2024-10-23 09:45:00
Geography books describe the Earth’s mantle beneath 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, studied by Cuban, Colombian, Spanish and Dutch geologists among others, suggests that the Earth’s mantle may 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 which is transformed into lead by radioactive decay. And because we know how quickly this happens, it is possible to measure how long ago these minerals 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 Christian Albrecht University Kiel in Germany, has found hundreds. 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?
Chemical analysis of zircons has shown that their isotopic composition is more or less the same in all cases. Therefore they must all 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 underlying plate is not that old. The only explanation is that ancient minerals were formed at the origin of volcanism, in the (upper) mantle beneath 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 (so-called mantle plumes), and as they approach the plates, the rocks in the plume and the mantle rocks around them melt and volcanoes form. Since the 1960s it has been known that mantle plumes remain in place for a long time as plates move across them. Every time the plate moves slightly, this mantle plume creates a new volcano. Hence the rows of extinct underwater volcanoes in the Pacific Ocean, with one or a few active 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 of Utrecht University in the Netherlands. “The difficulty is that the 165-million-year-old plates disappeared a long time ago in these subduction zones,” explains Van Hinsbergen, who 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, he found that this plateau must have disappeared beneath the Antarctic Peninsula about 110 million years ago. «An inexplicable phase of mountain construction took place there. “This mountain range, whose vestiges are still clearly visible, could be the heir of a volcanic plateau formed 165 million years ago.” Their reconstruction therefore demonstrated that the Easter Island mantle plume could well have been active at that time. The geological mystery of Easter Island would thus be solved: the ancient zircon minerals are then remains of previous magmas, 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 reconcile with the observation that the mantle feathers remain more or less in place while everything around them would simply continue to move. This has been explained by stating that the feathers rise so quickly that they are unaffected by such a swirling coat. “And that new material was constantly being supplied under the plate to form new volcanoes.” But in this case the old pieces of the plume, with the ancient 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 immobile as the plume itself. The discovery of ancient minerals on Easter Island therefore 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 offers new clues.
The study is titled “Zircon xenocrysts from Easter Island (Rapa Nui) reveal hotspot activity from the Middle Jurassic.” And it was published in the academic journal AGU Advances. (Source: Utrecht University)
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