Remnants of lost oceans

A few days ago I ran across some new research out of Utrecht University about the Pontus Plate, which because (1) I was raised Catholic, (2) I have a thing for Roman history, and (3) apparently I need new glasses, I keep misreading as "Pontius Pilate."

The Pontus Plate has nothing to do with New Testament Bad Guys.  It's a remnant of a (very) old tectonic plate that mostly vanished on the order of 120 million years ago, during a time when the Pacific Ocean was a great deal bigger than it is now, and the Atlantic Ocean had just started to form, rifting apart North and South America from Europe and Asia.

To see what's going on here, first a bit of background.

In general, there are three kinds of boundaries you find on the edges of plates.  A real geologist (not just a dilettante layperson like myself) would tell you it's way more complex than this, which is certainly true, but this is at least a broad-brush categorization:

1. Divergent zones, also called rifts, which are where magma is upwelling from the mantle, creating drag that moves plates apart.  Examples are the Mid-Atlantic Rift Zone and the East African Rift Valley, the latter of which was the subject of a post here at Skeptophilia only a few days ago.
2. Convergent zones, where plates move together.  When one or both of the plates is an oceanic plate -- which are thinner and more brittle -- one will dive underneath the other, causing a trench or a thrust fault.  The plate that dives down (subducts) eventually melts, giving rise to volcanoes, such as the ones in Japan, Indonesia, the Caribbean, and the Cascade Range.  When both of the colliding plates are continental plates -- thick, stiff, and cold, kind of like Ron DeSantis -- the two simply pile up against each other until friction slows them down.  This is the process that formed -- and is still forming -- the Himalayas and the Alps.
3. Transform faults, also called strike-slip faults, where two plates slide more-or-less parallel to each other.  An example is the San Andreas Fault in California, amongst many others.

New oceanic plate is constantly being formed at divergent zones and destroyed at convergent zones, so the entire tectonic map of the Earth is always shifting, the pieces breaking up and reassembling a bit like sheets of ice on a flowing river in March.

Sometimes, when the process of destroying a particular plate exceeds the process of forming it, the plate is doomed to disappear eventually.  This is happening right now to the Juan de Fuca Plate, off the northwest coast of North America:

The Juan de Fuca Plate, sandwiched between the much larger Pacific and North American Plates. The blue line is a convergent zone, the red lines are divergent zones, and the green lines are transform faults. [Image licensed under the Creative Commons Alataristarion, JuanDeFucaPlate, CC BY-SA 4.0]

The Juan de Fuca Plate is one of five small chunks that are all that are left of the enormous Farallon Plate, which once extended under much of the eastern half of the Panthalassa Ocean, the enormous mega-ocean covering seventy percent of the Earth's surface when the continents were locked up as Pangaea.  (The other four pieces are the Explorer, Gorda, Nazca, and Cocos Plates.)

The rather roundabout point I'm trying to make here is that the plates don't last forever, and there are some of them that have undoubtedly disappeared entirely.  Which makes what geologist Suzanna van de Lagemaat and her team did pretty astonishing.

Using data on remnants of oceanic rock in  Japan, Borneo, the Philippines, New Guinea, and New Zealand, van de Lagemaat was able to reconstruct one of the huge oceanic plates that was on the opposite side of Panthalassa from the aforementioned Farallon Plate, a now mostly-vanished plate she christened Pontus.  The biggest hints came from the northern region of the island of Borneo and from the highly active plate margin near the Philippines (which is responsible for the earthquakes and volcanic eruptions that strike the island chain with clocklike regularity).

"We... conducted field work on northern Borneo, where we found the most important piece of the puzzle," van de Lagemaat said.  "We thought we were dealing with relicts of a lost plate that we already knew about.  But our magnetic lab research on those rocks indicated that our finds were originally from much farther north, and had to be remnants of a different, previously unknown plate...  The Philippines is located at a complex junction of different plate systems.  The region almost entirely consists of oceanic crust, but some pieces are raised above sea level, and show rocks of very different ages."

The research is pretty impressive.  "Eleven years ago, we thought that the remnants of Pontus might lie in northern Japan, but we’d since refuted that theory," said Douwe van Hinsbergen, Van de Lagemaat’s Ph.D. supervisor, and senior author of the study.  "It was only after Suzanna had systematically reconstructed half of the 'Ring of Fire' mountain belts from Japan, through New Guinea, to New Zealand that the proposed Pontus Plate revealed itself, and it included the rocks we studied on Borneo."

[Image from van de Lagemaat et al., Nature, October 2023]

The whole thing is fascinating.  Geologists studying what are now widely-separated rock formations are able to reconstruct the remnants of a lost oceanic plate from over a hundred million years ago, a time when our ancestors were still small, scurrying furry creatures, and the asteroid collision that would end the dinosaurs' hegemony was still a good sixty million years in the future.  Reconstructing a puzzle of that magnitude is an amazing feat -- making me wonder what pieces of the past still lie undiscovered, waiting for some brilliant researcher like Suzanna van de Lagemaat to reassemble.

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