Schematic of three volcanoes now thought to make up the region of O‘ahu. (credit: J. Sinton)

Schematic of three volcanoes now thought to make up the region of O‘ahu. (credit: J. Sinton)

Researchers from the University of Hawaiʻi at Mānoa, Laboratoire des Sciences du Climat et de l’Environment in France and Monterey Bay Aquarium Research Institute in California recently discovered that Oʻahu actually consists of three major Hawaiian shield volcanoes, not two, as previously thought. “Kaʻena Volcano—A precursor volcano of the island of Oʻahu, Hawaiʻi” was published in the Geological Society of America Bulletin.

The island of Oʻahu, as we know it today, is the remnants of two volcanoes, Waiʻanae and Koʻolau. But extending almost 100 km WNW from Kaʻena Point, the western tip of the island of Oʻahu, is a large region of shallow bathymetry, called the submarine Kaʻena Ridge. It is that region that has now been recognized to represent a precursor volcano to the island of Oʻahu, and on whose flanks the Waiʻanae and Koʻolau Volcanoes later formed.

Prior to the recognition of Kaʻena Volcano, Waiʻanae Volcano was assumed to have been exceptionally large and to have formed an unusually large distance from its next oldest neighbor, Kauaʻi. “Both of these assumptions can now be revised: Waiʻanae is not as large as previously thought and Kaʻena Volcano formed in the region between Kauai and Waiʻanae,” noted John Sinton, lead author of the study and emeritus professor of geology and geophysics at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

In 2010 scientists documented enigmatic chemistry of some unusual lavas of Waiʻanae. “We previously knew that they formed by partial melting of the crust beneath Waiʻanae, but we didn’t understand why they have the isotopic composition that they do,” said Sinton. “Now, we realize that the deep crust that melted under Waianae is actually part of the earlier Kaʻena Volcano.”

High quality data key to new understanding

Among the most important developments was the acquisition of high-quality bathymetric data of the seafloor in the region. This mapping was greatly accelerated after UH acquired the Research Vessel Kilo Moana, equipped with a high-resolution mapping system. The new data showed that Kaʻena Ridge had an unusual morphology, unlike that of submarine rift zone extensions of on-land volcanoes. Researchers then began collecting samples from Kaʻena and Waiʻalu submarine Ridges. The geochemical and age data, along with geological observations and geophysical data confirmed that Kaʻena was not part of Waiʻanae, but rather was an earlier volcanic edifice; Waiʻanae must have been built on the flanks of Kaʻena.

“What is particularly interesting is that Kaʻena appears to have had an unusually prolonged history as a submarine volcano, only breaching the ocean surface very late in its history,” said Sinton. Much of our knowledge of Hawaiian volcanoes is based on those that rise high above sea level, and almost all of those formed on the flanks of earlier ones. Kaʻena represents a chance to study a Hawaiian volcano that formed in isolation on the deep ocean floor.

Despite four different cruises and nearly 100 rock samples from Kaʻena, researchers say they have only begun to observe and sample this massive volcanic edifice. While this article was in press, SOEST scientists visited Kaʻena Ridge again—this time with the UH’s newest remotely operated vehicle, ROV Luʻukai—and collected new rock samples from some of its shallowest peaks. With these new samples Sinton and colleagues hope to constrain the timing of the most recent volcanism on Kaʻena.