We found the hidden force that created two chains of underwater volcanoes running side by side

Lord Howe Island, located about 600 kilometres off the coast of New South Wales, is a unique crescent of rainforest and sea cliffs that attracts visitors with its picturesque beaches and distinctive birdlife. However, few realize that they are standing on the weathered remains of a volcano. In fact, Lord Howe is part of a chain of underwater volcanoes, known as "seamounts", with its counterpart, the Tasmantid seamounts, situated roughly 650 kilometres west.

The intriguing phenomenon of two nearly identical chains of volcanoes running side by side in the middle of a tectonic plate puzzled researchers until a new study published in the journal Gondwana Research provided an explanation.

The traditional understanding of volcanic formation revolves around the meeting of tectonic plates. However, Hawaii is an exception, formed by a plume of hot rock rising from deep within the Earth and melting its way through the Pacific plate. As the tectonic plate moves over the stationary plume, a chain of seamounts emerges. The Lord Howe and Tasmantid seamount chains share this characteristic, suggesting a single mantle plume is responsible for their formation.

The study found that the plume gets deflected by a slab of old seafloor that has stalled about 500 kilometres deep, where the mantle becomes rigid. Unable to pass through the dense slab, the plume bends around it and escapes through gaps on either side, giving birth to the two chains.

The researchers backed their findings with three lines of evidence. Firstly, numerical simulations of Earth's interior revealed that the plume splitting observed in the models closely matched the spacing between the Tasmantid and Lord Howe seamount chains. Secondly, reconstructing the movement of tectonic plates over the past 200 million years exposed a ribbon of slab material with gaps on either side, aligning perfectly with the model predictions.

Finally, the chemistry of lavas from each chain exhibited a chemical fingerprint that originated from the deep mantle, confirming that the eruptions were sourced from the mantle rather than the nearby subducted slab.

One of the most intriguing implications of this discovery is the prediction of the future fate of the Tasmantid and Lord Howe seamount chains. As the slab obstructing the plume sinks deeper into the Earth's mantle, one branch will become dominant while the other will cease. The Lord Howe chain has been producing smaller eruptions for the past 23 million years, while the Tasmantid volcanoes have been generating larger eruptions over the same period.

Consequently, the study predicts that the Tasmantid chain will become the main conduit, while Lord Howe will eventually terminate many millions of years from now.

The findings also raise questions about the traditional understanding of plumes in the deep Earth. While plumes are typically depicted as straight conduits from the deep mantle to the surface, this study demonstrates that they can evolve dynamically as they encounter obstacles within the mantle. This revelation may necessitate revisions in our reconstructions of absolute plate motion, potentially altering our understanding of the Earth's past movements.

Furthermore, the study suggests that similar mechanisms may be responsible for other closely spaced volcanic chains, including Yellowstone in the United States. As seismic images beneath the Tasman Sea remain unclear, further research is needed to fully comprehend the branching structure of these volcanic hotspots. Additionally, obtaining more rock samples from these underwater volcanoes could provide valuable insights into the geochemistry of the eruptions and the frequency of recent eruptions in the southern Tasman Sea.