The 60-Million-Year-Old Ridge That Drifted Away From Australia

On the floor of the Indian Ocean, 1,500 kilometres west of Perth, a slab of continent lies two kilometres deep. It was once attached to Australia. Now it is the only piece of a vanished landmass that explorers ever mapped while it still existed.

The Lost Plateau

In 1926, a British oceanographic expedition aboard the RRS Discovery sailed south of Java to survey the seafloor. The crew's echo sounder traced a shallow plateau rising from the abyssal plain. They named it Broken Ridge, after its jagged northern escarpment, and assumed it was a submarine bank like any other.

The ridge was not a bank. It was a fragment of continental crust — granite, not basalt — that had once been part of Australia.

Sixty million years ago, Broken Ridge and Australia's Naturaliste Plateau were a single block. Then the Indian Ocean began to open, and a spreading centre ripped them apart. The ridge drifted west; the plateau stayed. Today they sit 1,000 kilometres apart, their matching geology the only evidence of the split.

The Basalt That Betrayed the Breakup

The clue came from drill cores. In the 1970s, the Deep Sea Drilling Project punched through the sediment atop Broken Ridge and hit basalt — but not the ordinary basalt of oceanic crust. The rock was chemically identical to the Bunbury Basalt of southwestern Australia, a 132-million-year-old lava flow that erupted as the Indian Ocean began to open.

The match was exact. The same magma chamber had fed both eruptions, one on each side of the future ocean. When the continent split, the lava flow was severed in half.

The ridge is a 2,000-kilometre-long scar left by the moment Australia tore itself free from India and Antarctica.

A World Above the Waves

Before it sank, Broken Ridge was a mountain range. For tens of millions of years after the breakup, the ridge stood above sea level, its granite peaks exposed to wind and rain. Rivers carved valleys into its flanks. Forests grew on its slopes.

Then, around 40 million years ago, the ridge cooled and subsided. The crust beneath it contracted as it drifted away from the spreading centre, and the entire landmass slipped beneath the waves. Today the highest point of Broken Ridge lies 1,000 metres below the surface. The valleys that were once river gorges are now submarine canyons.

The Living Coral That Grew on a Dead Continent

As Broken Ridge sank, it passed through the photic zone — the sunlit layer where coral can grow. On the ridge's western edge, a massive coral reef built itself on the drowned granite. The reef grew for millions of years, keeping pace with the subsidence, until the ridge sank too deep for photosynthesis. Then the coral died, leaving a limestone cap 500 metres thick.

That cap is now the only part of Broken Ridge that oceanographers can map with sonar. The granite beneath is invisible, buried under the reef's calcified skeleton.

In 2017, an Australian research vessel surveyed the ridge for the first time in decades. The team found that the coral cap is still intact — a fossil reef, perfectly preserved, sitting on a piece of a continent that no longer exists.

The Memory in the Rock

Broken Ridge is not unique. The world's ocean floors are littered with continental fragments — Zealandia, the Kerguelen Plateau, the Jan Mayen Microcontinent — that were torn away during the breakup of Pangea and Gondwana. But Broken Ridge is the only one that was charted while it was still above water, even if no living person saw it.

The ridge's granite, its basalt, and its fossil reef are a single record: the moment a continent fractured, the mountain range that rose in the aftermath, and the slow drowning that erased it. All of it is still there, two kilometres down, waiting for a drill bit to bring it back to light.