They clarify a geological mystery of Easter Island

Geography textbooks describe the Earth’s mantle located under the tectonic plates as a viscous, well-mixed rock that moves along the plates like a conveyor belt. This idea, first put forward in the 1910s, has proven surprisingly difficult to prove. A discovery on Easter Island, investigated by Cuban, Colombian, Spanish and Dutch geologists, among others, suggests that the Earth’s mantle could behave differently.

Easter Island is made up of several extinct volcanoes. The oldest lavas formed about 2.5 million years ago on an oceanic plate not much older than the volcanoes themselves. In 2019, a team of Cuban and Colombian geologists set out for Easter Island to precisely date the island. To do this, they turned to a proven recipe: dating zircon minerals. When the magma cools, these minerals crystallize. They contain a small amount of uranium that is transformed into lead by radioactive decay. And since it is known how quickly it happens, it is possible to measure how long ago these minerals were formed. For this reason, a team from the University of Los Andes, in Colombia, led by Cuban geologist Yamirka Rojas-Agramonte, went in search of these minerals. Rojas-Agramonte, now affiliated with the Christian Albrecht University of Kiel in Germany, found hundreds of them. But, surprisingly, not only from 2.5 million years ago (the age of the islands), but also from much further back in time, up to 165 million years. How was this possible?

A chemical analysis of the zircons showed that their isotopic composition is more or less the same in all cases. Therefore, all of them must have come from magma with the same composition as that of current volcanoes. However, these volcanoes cannot have been active for 165 million years, because the plate beneath them is not that old. The only explanation is that ancient minerals were formed at the origin of volcanism, in the (upper) mantle under the plate, long before the formation of today’s volcanoes. But this presented the team with another conundrum.

Easter Island is famous for its iconic statues, the moai. (Photo: Douwe van Hinsbergen)

Volcanoes like those on Easter Island are so-called “hot spot volcanoes.” They are common in the Pacific Ocean; Let’s think about Hawaii. They form from large bubbles of rock that slowly rise from the depths of the mantle (the so-called mantle plumes) and when they approach the plates, the rocks in the plume, and the mantle rocks around them, melt. and volcanoes are formed. Since the 1960s, it has been known that mantle plumes stay in place for a long time as plates move over them. Every time the plate moves a little, such a mantle plume creates a new volcano. Hence the rows of extinct underwater volcanoes in the Pacific Ocean, with one or a few active ones at the end. Had the team found evidence that the mantle plume beneath Easter Island had been active for 165 million years?

To answer this question, Rojas-Agramonte had to look for evidence in the geology of the “Ring of Fire,” an area around the Pacific Ocean with many earthquakes and volcanism, where oceanic plates subduct (subduct) into the Earth’s mantle. . Therefore, he contacted geologist Douwe van Hinsbergen from the University of Utrecht in the Netherlands. “The difficulty is that the plates from 165 million years ago disappeared long ago in these subduction zones,” explains Van Hinsbergen, who has reconstructed the missing pieces in detail. When he added to those reconstructions a large volcanic plateau at the location 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 ago. «An inexplicable phase of mountain building took place there. “This mountain range, whose vestiges are still clearly visible, could well be the heir to a volcanic plateau that was formed 165 million years ago.” Their reconstruction therefore demonstrated that the Easter Island mantle plume could very well have been active during that time. The geological mystery of Easter Island would thus be solved: the ancient zircon minerals are then remains of previous magmas that were brought to the surface from the depths of the Earth together with younger magmas during volcanic eruptions.

But another problem immediately arose. The classic “conveyor belt theory” was already difficult to square with the observation that mantle feathers remain more or less in place while everything around them would simply continue to move. This was explained by stating that the feathers rise so quickly that they are not affected by such a swirling mantle. “And that new plume material was constantly being supplied under the plate to form new volcanoes.” But in that case the old pieces of the plume, with the old zircons, must have been carried by mantle currents away from the location of Easter Island, and cannot now lie there on the surface. From this, specialists concluded that these ancient minerals can only have been preserved if the mantle surrounding the plume is as still as the plume itself. The discovery of ancient minerals on Easter Island thus indicates that the Earth’s mantle behaves fundamentally differently than has always been assumed, a possibility that both Rojas-Agramonte and Van Hinsbergen and their team suggested a few years ago. in studies on the Galapagos Islands and New Guinea, and for which Easter Island now provides new clues.

The study is titled “Zircon xenocrysts from Easter Island (Rapa Nui) reveal hotspot activity since the middle Jurassic.” And it has been published in the academic journal AGU Advances. (Source: Utrecht University)