The Pacific plate, with lines of equal age marked. Boschman and van Hinsebergen/Science Advances

The age of oceanic crust is measured from its magnetism, which provides a record of the alignment of the magnetic poles at the time the crust was formed. The triangle is thought to indicate that the Pacific Plate was once surrounded by three pre-existing plates, now almost entirely consumed.

Reconstructing the locations of these former plates shows “the Pacific Plate originated virtually at a point,” the paper argues. This is remarkable because every other plate we know of, ocean plates included, formed when breaks appeared in pre-existing plates.

Previous researchers exploring the Pacific Plate's formation imagined it started as the meeting of three ridges, known as an RRR triple junction. Puzzingly, however, existing RRR triple junctions have been stable for millions of years, so it is not clear how one could grow to be the giant plate we now see.

Boschman and van Hinsbergen propose instead that the giant plate's origins lie in the meeting point of three transform faults, similar to California's famous San Andreas Fault. If one of these faults had a kink in it, with a subduction zone, where one plate slides under another, the meeting point would migrate towards the kink.

When the triple junction met the kink, a triangle of crust formed, which grew as the fault lines released magma that became new crust. The authors used magnetic indications of crustal age to locate the triangle's origins. They place it in the eastern Pacific, south of modern-day Hawaii, from where it migrated erratically westward.

content-1469629747-boschman1hr.jpgThe three-step process by which the plates that once surrounded the Pacific Plate produced a triangle that grew. Boschman and van Hinsebergen/Science Advances

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