The oceanic plate that underlies most of the Pacific Ocean is the world's largest, yet its formation remains something of a mystery. A new theory attributes its birth to a kink in a plate boundary, from which came a plate that reshaped the whole world.
In the early Jurassic, all the continents were joined together into the mighty amalgamation known as Pangea. Three-quarters of the world was taken up by the vast ocean Panthalassa. The processes that caused the break-up of Pangea have attracted much attention from geologists, but what was happening in Panthalassa, the ocean that surrounded Pangea?
In Science Advances, PhD student Lydian Boschman and Dr Douwe van Hinsbergen, both of Utrecht University, present a theory that the meeting of three plate boundaries beneath this ocean spawned the Pacific Plate.
The oldest continental crust in the world dates back 4.4 billion years, almost to the formation of Earth. The floor of the deep sea is a different matter, with little older than about 200 million years, as older crust is pushed beneath other plates and new material is made at mid-ocean ridges. Consequently, Boschman and van Hinsbergen describe models of the behavior of ancient oceanic plates as “conceptual and inevitably speculative.”
Nevertheless, the pair argue, modeling Panthalassa's plates is important if we are to understand how Earth’s mantle responds to the unusual situation of having all the continents combined. To do this, they started with the oldest marine material we have, an area known as the Pacific Triangle, just east of the Marianas Trench.
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.
The three-step process by which the plates that once surrounded the Pacific Plate produced a triangle that grew. Boschman and van Hinsebergen/Science Advances
