Evidence of frequent reversals in the direction of Earth’s magnetic field is preserved in the rock record, and it has played a fundamental role in deciphering our planet’s tectonic history. However, the processes that cause these polarity reversals—and the properties of the transitional magnetic field itself—remain topics of vigorous debate.
Now, Valet and Fournier review the major features of reversals and evaluate their compatibility with recent numerical modeling results. They conclude that in spite of the increasing number of paleomagnetic records collected during the last 50 years, varying interpretations of the transitional fields still exist. Uncertainties, the authors argue, arise from the rapidity of the reversals—which often occur faster than the age resolution of the rocks that record them. In addition, the observed decrease in transitional field intensity can reduce the fidelity of the magnetic record, particularly in sedimentary rocks.
Despite multiple attempts to collect and model the data from the last reversal, which occurred about 779,000 years ago, the authors conclude that the behavior of the transitional field during that event is still poorly constrained, partly because of the varying resolution of each paleomagnetic record. In addition—and in spite of recent progress—it is still not possible to predict when the next magnetic reversal could occur. The researchers argue that the science is not yet advanced enough to evaluate whether the decreased magnetic field strength measured during the last 800 years is an indication that the next reversal is imminent or if it falls within the typical range of long-term variability.
Present-day reversal forecasts are still a challenge because of the limited scientific understanding of the processes that generate Earth’s magnetic field. These processes are estimated to occur on the scale of several decades to a century—at least an order of magnitude shorter than the time frame during which a reversal occurs. However, advanced physics-based models could increase the predictability of the processes that scientists believe are responsible for the reversals. Likewise, studies that characterize the Sun’s magnetic reversals, which occur about every 11 years, could also shed light on Earth’s longer-term events.
Future progress in understanding Earth’s magnetic reversals will depend upon several types of additional studies, according to the researchers. These include estimates of cosmogenic radionuclide production—which may confirm the existence and characterize the strength of oscillations prior to reversals—as well as advanced technologies like new, highly sensitive magnetometers that are capable of scanning magnetization at submillimeter scales. (Reviews of Geophysics, doi:10.1002/2015RG000506, 2016)
—Terri Cook, Freelance Writer