locked into rocks associated with salts from all over the globe that have been deposited over the past 2,500 million years. Taking as a working model the geocentric axial dipole' — the idea that, averaged over thousands of years, the magnetic field at Earths surface resembles the field of a magnet, or dipole, at Earths centre2'3 — these magnetizations and this model provide clues to the past evolution and interplay of Earths magnetism, climate and geography. In the geocentric axial dipole model, the north and south poles of Earths internal magnet are aligned along Earths axis of rotation. This simple axial form is thought to be caused by rotational forces that guide the motions of Earths conducting liquid core, and so constrain the average surface field. Under favourable circumstances, rocks become magnetized along the direction of the ambient geomagnetic field as they are formed. Thus, by sampling sequences of rocks with formation dates spanning several thousands of years, one can determine the past average direction of the field, the 'palaeolatitude' of the sampling locality and the position of the palaeomagnetic pole at the time. For the past 5 million years, these poles coincide with the present rotational pole; the giant dipole model has therefore been valid for at least this long.