In the early 1980s, researchers collected cores from igneous basalt in an extinct submarine volcano in the Hawaiian-Emperor seamount chain. The samples indicated that over the preceding 60 million years, the Hawaiian hot spot had migrated approximately 7° southward. The investigation—along with other paleomagnetic studies—used the cores to analyze the orientations of iron-bearing minerals as they cooled below the Curie temperature. The resulting data offered a snapshot of the orientation of Earth’s magnetic field at the time of cooling and were thus used to infer the paleolatitude.
Recently, however, the accuracy of the so-called drill hole paleomagnetism research has been called into question. Cores from sedimentary rocks tend to be unreliable because of inherent compaction, or induced shallowing of the recorded magnetization. Likewise, igneous cores can yield highly uncertain values because the rocks do not record enough time to accurately represent magnetism averaged over time; researchers rely on this 100,000-year average because magnetic north is expected to be the same as true north.
In a new study, Zheng et al. outline an improved paleomagnetic methodology to avoid these pitfalls and offer evidence for true polar wander that occurred during the late Cenozoic.
The researchers used observations made from airplane and shipboard equipment to evaluate asymmetry, or skewness, of marine magnetic anomalies resulting from seafloor spreading. The authors argue that when the ship or airplane crosses magnetic anomalies, which occur in suspected low paleolatitudes between the Murray and Marquesas fracture zones on the Pacific plate, these crossings offer the most information for constraining the location of the plate’s paleomagnetic poles.
The results yielded updated locations for the ancient poles, suggesting that Pacific hot spots have not been fixed relative to Earth’s spin axis for the past 32 to 44 million years at least. The researchers argue that this hot spot motion is best explained as a recent episode of true polar wander; an approximately 4° shift of the paleomagnetic poles likely began 4 to 11 million years ago.
Previous research provides multiple explanations for the migration of Earth’s spin axis. One interpretation suggests that it stems from a change in the global geometry of subducting plate tectonic slabs of lithosphere. Alternatively, the shift may be related to the detachment of a piece of the Pacific plate in the southwest Pacific basin. The cause of true polar wander may not even have anything to do with observable tectonics; rather, slow and subtle flow in Earth’s lower mantle may be the culprit.
With these new findings, the authors join the long-simmering debate over polar wander and the paleomagnetic poles of the Pacific plate. This new study offers exciting alternative techniques for paleomagnetism that are sure to generate discussion. (Tectonics, https://doi.org/10.1029/2017TC004897, 2018)
—Aaron Sidder, Freelance Writer