Though scientists have noted Ka'ena volcano's massive underwater bulk for decades, it took several underwater expeditions to confirm geologists' suspicions that it was separate from Wai'anae.
Long, shallow ridges (Ka'ena Ridge and Wai'anaa) that stretch to the northwest mark the topography offshore of Oahu. When the U.S. Navy mapped the seafloor during World War II, scientists thought the ridges were extensions of Wai'anae volcano.
Sinton and his colleagues distinguished the volcanoes with analysis of rock samples and detailed mapping of the seafloor's bumps and saddles. The lava at each volcano is chemically distinct, with different ages; gravity surveys suggest the different groups of lava had unique sources.
"Once better maps started to appear, the idea that there might be an earlier volcano came to our minds," Sinton said. "It was obvious this was something unusual."
The new volcano fills a puzzling gap between Oahu and its nearest neighbor, the island of Kauai. Except for these two islands, volcanoes in the Hawaiian chain are spaced within 20 to 40 miles (32 to 64 kilometers) of their older neighbor. But Wai'anae volcano was 90 miles (145 km) from Kauai. Ka'ena volcano fixes the chain's missing link.
The Hawaiian Islands are hotspot volcanoes, a chain formed as the Pacific plate moves over a plume of hot magma in the Earth's mantle. As the plate shifts, new volcanoes appear.
But there's one mystery Ka'ena volcano can't solve. Despite an intense search, the research team never found the source of an enigmatic underwater eruption in 1956 offshore of Oahu, Sinton said. However, the scientists did find young lava fields that erupted as recently as 300,000 to 400,000 years ago on the south side of Ka'ena Ridge.
The lava fields show eruptions are possible near the region of the mysterious 1956 event, the study concludes. "The best I can say is it's unconfirmed," Sinton said.
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