The Pumice That Carried Life Across a Drowned Continent: Western Australia's Gascoyne Seamounts

Off the coast of Western Australia, a chain of submerged volcanoes rises from the abyssal plain like a staircase into the open ocean. The Gascoyne Seamounts—a string of extinct submarine volcanoes stretching northwest from Cape Range—never break the surface today. But seventy million years ago, during the twilight of the Cretaceous, they were islands.

The Volcanoes That Refused to Sink

The seamounts formed as the Indian Plate tore away from Australia, a process that began around 130 million years ago when Gondwana fractured. As the continent stretched, magma pushed through the thinning crust, building volcanic cones on the seafloor. Unlike the explosive eruptions of continental volcanoes, these were quiet, effusive events—basaltic lava meeting seawater, building pillow lavas that stacked like a pile of cushions.

The Gascoyne chain is part of a larger volcanic province that includes the Wallaby Plateau and the Zeewyck Sub-basin. Radiometric dating of dredged basalt samples gives ages between 70 and 65 million years. At that time, sea levels were higher than today, and the western margin of Australia was a drowned landscape of shallow seas and scattered volcanic islands. The seamounts were among the few dry points on a submerged continent.

A pumice raft can carry seeds, insects, and small vertebrates across thousands of kilometres of open ocean. A volcanic island is a pumice raft made permanent.

Stepping Stones Across an Opening Ocean

The Gascoyne Seamounts sat directly in the path of the newly forming Indian Ocean, which widened as Australia drifted north. For organisms adapted to shallow-water environments, the open ocean was a barrier. But islands—even small, temporary ones—could serve as stepping stones.

Pumice, the frothy glass ejected during explosive eruptions, floats. It can drift for months, carrying seeds, spiders, and snails. But a volcanic island that persists for millions of years does something more: it provides stable habitat. The Gascoyne seamounts, once above water, would have been colonised by seabirds, which brought guano, which brought nutrients, which brought plants, which brought insects. Each island became a tiny ark.

Evidence for this dispersal comes from the distribution of modern marine organisms. Genetic studies of certain tropical reef fish and crustaceans show closer relationships between Western Australian and Indian Ocean populations than expected, suggesting a chain of intermediate islands that no longer exist. The Gascoyne Seamounts, now submerged, are the most plausible candidates.

The Drowning of the Arks

As Australia continued its northward drift, the tectonic plate that carried the seamounts cooled and subsided. Combined with rising sea levels at the end of the Cretaceous, the islands slowly sank. By 55 million years ago, most were submerged. Today they sit between 1,000 and 2,500 metres below the surface, their flat tops—wave-cut platforms from when they were at sea level—still visible on bathymetric maps.

The seamounts are now covered in ferromanganese crusts, slow-growing mineral deposits that accumulate millimetres per million years. These crusts contain cobalt, nickel, and rare earth elements, and there is interest in mining them. But the seamounts also support deep-water coral communities, some of which may be centuries old.

What the Seamounts Still Hold

The Gascoyne chain has never been drilled. No one has sampled the fossil soils that may still be preserved beneath the basalt caps. If any record of the islands' terrestrial life survives—pollen, charcoal, the bones of birds—it lies buried under millions of years of marine sediment.

What we know comes from dredged rocks and seismic profiles. The lavas are tholeiitic basalts, chemically similar to those erupting today at mid-ocean ridges. The seamounts are aligned along fracture zones in the underlying plate, evidence that the magma exploited pre-existing weaknesses in the crust. Their ages become younger to the south, suggesting the volcanic source moved relative to the plate—or that the plate drifted over a fixed hotspot.

The Gascoyne Seamounts are a reminder that most of Australia's geological story is not visible from land. The continent extends far beyond the shoreline, and its edges are littered with the remnants of vanished worlds—islands that rose, hosted life, and sank without a single human witness.