Extinct volcanoes that lurk around Rapa Nui, aka Easter Island, may challenge some parts of the standard explanation about how tectonic plates move around our planet.
It's often taught that Earth's rocky tectonic plates sit on top of a syrup-like bed of viscous rock, known as the mantle, that moves along with those plates like a conveyor belt. This is loosely the idea that started in 1912 when Alfred Wegener published a paper on his theory of continental drift. It proved highly controversial but eventually became accepted by the overwhelming majority of scientists by the latter half of the 20th century.
However, parts of this theory are still causing confusion and debate among scientists. One point of contention may have recently been unearthed at Rapa Nui, the iconic island 3,600 kilometers (2,237 miles) off the coast of Chile in eastern Polynesia that was formed by volcanic eruptions millions of years ago.
In 2019, a team of Cuban and Colombian geologists traveled to Rapa Nui to accurately date its volcanic origins using zircon. This mineral is particularly useful because it crystallizes as magma cools and can be dated based on the radioactive decay of uranium to lead.
While the team expected to find the island's zircons formed around 2.5 million years ago, when the island's earliest lava deposits formed, they were surprised to discover the minerals dated as far back as 165 million years ago.
Chemical analysis of the zircon crystals revealed that all had broadly the same composition, indicating they came from magma with the same composition as today's volcanoes. However, these volcanoes couldn't have been active 165 million years ago - the plate below them is not even that old.
The most likely explanation, the team says, would be that the ancient minerals come from the source of volcanism in the Earth's mantle beneath the plate, long before the volcanoes were formed.
However, this explanation also raises problems. Rapa Nui is thought to be the product of a mantle plume, a giant column of hot molten rock that allows material to move from deep within Earth's mantle to the surface. It's believed that mantle plumes remain in the same place as Earth's plates move over them, producing new volcanoes each time the plate shifts (as you can see in the diagram above).
For further insights, the team reached out to geologist Douwe van Hinsbergen from the University of Utrecht in the Netherlands, who added a large volcanic plateau to tectonic simulations of the region.
The findings imply that the Rapa Nui mantle plume could have potentially been there some 165 million years ago. The ancient zircon, therefore, could be remnants of earlier magmas that were "recycled" and brought to the surface from deep inside the earth, along with younger magmas in volcanic eruptions.
More questions remain, though. The researchers note that the traditional "conveyor belt theory" is hard to fit with the fact that mantle plumes remain stationary while everything else around them moves.
Likewise, with this new observation, it doesn't make sense that the ancient zircon wasn't carried off by currents in the mantle currents and moved far away from the location of Rapa Nui. One explanation could be that Earth's mantle moves significantly slower than the plates that move over it.
The team's previous research on the Galapagos Islands and New Guinea has also hinted that Earth's mantle may be slower than expected, but further work is needed before it can be considered gospel.
The study, which is yet to be peer-reviewed, is published on the pre-print server ESS Open Archive.