The volcanic history of Easter Island suggests that the Earth’s mantle behaves very differently than previously thought.

Geography textbooks describe the Earth’s mantle beneath its plates as a viscous, well-mixed rock that moves with these plates like a conveyor belt. But this idea, first formulated about 100 years ago, is surprisingly difficult to prove. A mysterious discovery on Easter Island, studied notably by Cuban, Colombian and Utrecht geologists, suggests that the Earth’s mantle seems to behave quite differently.

Easter Island consists of several extinct volcanoes. The oldest lava deposits formed about 2.5 million years ago on top of an oceanic plate not much older than the volcanoes themselves. In 2019, a team of Cuban and Colombian geologists went to Easter Island to precisely date the volcanic island.

To do this, they used a proven recipe: dating zircon minerals. When the magma cools, these minerals crystallize. They contain some uranium, which “transforms” into lead by radioactive decay. Their findings are available in preprint form in ESS Open Archives.

Because we know how quickly this process occurs, we can measure how long ago these minerals formed. The team from the University of Los Andes in Colombia, led by Cuban geologist Yamirka Rojas-Agramonte, therefore set out in search of these minerals. Rojas-Agramonte, now at Christian Albrechts University in Kiel, has found hundreds. But surprisingly, not only from 2.5 million years ago, but also from much further back in time, to 165 million years ago. How is this possible?

Chemical analysis of the zircons showed that their composition was essentially the same in all cases. They must therefore all come from a magma of the same composition as that of current volcanoes. Yet these volcanoes cannot have been active for 165 million years, because the plate beneath them isn’t even that old. The only explanation then is that the ancient minerals came from the source of volcanism, in the Earth’s mantle beneath the plate, well before the formation of current volcanoes. But this presented the team with another conundrum.

Hotspot volcanoes and their origins

Volcanoes like those on Easter Island are what are called “hot volcanoes”. These are common in the Pacific Ocean; Hawaii is a famous example. They form from large blobs of rock that slowly rise from deep in the Earth’s mantle, called mantle plumes. When they approach the base of the Earth’s plates, the rocks of the plume as well as those of the surrounding mantle melt and form volcanoes.

Scientists have known since the 1960s that mantle plumes stay in place for a very long time while the Earth’s plates move above them. Every time the plate moves a little, the mantle plume produces a new volcano. This explains the rows of extinct underwater volcanoes in the Pacific Ocean, with one or a few active volcanoes at the end. Had the team found evidence that the mantle plume beneath Easter Island had been active for 165 million years?

Subduction zones

To answer this question, Rojas-Agramonte needed geological evidence from the “Ring of Fire”, an area around the ocean with many earthquakes and volcanism, where ocean plates plunge (“subduct”) into the terrestrial mantle. So she contacted Utrecht geologist Douwe van Hinsbergen.

“The difficulty is that the plates from 165 million years ago have long since disappeared in these subduction zones,” explains Van Hinsbergen, who had reconstructed the missing pieces in detail. When he added a large volcanic plateau to these reconstructions at the site of present-day Easter Island 165 million years ago, it turned out that this plateau must have disappeared beneath the Antarctic Peninsula about 110 million years old.

“And this precisely coincides with a poorly understood phase of mountain formation and crustal deformation at this precise location. This mountain range, the traces of which are still clearly visible, could well be the effect of the subduction of a volcanic plateau which formed 165 million inhabitants years ago,” he adds.

Its reconstruction therefore showed that the Easter Island mantle plume could very well have been active for that long. This would solve the geological mystery of Easter Island: the ancient zircon minerals are believed to be remnants of earlier magmas brought to the surface from deep within the earth, as well as younger magmas during volcanic eruptions.

Inconsistencies

But then another problem presents itself. The classic “conveyor belt theory” was already difficult to reconcile with the observation that mantle plumes stay in place while everything around them continues to move. Van Hinsbergen explains: “People have explained this by saying that the plumes rise so quickly that they are not affected by a mantle that moves with the plates. And that new plume material is constantly being supplied beneath the plate to form new volcanoes.”

But in this case, ancient fragments of the plume, along with the ancient zircons, would have had to be carried away by these mantle currents, away from the location of Easter Island, and could no longer be found on the surface now.

“From this we draw the conclusion that these ancient minerals could only have been preserved if the mantle surrounding the plume was fundamentally as stationary as the plume itself,” he adds.

The discovery of ancient minerals on Easter Island therefore suggests that the Earth’s mantle behaves fundamentally differently and moves much more slowly than has always been assumed; a possibility that Rojas-Agramonte and Van Hinsbergen and their teams raised a few years ago in studies of the Galapagos Islands and New Guinea, and for which Easter Island now provides new clues.