The linear chains of islands running across the Pacific Ocean aren’t improbable coincidences of orderliness—they’re the product of hot towers of mantle rock punching volcanic holes through a tectonic plate sliding overhead. But if you follow the Hawaiian chain back to where the older seamounts no longer rise above the waves, you find a sharp dogleg, as you can see above.

We haven't had a satisfactory explanation for this sudden turn. One idea was that, given a stationary mantle hotspot, the tectonic plate must have changed direction at one point in time. This theory has never been entirely satisfactory, however—not least because the Louisville seamount chain in the South Pacific sports a gentler kink.

We still have a lot to figure out about how mantle hotspot plumes work, but we do know that the Hawaii and Louisville plumes go all the way down to the deepest part of the Earth’s mantle. Plumes like these are rooted near the edges of unusual, lumpy regions of rock at the base of the mantle beneath the Pacific (as well as Africa). These structures are known as large low-shear-velocity provinces—for lack of any reasonable alternative, we’ll grit our teeth and refer to them as LLSVPs.

Here’s what we know about LLSVPs: At that depth in the mantle, near the liquid-hot outer core of the Earth, the rock suddenly becomes a little denser. Because the proximity to the outer core also raises the temperature, this denser rock is also a little squishier in the ultra-high-pressure world of the mantle, where solid rock can deform like putty.

A group of researchers led by the University of Sydney’s Rakib Hassan and Dietmar Müller turned a model of the Earth’s interior toward the question of the kink in the Hawaiian chain. Instead of focusing on the plate above, they wanted to see what the base of the plume (and that weird LLSVP) might have done.

Starting with a uniform layer of denser rock at the base of the mantle, the researchers let their model run for a while, simulating the slow convection of the mantle and the sinking of subducting oceanic plates all around the Pacific Ring of Fire. As the slow-to-heat-up oceanic crust sinks toward the bottom of the mantle, it actually squishes that dense bottom layer around a bit. Like toothpaste squeezed into one end of the tube, this bottom layer bulges up beneath the Pacific plate, building a region that matches the real LLSVP pretty well.

The edges of this squished-up LLSVP form ridges, and the model produces some hotspot plumes rising from the ridges—a bit like the way bubbles in a carbonated beverage can nucleate and rise from specific points at the bottom of a glass. This is where we find these deep plumes in the real mantle. The simulated plumes included a pair in just about the right spots to build the Hawaiian and Louisville chains.

Running the simulation with what we know about the last couple hundred million years of tectonic plate motion, the Hawaiian hotspot plume doesn’t stay in the same spot. As subducting plates and mantle flow push the LLSVP around, the base of the plume migrates. That motion also tilts the plume from vertical into a slight angle, like chimney smoke caught in a gentle breeze.

Between 100 million and 50 million years ago, the Pacific Ring of Fire shrunk as continents continued spreading out from the Pangaea supercontinent, encroaching on the Pacific Ocean. In the model, this inward migration of the subduction zones around the northern half of the Pacific squeezes the LLSVP toothpaste toward the center. The edge of the LLSVP where the Hawaiian hotspot plume was anchored moved southward several centimeters per year. Around 50 million years ago, the continents slowed their invasion of the Pacific’s territory, so the edge of the LLSVP slowed its migration as well.

The base of the hotspot plume only moves south by about 6° latitude over this time period, yet the top of the plume moves 10°. That’s because the top finally catches up with the base as it slows down, straightening up a plume that was initially more tilted. This rapid straightening is key to the shape of the Hawaiian chain. After the top of the plume catches up with the base, it suddenly stops migrating so quickly, accounting for the sharp bend in the chain.

But what about the Louisville seamount chain in the South Pacific? The subduction zones around the South Pacific were closing in more slowly, and that edge of the LLSVP didn’t move north very much or very abruptly. The end result is a chain of seamounts with a much more subtle bend.

There are other details that could be added to the model to see if they make a significant difference, but this a pretty appealing hypothesis. Although tectonic plates move much faster than hotspots can, you can’t assume that hotspots are fixed in place and passively marking the absolute motion of the plate above. In this case, accounting for the forces that can push some hotspots around appears to provide a solution to the kinked-Pacific-seamount-chain puzzle.

Nature, 2016. DOI: 10.1038/nature17422 (About DOIs).