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Towards the end of the dinosaurs' reign the Earth tilted before righting itself, new evidence suggests. The movement took millions of years, so it wasn't like giant sauropods were suddenly sent sliding. However, the findings in Italy's Apennine mountains could settle a long-running geological debate.
In theory, the liquid nature of the Earth's outer core provides an opportunity for the mantle and crust to slide around it, potentially seeing them slip large distances compared to the poles. However, it is much harder to determine when and if this has happened, in contrast to the migrations of the magnetic poles, which we know occur even if we don't really understand why.
Professor Joe Kirshvink of Caltech examined magnetic deposits in search of evidence for the most recently proposed example of such a tip. In Nature Communications, Kirshvink and co-authors not only claim to have found clear signs the Earth moved, but to be able to measure the shift at 12 degrees over around 4 million years, followed by a reversal.
The adjustment Kirshvink is studying is called true polar wander, to distinguish it from movement of the magnetic poles alone. "Imagine looking at Earth from space," Kirschvink explained in a statement. "True polar wander would look like the Earth tipping on its side, and what's actually happening is that the whole rocky shell of the planet—the solid mantle and crust—is rotating around the liquid outer core."
We know that true polar wander exists on a small scale – we can measure it with satellites, but whether movements of many degrees have taken place is still debated, with scientific papers both for and against a late Cretaceous tip.
The Apennine mountains are a particularly good place to test the idea because Cretaceous-era limestone deposits known as Scaglia Rossa are veined with iron-rich bacteria that formed magnetite deposits. As the name suggests, magnetite is magnetic and preserves the magnetic field at the time at which it formed.
Just as magnetic basalt around mid-ocean ridges record the flips in orientation of the Earth's magnetic poles, leading to the discovery of tectonic plates and enabling the dating of important fossils, these magnetite veins record orientation relative to the poles.
Between 87 and 78 million years ago these mountains swung 12 degrees, before returning to almost the exact same place. Movement peaked 84-82 million years ago. Although the magnetic measurements can only compare movements compared to the magnetic poles, the paper's authors maintain this was true polar wander, with the whole crust tilting compared to the Earth's axis of spin.
The paper's findings are significant because previously the Scaglia Rossas limestones have been used as key evidence against a major wandering event in the Cretaceous. The authors argue previous researchers relied on just three average inclinations and may have also used less reliable techniques. The paper is based on 1,090 samples from two parallel stratigraphic sections adjusted for the movements of nearby tectonic plates.
Planets experience true polar wander because their moment of inertia is minimized when mass concentrations are near the equator. If clusters of dense material, such as Hawaii's mountains, are at high latitudes it can cause the whole body to tip to regain stability. The forces are easier to imagine if one thinks of how an ice skater spins fastest, and therefore is most balanced, when their hands are close to their body. What caused the reversal Kirschvink and co-authors report is less clear.
