Rice University study finds evidence of ancient reorientation of Earth’s solid surface

Rice University highlighted new evidence on October 7 that Earth’s solid surface may have reoriented relative to its spin axis between 56 million and 48 million years ago. The conclusion comes from a study of magnetic records in the Pacific Ocean and matters because it changes how researchers interpret the motion that shaped the Hawaiian–Emperor chain of islands and underwater volcanoes.

The researchers identify an approximately 6°–7° episode of true polar wander as the best explanation for an apparent southward shift in their reconstruction. That is an inference about the distant past, rather than a directly observed movement. Their study also allows for some motion of Pacific hot spots through the mantle.

What the Pacific magnetic record shows

The research, by Daniel Woodworth, Richard G. Gordon and Kevin M. Gaastra, was first published on June 15 in the Journal of Geophysical Research: Solid Earth. It adds a Pacific plate paleomagnetic pole for a period known as chron C24r, about 56 million years ago, inferred from the skewness of marine magnetic anomalies.

As molten rock cools, magnetic information can be preserved in it. Researchers use that record to reconstruct where a plate lay relative to Earth’s magnetic pole. In the Rice University account, Gordon and Woodworth explain that comparing this record with other evidence for plate motion produced an apparent disagreement about which way the Pacific plate had moved.

In the researchers’ reconstruction, the Pacific plate appears to have shifted south relative to the spin axis between about 56 million and 48 million years ago. The paper’s plain-language summary describes the inferred displacement as about 800 kilometres relative to Earth’s solid layers. That figure describes a geological reconstruction; it is not a measurement of a present-day shift.

Why the researchers favour true polar wander

The study considers two broad explanations for the apparent southward displacement. One involves rapid southward movement of the Pacific plate and hot spots relative to the mantle. The other is true polar wander: reorientation of the solid Earth relative to its spin axis. The authors say reconstructed plate geometry and relative plate motions do not fit the ancient forces driving plates as an explanation for the southward motion seen relative to the spin axis.

That comparison leads them to favour a roughly 6°–7° reorientation between two intervals of what they describe as true polar stillstand. It does not require the Hawaiian hot spot to have remained perfectly fixed. Woodworth said the team detected small movements of that hot spot, while reconsidering the assumption that the spin axis had stayed stationary relative to the solid Earth and its hot spots.

The interpretation also bears on the bend in the Hawaiian–Emperor chain, which formed around 47 million years ago. According to Rice University’s account, the results strongly favour a change in the Pacific plate’s movement over movement of the Hawaiian hot spot as the cause of that bend. The chain’s shape alone had left room for debate about which had moved.

What the finding does and does not establish

Gordon said the results indicate that substantial true polar wander could have occurred as recently as 48 million to 56 million years ago. The wording matters: the event is inferred from magnetic and plate-reconstruction evidence, and the researchers still allow that movement between hot spots and southward motion of Pacific hot spots through the mantle may have contributed.

The study places the proposed reorientation near the end of the Early Eocene Climate Optimum and the onset of Cenozoic cooling. That timing does not establish that reorientation caused the climate transition. The paper’s abstract and plain-language summary available for this account also do not give a quantified uncertainty range for the 6°–7° estimate.

The authors state that the magnetic profiles they analysed are available through the National Oceanic and Atmospheric Administration’s marine geophysical track-line database. Those underlying records and the competing motion explanations leave the proposed ancient reorientation open to further testing.