Found! New Underwater Volcano Discovered in Hawaii

A newly discovered underwater peak, called Ka'ena volcano, brings Oahu island's volcano count to three, up from two.

The sprawling chain of Hawaiian volcanoes just added another underwater branch.

The discovery means Oahu once towered above the ocean with three volcanic peaks, the researchers said. Until now, scientists thought Oahu was built by two volcanoes - Wai'anae on the west and Ko'olau on the east.

"I think we may very well have had three active volcanoes in the Oahu region," said lead study author John Sinton, a geologist at the University of Hawaii at Manoa.

The new volcano, named Ka'ena, was born in the deep underwater channel south of Kauai about 5 million years ago, according to the study, published May 2 in the Geological Society of America Bulletin. Sometime later, Wai'anae rose on Ka'ena's flanks and therefore breached the sea first, breaking through the waves 3.9 million years ago. The researchers think Ko'olau surfaced after that, about 3 million years ago. [See Photos of the Newfound Underwater Volcano]

Ka'ena volcano is about 2.5 miles (4 kilometers) high, but only reached about 3,000 feet above sea level, Sinton said. As Oahu's first-born, Ka'ena is the shortest of the three volcanoes because it had to grow farthest from the seafloor to the ocean surface. But the researchers know Ka'ena was once an island peak, because the underwater mountain is capped by lavas with textures that only form in air. With a remotely operated vehicle, the researchers also spied a sandy beach strewn with shark teeth.

Oahu's volcanoes died out about 2 million years ago, and like all of Hawaii's islands, their massive bulk is slowly sinking, hiding Ka'ena beneath the sea.

Disappearing islands The weight of the Hawaiian volcanoes has pressed down the Earth's crust. The flexing resembles a person standing on a trampoline, causing the springy surface to sag.

"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," he told Live Science's Our Amazing Planet.

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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Pillow lavas at Ka'ena volcano, a type of lava that only forms underwater.

A new cinder cone breaks the sea surface off the coast of the uninhabited Nishinoshima island on Nov. 20, 2013.

An aerial photograph on Nov. 21, 2013 shows the eruption of the new cinder cone continues off the coast of Nishinoshima island.

Discoloration of the water surrounding the newly erupting cinder cone on Nov. 21, 2013.

Japan's new cinder cone eruption occured some 620 miles south of Tokyo.

The erupting cinder cone may build a small new island in the Pacific. The Japan Coast Guard is monitoring the eruption as it continues and has asked vessels to stay clear of the area.

Nishinoshima island and its new neighbor are part of Japan's Ogasawara island chain or Bonin Islands.

One of the first images of the new erupting cinder cone in Japan's southern territorial waters. Note the hot lava bursts mixed with the steam, ash and smoke in the erupting plume.

Nishinoshima island, Japan, in 2005.

This graphic from 2003, shows Nishinoshima island's adjacent cinder cone as it rises above the average seafloor depth prior to breaking the sea surface.

A bathymetry map of Nishinoshima island showing its elevation from the depths of the seafloor to just above sea surface.

A Sep. 14, 1973, photograph shows the beginning of the eruption that eventually led to the enlargement of Nishinoshima island in 1974.

Nishinoshima island is seen here on Dec. 21, 1973, with a plume of ash billowing from its second crater.

In May 1973, the first signs of of undersea volcanic activity around Nishinoshima island discolor the water off its southern tip.