The long-accepted understanding of Oahu’s geology is turned on its head with a new report from the University of Hawaii that a third major volcano contributed to the island’s formation.
For decades the common notion was that Oahu was built by two volcanoes — Waianae and Koolau, which surrendered their flanks to giant submarine landslides, giving the island the shape it has today.
But now a research team led by John Sinton, a UH professor of volcanology, concludes that a third major volcano coexisted with the other two perhaps for millions of years, lending the island contours that would be unrecognizable today.
The undersea remnants of Kaena Volcano, as it is called, extend westward from Kaena Point more than two-thirds of the way across the Kauai Channel, indicating that Oahu was once about twice its current size.
"It would have been a three-volcano island, with three distinct mounds, of which Waianae was probably the biggest and tallest," Sinton said in a telephone interview Thursday. "I see this as a precursor volcano to the island of Oahu. When we look at Oahu now, we have a new understanding of the mountain ranges."
The team’s research was published online May 2 in the Geological Society of America Bulletin. Co-authors include UH postdoctoral researcher Deborah Eason and former UH researchers Mary Tardona, Douglas Pyle and Iris van der Zander.
The existence of Kaena Ridge and its neighboring Waialu Ridge has been known since the Navy began mapping the seafloor around Hawaii in 1939.
But it was long assumed that the ridges were simply extensions of Waianae Volcano, much like those that extend offshore from western Molokai (Penguin Bank); Hana, Maui; and Puna, Hilo and South Point on Hawaii island.
The UH researchers, after several bathymetric surveys and rock-collecting missions, conclude that the lava from Kaena Volcano is chemically distinct from lava from Waianae. Further, measurements of the gravity fields around Kaena show areas of density that suggest it had its own core magma source, Sinton said.
"It doesn’t look like a simple extension of one of the on-land volcanoes of Oahu or elsewhere," said Sinton. "It has a different profile."
Typical submarine ridges in Hawaii drop off dramatically. South Point’s ridge, for instance, passes a depth of 14,000 feet within about 20 miles of shore.
In striking contrast, the Kaena Ridge is much shallower and more gradual in its descent, even rising up again to form two cones, the taller one less than a half-mile below the surface — also 20 miles from shore, Sinton said.
Kaena Volcano began growing on the seafloor about 5 million years ago, making it older than Waianae, Sinton said.
However, Waianae, growing on Kaena’s flanks, breached the surface first, about 3.8 million years ago.
Kaena joined it about 300,000 years later and probably reached a maximum elevation of 3,000 feet, about the same as Lanai today, Sinton estimates.
Koolau emerged about 3 million years ago.
Just how long Kaena remained above the surface is uncertain, Sinton said.
But the three mountain monarchs could have reigned for eons.
"Almost certainly it was a single island," Sinton said.
There would have been a saddle between Waianae and Kaena much like the Central Oahu highlands between the Waianaes and Koolaus, he said. In fact, such a saddle remains in evidence today between Kaena Point and Kaena Ridge’s first peak, he added.
"Hawaiian volcanoes, as soon as they grow, they begin to sink, to subside," he said. "When they stop growing, they continue to sink."
For Kaena the growth of Waianae on its flank probably accelerated the process, he said.
The first cruise to Kaena Ridge was in 2006 aboard the UH research vessel Kilo Moana in concert with some undergraduate teaching at UH, Sinton said.
"That was the first investigation to start looking at: What is Kaena Ridge, and what is going on out there?" Sinton said. "We kind of kept hammering on it for the next few years."
A National Science Foundation grant led to another expedition in 2011.
"We got more data, more rocks to analyze in various ways, and that led us to the paper that has just come out," he said.
Rock samples were dated by looking at the ratio of isotopes of potassium and argon. Isotopes decay at different rates, so the ratios serve as a radiometric clock.
A third Oahu volcano also helps fill the Kauai-Oahu "gap."
Spacing among the volcanoes in the main Hawaiian Islands shows that all but one formed within 20 to 40 miles of its older neighbor. At 90 miles from Central Kauai, Waianae was the single departure from that pattern.
With Kaena Volcano the pattern now is complete.
Sinton said he found no sign of modern volcanic activity.
In May 1956 a Navy pilot spotted discolored water about 37 miles west-northwest of Kaena Point, which many at the time saw as evidence of a submarine eruption.
The report triggered a "frenzy of observations," both military and private. In addition to the brownish-yellow or yellow-green water, there were reports of small floating fragments and three dead whales that appeared to have been poisoned.
The eruption theory strengthened when, a few days later, pieces of pumice washed ashore at two spots on Windward Oahu.
But Sinton said nothing in the bathymetry data supports an eruption in the last 60 years.
"We didn’t find any evidence for it," he said.
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