A fascinating discovery on Easter Island, studied by a group of geologists, indicates that the Earth’s mantle may not operate as previously believed.
Traditionally, geography textbooks describe the Earth’s mantle beneath its tectonic plates as a thoroughly mixed, viscous rock that shifts along with these plates, similar to a conveyor belt. This concept, introduced about a century ago, has proven to be challenging to validate. However, a surprising find on Easter Island, examined by geologists from Cuba, Colombia, Utrecht, and other locations, suggests that the behavior of the Earth’s mantle might be fundamentally different.
Easter Island is composed of several extinct volcanoes, with its oldest lava flows dating back approximately 2.5 million years, situated atop an oceanic plate that is not significantly older than the volcanoes themselves. In 2019, a team of geologists from Cuba and Colombia set out to accurately date the volcanic island. They turned to a well-established method: dating zircon minerals. When magma cools, these minerals form crystals that incorporate a small amount of uranium, which transforms into lead via radioactive decay.
By understanding the rate of this process, the team could determine when these minerals were formed. Led by Cuban geologist Yamirka Rojas-Agramonte from Colombia’s Universidad de Los Andes, the team sought out these minerals. Rojas-Agramonte, now affiliated with the Christian Albrechts-University Kiel, discovered hundreds of zircon minerals. Surprisingly, these were not only from 2.5 million years ago but also from as far back as 165 million years. How is that possible?
The Earth’s Mantle
Chemical analysis of the zircons revealed a consistent composition across the samples, suggesting they all originated from magma similar to that of the current volcanoes. However, the volcanoes could not have been active 165 million years ago, as the tectonic plate beneath is younger than that. The only plausible explanation is that these ancient minerals came from the source of the volcanism in the Earth’s mantle, well before today’s volcanoes formed. This raised another perplexing question for the team.
Hotspot Volcanoes and Their Origins
Volcanoes like those found on Easter Island are classified as ‘hotspot volcanoes,’ a type commonly found in the Pacific Ocean, with Hawaii being a notable example. These volcanoes are formed by large masses of rock ascending from deep within the Earth’s mantle, known as mantle plumes. When these plumes approach the base of the tectonic plates, the surrounding mantle and the plume itself melt, resulting in the formation of volcanoes. It has been known since the 1960s that mantle plumes remain stationary as tectonic plates move over them. Each time the plate shifts slightly, the manta plume generates a new volcano. This explains the linear arrangement of extinct underwater volcanoes across the Pacific, with one or more active ones at the end. Did the team’s findings indicate that the mantle plume under Easter Island has been active for 165 million years?
Subduction Zones
To investigate this, Rojas-Agramonte required geological evidence from the ‘Ring of Fire’—a region marked by numerous earthquakes and volcanic activity, where oceanic plates subduct into the Earth’s mantle. She reached out to Utrecht geologist Douwe van Hinsbergen for assistance. “The challenge lies in the fact that the tectonic plates from 165 million years ago have long since been subducted,” explains Van Hinsbergen, who meticulously reconstructed these lost sections. When he integrated a large volcanic plateau into his reconstruction of the area where Easter Island is located, dating back to 165 million years ago, he found that this plateau likely submerged beneath the Antarctic Peninsula about 110 million years ago. Coincidentally, this aligns with a poorly understood phase of mountain formation and crustal shifts at that time. This mountain range, which is still visible, might have been influenced by the subduction of the volcanic plateau formed 165 million years ago. His reconstruction strongly suggested that the mantle plume beneath Easter Island could indeed have been active for that long. This could potentially solve the geological enigma of Easter Island; the ancient zircon minerals might be remnants of earlier magmas that came to the surface alongside younger magmas during volcanic eruptions.
Inconsistencies
However, this discovery brings forward further complications. The traditional ‘conveyor belt theory’ has already struggled to account for the observation that mantle plumes remain stationary while the surrounding material shifts. Van Hinsbergen notes, “People have theorized that plumes rise rapidly enough to escape the influence of a moving mantle. New materials are continuously supplied beneath the plate, causing volcanoes to form.” But if this were the case, any older parts of the plume and the ancient zircons should have been relocated by mantle currents, meaning they could not possibly remain at Easter Island’s surface. “This leads us to conclude that those ancient minerals could only be preserved if the mantle surrounding the plume is virtually as motionless as the plume itself.” Consequently, the discovery of these ancient minerals on Easter Island implies that the behavior of the Earth’s mantle may be far more complex and slower-moving than previously assumed; a notion that both Rojas-Agramonte and Van Hinsbergen, along with their teams, proposed a few years ago in their studies conducted in the Galapagos Islands and New Guinea, with Easter Island now providing further insights.
