Earth

Easter Island’s volcanic history suggests that Earth’s mantle behaves very differently than previously assumed

The structure of the mantle plume beneath Easter Island. Credit: Douwe van Hinsbergen

Geography textbooks describe the Earth’s mantle, which lies beneath the plates, as a well-mixed, viscous rock that moves with the plates like a conveyor belt. But this idea was first broached about 100 years ago, and it’s surprisingly difficult to prove. A mysterious discovery on Easter Island, investigated by geologists from Cuba, Colombia and Utrecht, suggests that Earth’s mantle appears to behave very differently.

Easter Island is made up of several extinct volcanoes. The oldest lava deposits formed about 2.5 million years ago on an oceanic plate not much older than the volcano itself. In 2019, a team of Cuban and Colombian geologists set out to Easter Island to determine the exact age of the volcanic island.

To do so, they relied on a tried-and-true recipe: dating zircon minerals. When the magma cools, these minerals crystallize. These contain small amounts of uranium, which is “transformed” into lead through radioactive decay. Their findings are available as preprints on the ESS Open Archive.

Because we know how fast that process occurs, we can measure how long ago those minerals were formed. So a team from Colombia’s Los Andes University, led by Cuban geologist Yamilca Rojas Agramonte, set out to find these minerals. Rojas Agramonte, now at Christian Albrecht University in Kiel, has discovered hundreds of them. But surprisingly, not only are some 2.5 million years old, but others go much further back, up to 165 million years old. How could that be?

Chemical analysis of zircons shows that their composition is more or less the same in all cases. Therefore, they all must have come from magma with the same composition as today’s volcanoes. However, these volcanoes could not have been active for 165 million years. Because the plate below it is not that old. Therefore, the only explanation is that ancient minerals were born in volcanic sources in the Earth’s mantle beneath the plates, long before today’s volcanoes were formed. But it posed an additional challenge for the team.

Hotspot volcanoes and their origins

Volcanoes like Easter Island are so-called “hotspot volcanoes.” These are common in the Pacific. Hawaii is a famous example. They form from so-called mantle plumes, large chunks of rock that slowly rise from the earth’s deep mantle. As they approach the base of the Earth’s plates, rocks in the plume and surrounding mantle rocks melt and form volcanoes.

Scientists have known since the 1960s that mantle plumes remain in place for a very long time while the Earth’s plates move over them. Every time the plates shift slightly, the mantle plume creates a new volcano. This explains why the Pacific Ocean has a series of dead underwater volcanoes, with one or a few active volcanoes at the end. Did the research team find evidence that the mantle plume beneath Easter Island has been active for 165 million years?

Echoes from the past: geological mysteries solved on Easter Island

Easter Island statue. Credit: Douwe van Hinsbergen

subduction zone

To answer that question, Rojas Agramonte needed evidence from the geology of the Ring of Fire. This Ring of Fire is an area around the ocean that is prone to seismic and volcanic activity, where oceanic plates subduct (“subduct”) into the Earth’s mantle. So she contacted Douwe van Hinsbergen, a geologist from Utrecht.

“The challenge is that the 165-million-year-old plates disappeared in those subduction zones a long time ago,” says van Hinsbergen, who has reconstructed the missing pieces in detail. When he added a large volcanic plateau to the 165-million-year-old ruins of present-day Easter Island, he found that the plateau must have disappeared beneath the Antarctic Peninsula about 110 million years ago.

“And it happened to coincide with a little-understood stage of mountain building and tectonic deformation in that very location. That mountain range, the traces of which are still clearly visible, was probably formed 165 million years ago. It could be the effect of subduction of a volcanic plateau “many years ago,” he added.

His reconstruction therefore showed that the Easter Island mantle plume was very likely active for that long. This will solve the geological mystery of Easter Island. Ancient zircon minerals may be the remains of early magma brought to the surface from deep within the Earth along with younger magma in volcanic eruptions.

mismatch

But then another problem arises. The classic “conveyor belt theory” was already difficult to reconcile with the observation that a mantle plume stays in place while everything around it continues to move. “People have explained this by saying that the rate of rise of the plume is so fast that it is unaffected by the mantle, which was moving with the plate, and the material in that plume is always moving along with the plate,” Van Hinsbergen said. , and are forming new volcanoes.”

However, in that case, the old fragments of the plume containing old zircons would have been carried away from the Easter Island location by mantle currents and should not be present at the surface.

“From there, we draw the conclusion that those ancient minerals could have been preserved only if the mantle surrounding the plume was as essentially stationary as the plume itself.” he added.

The discovery of ancient minerals on Easter Island therefore suggests that Earth’s mantle behaves fundamentally differently and moves much more slowly than previously thought. This is a possibility that both Rojas Agramonte and van Hinsbergen and their team raised in their research on the Galapagos Islands and New Guinea several years ago, and now Easter Island offers new clues. Provided.

Further information: Yamirka Rojas-Agramonte et al, Easter Island (Rapa Nui) zircon xenocrystals reveal hotspot activity since the Middle Jurassic, ESS Open Archive (2023). DOI: 10.22541/au.170129661.17646127/v1

Provided by Utrecht University

Quote: Easter Island’s volcanic history suggests Earth’s mantle is behaving much differently than previously assumed (October 16, 2024) Retrieved from https://phys.org/news/2024-10-easter-island-volcanic-history-earth). html

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