Zircons discovered in the sands of Easter Island indicate a volcanic history dating back 165 million years. Situated in the heart of the Pacific Ocean, far from any continent, Easter Island emerges from the ocean, revealing a narrative of volcanic activity that commenced around 2.5 million years ago. However, recent revelations suggest a much deeper and older geological story.
Geologists studying zircon crystals, sturdy minerals that form within volcanic rocks, have uncovered astonishing evidence that the volcanic processes beneath Easter Island may have origins stretching back 165 million years. This discovery has the potential to reshape our understanding of the Earth’s deep interior.
When lava cools, minerals like zircon get trapped within it. These crystals contain uranium, which gradually transforms into lead through radioactive decay over time. By analyzing the uranium-lead ratio in zircon, scientists can pinpoint the exact time the crystal formed, acting as a time capsule to reveal the circumstances of its solidification.
In 2019, a team led by geologist Yamirka Rojas-Agramonte traveled to Easter Island to gather samples of sand and red soil, hoping to find zircons to date the volcanic eruptions that shaped the island. While the volcanic rocks they examined were expected to be approximately 2.5 million years old, given that they overlay seafloor no older than 4.8 million years, the discovery was much more enigmatic.
Hundreds of zircon grains displayed ages dating back as far as 165 million years. These crystals could not have formed during the recent volcanic activity on the island, yet their composition closely resembled the modern lavas of Easter Island. This disparity begged the question of how they ended up there.
To unravel this mystery, one must delve deep below the Earth’s surface into the mantle, a thick layer of hot rock located between the crust and the core. Certain volcanoes, like those on Easter Island and Hawai’i, originate not at plate boundaries but within ocean plates, fueled by rising columns of hot rock known as mantle plumes.
Mantle plumes can remain stationary for millions of years while tectonic plates drift above them. As the plates move, the plume continues to generate volcanoes, forming chains of islands. This mechanism explains the extensive strings of volcanic islands found in regions such as the Pacific.
Although Easter Island’s mantle plume has been active for only the past few million years, the presence of zircon grains dating back 165 million years poses a conundrum. Rojas-Agramonte and her team propose that ancient magma formed deep within the mantle in the distant past. As new volcanic activity occurred, these aged zircon grains were carried to the surface by the younger magmas.
This hypothesis prompts further inquiry: could these ancient grains be remnants of a previous phase of volcanic activity, perhaps originating from a vanished volcanic plateau?
To explore this theory, Rojas-Agramonte consulted geologist Douwe van Hinsbergen and embarked on a journey of
Research conducted by van Hinsbergen at Utrecht University focuses on the movement and changes of tectonic plates spanning hundreds of millions of years. By utilizing plate reconstruction models, van Hinsbergen has been able to rewind Earth’s historical movements and gain insight into the geological activity beneath Easter Island 165 million years ago. His findings suggest that during this period, a significant volcanic plateau may have emerged on the Phoenix Plate at that same location. Approximately 110 million years ago, this plateau would have been subducted under the Antarctic Peninsula, aligning with a mountain-building phase known as the Palmer Land deformation event in the region.
The discovery of ancient zircons on Easter Island presents a potential resolution to two geological enigmas. Firstly, it offers new evidence supporting the existence of an ancient volcanic plateau. Secondly, it establishes a connection between the disappearance of this plateau and the mountain-building processes in Antarctica, shedding light on a previously perplexing geological event.
Unearthing ancient zircons within young volcanic islands is not a novel concept. Researchers have identified similar ancient zircon populations in unexpected locations such as mid-ocean ridges and isolated hotspot islands like Hawai’i and the Galápagos over the past few decades. These zircons, often referred to as “xenocrysts,” are distinct in that they were not formed alongside the host magma but instead originated from deeper layers within the Earth.
This research challenges the conventional view that Earth’s mantle moves akin to a conveyor belt in conjunction with the plates above it. The revelation that ancient zircons remain preserved beneath Easter Island suggests that the surrounding mantle may have exhibited minimal movement, deviating from the assumption that it moves in tandem with the plates. This paradigm shift underscores the complexity of deep-Earth processes and the intricate interplay between mantle dynamics and plate tectonics.
In certain locations, it appears that the stability of the mantle may be greater than previously believed. This discovery aligns with a hypothesis suggesting that mantle plumes and the surrounding sub-lithospheric mantle, where these crystals resided for tens of millions of years, can remain undisturbed for extended periods. Similar observations have been made in Galápagos and New Guinea, where ancient zircons contradict conventional mantle flow models.
A geological map of Easter Island (Rapa Nui) has been developed by integrating data from previous studies by Vezzoli and Acocella (2009) and Gioncada et al. (2010), showcasing the sampling sites of this current research project (IP). The map separates information on the island’s three primary polygenetic volcanoes (Poike, Rano Kau, and Terevaka) from data on smaller monogenetic vents.
This zircon-dating project initially conducted on a secluded Pacific island has evolved into a narrative with global significance. The grains extracted from Easter Island’s beaches and soils are not merely remnants of recent volcanic activity; they are remnants from an era when dinosaurs roamed the planet and continents had different configurations.
These discoveries challenge conventional notions of the Earth’s interior dynamics, suggesting that the mantle may have enduring, non-uniform regions that retain evidence of ancient volcanic events. Zircon minerals, previously viewed solely as age indicators, now serve as valuable tools for deciphering Earth’s deep volcanic history, connecting surface geology to vanished tectonic plates, and reshaping our understanding of mantle processes. Easter Island, renowned for its enigmatic statues, now harbors a new legacy concealed beneath its sandy terrain.
The research findings discussed in this article are accessible online via the ESS Open Archive.
