Scientists Solve Mystery of Twin Underwater Volcano Chains in Tasman Sea

Lord Howe Island, a crescent of rainforest and sea cliffs 600 kilometres off the New South Wales coast, draws visitors for its beaches and rare birds. Most who walk its shores don't realize they're standing on the weathered remains of a volcano. That volcano, it turns out, has a twin.

Beneath the Tasman Sea lie two chains of underwater mountains—seamounts—running nearly parallel to each other, separated by roughly 650 kilometres. Lord Howe is the only peak that breaks the surface; the rest of its chain stays submerged. A second chain, the Tasmantid seamounts, traces an almost identical path to the west. For decades, this pairing puzzled geologists. Two nearly identical volcanic chains side by side in the middle of a tectonic plate shouldn't exist. A new study, published in Gondwana Research, finally explains how they formed.

The answer lies in a single plume of hot rock rising from Earth's deep mantle. Normally, such plumes behave like blowtorches, burning through the moving tectonic plate above them as it drifts overhead, creating a single chain of volcanoes—Hawaii is the classic example. But in the Tasman Sea, something unusual happens. The plume encounters a slab of old seafloor that sank at a subduction zone, where oceanic crust is recycled into Earth's interior. This dense slab stalls about 500 kilometres down, where the mantle becomes abruptly stiffer. The plume cannot push through. Instead, it bends around the obstacle, splitting into two branches that escape through gaps on either side. Two chains of volcanoes form from one source.

Three independent lines of evidence confirm this mechanism. Computer models of Earth's interior, set up without any instruction to reproduce the Tasman Sea, spontaneously generated plume splitting that matched the observed 650 to 900 kilometre spacing between the chains—and sustained it for roughly 70 million years. Plate reconstructions mapping the last 200 million years of motion revealed a ribbon of subducted slab material with gaps on either side, exactly where the model predicted each branch should rise. Chemical analysis of lava samples from both chains provided the third proof: lead isotopes in the rock carry a fingerprint that takes more than a billion years to develop, far too ancient to have come from the nearby slab, which has only been subducting for 60 million years. The material erupting at the surface must have been delivered from the deep mantle to both chains.

The simulation also reveals the future. As the slab sinks deeper into Earth's mantle over millions of years, one branch will become uncovered and dominant while the other shuts down. The Lord Howe chain has already begun to decline, producing smaller eruptions for the past 23 million years, while the Tasmantid volcanoes have grown more active. The prediction is stark: the Tasmantid chain will become the main conduit, and Lord Howe will eventually cease activity entirely, many millions of years from now.

This discovery reshapes how geoscientists understand both the surface and the depths. Volcanic hotspots have long served as tools for reconstructing how continents moved—a plume stays roughly fixed while plates slide over it, leaving a trail of volcanoes. But if a single plume can branch and shift as it encounters obstacles in the mantle, then existing reconstructions of absolute plate motion may need refinement. The canonical picture of plumes as straight conduits from deep Earth to the surface must be redrawn. Plumes, it turns out, are dynamic and evolve as they navigate the mantle's obstacles. The same mechanism may explain other closely spaced volcanic chains, including Yellowstone in the United States. Some hotspots that appear ordinary today may have branched in their past, their twin chains now hidden or long extinct.