Previously, geologists believed the island’s current profile is the remnants of two volcanoes, Waiâ€˜anae and Koâ€˜olau. But extending almost 100 km WNW from Kaâ€˜ena Point, the western tip of the island of Oâ€˜ahu, is a large region of shallow bathymetry, called the submarine Kaâ€˜ena Ridge. It is that region that has now been recognized to represent a precursor volcano to the island of Oâ€˜ahu, and on whose flanks the Waiâ€˜anae and Koâ€˜olau Volcanoes later formed.
The team included scientists from the University of Hawaiâ€˜iâ€“MÄnoa, Laboratoire des Sciences du Climat et de Lâ€™Environment in France, and Monterey Bay Aquarium Research Institute.
Prior to the recognition of Kaâ€˜ena Volcano, Waiâ€˜anae Volcano was assumed to have been exceptionally large and to have formed an unusually large distance from its next oldest neighbor: Kauaâ€˜i.
â€œBoth of these assumptions can now be revised: Waiâ€˜anae is not as large as previously thought and Kaâ€˜ena Volcano formed in the region between Kauai and Waiâ€˜anae,â€ noted John Sinton, lead author of the study and Emeritus Professor of Geology and Geophysics at the UHM School of Ocean and Earth Science and Technology (SOEST).
In 2010 scientists documented enigmatic chemistry of some unusual lavas of Waiâ€˜anae. â€œWe previously knew that they formed by partial melting of the crust beneath Waiâ€˜anae, but we didn’t understand why they have the isotopic composition that they do,â€ said Sintonâ€œ Now, we realize that the deep crust that melted under Waianae is actually part of the earlier Kaâ€˜ena Volcano.â€
This new understanding has been a long time in the making.
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 Kaâ€˜ena Ridge had an unusual morphology, unlike that of submarine rift zone extensions of on-land volcanoes. Researchers then began collecting samples from Kaâ€˜ena and Waiâ€˜alu submarine Ridges. The geochemical and age data, along with geological observations and geophysical data confirmed that Kaâ€˜ena was not part of Waianae, but rather was an earlier volcanic edifice; Waiâ€˜anae must have been built on the flanks of Kaâ€˜ena.
â€œWhat is particularly interesting is that Kaâ€˜ena appears to have had an unusually prolonged history as a submarine volcano, only breaching the ocean surface very late in its history,â€ 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. Kaâ€˜ena 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 Kaâ€˜ena, researchers say they have only begun to observe and sample this massive volcanic edifice. While this article was in press, SOEST scientists visited Kaâ€˜ena Ridge again â€“ this time with the UHâ€™s newest remotely operated vehicle, ROV Luâ€˜ukai â€“ 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 Kaâ€˜ena.
The School of Ocean and Earth Science and Technology at the University of Hawaii at Manoa was established by the Board of Regents of the University of Hawaiâ€˜i in 1988 in recognition of the need to realign and further strengthen the excellent education and research resources available within the University. SOEST brings together four academic departments, three research institutes, several federal cooperative programs, and support facilities of the highest quality in the nation to meet challenges in the ocean, earth and planetary sciences and technologies.