Three active volcanoes, not two as previously believed, formed the Hawaiian island of Oahu, say researchers who've uncovered evidence of an underwater volcano that once helped form the island.

It had been previously assumed Oahu was formed as two volcanoes -- Ko'olau in the island's east and Wai'anae in the west -- broke above the waters of the Pacific Ocean between 3 and 4 million years ago.

Now, researchers from the University of Hawaii at Manoa say, evidence suggests Wai'anaei actually rose 3.9 million years ago from the flanks of an earlier precursor volcano dubbed Ka'ena -- the "hottest part of the flame" in Hawaiian -- which formed deep underwater south of the island of Kauai a million years before that.

It is likely Ko'olau then surfaced about a million years later, also from the flanks of Ka'ena, they report in the Geological Society of America Bulletin.

Ka-ena also briefly rose above the sea, although it is now completely underwater, the researchers said.

They know it was once above sea level because it is now capped with lavas showing textures that form only in open air, they said.

"What is particularly interesting is that Ka'ena appears to have had an unusually prolonged history as a submarine volcano, only breaching the ocean surface very late in its history," geology and geophysics Professor John Sinton says.

Volcanoes that rise above the ocean provide most of what's currently known about Hawaiian volcanoes, so the discovery of Ka-ena provides an opportunity to study at least one Hawaiian volcano whose formation occurred deep beneath the waves on the ocean floor, the researchers said.

Oahu's volcanoes have been extinct for around 2 million years, and their massive weight on the crust of the Earth is causing them to slowly sink, pushing Ka'eana further underwater.

"The first ones, because they form in deep water, they kind of escape notice," Sinton said. "We like to think we know how many Hawaiian volcanoes there are, but what we know about what's underwater is a huge area of ignorance."

The discovery also answers another puzzle, that of the considerable gap existing between Oahu and Kauai, its closest neighbor island. The Wai'anae volcano is 90 miles from Kauai, whereas all the other volcanoes in the Hawaiian chain of islands are within 20 to 40 miles of each other.

The discovery of Ka'ena fills in the gap nicely, the researchers said.

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