(credit: Google Earth)
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.
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Why the Hawaiian seamount chain took a sudden turn
