hawaii oahu volcano
May 17, 2014

Oahu Precursor Volcano Discovered

April Flowers for redOrbit.com - Your Universe Online

O'ahu, the "gathering place," is the most developed island in the Hawaiian chain. Until recently, scientists believed that O'ahu consisted of the remnants of two major Hawaiian shield volcanoes: Wai'anae and Ko'olau. A new study from University of Hawai'i at Mānoa (UHM), Laboratoire des Sciences du Climat et de L'Environment (France), and Monterey Bay Aquarium Research Institute has found that O'ahu actually consists of three shield volcanoes, not two.

Extending approximately 62 miles from the western tip of O'ahu, known as Ka'ena Point, is the submarine Ka'ena Ridge, a large region of shallow bathymetry (underwater topography). The researchers have discovered that this region represents a precursor volcano to the island of O'ahu. The two previously recognized volcanoes, Wai'anae and Ko'olau, formed on the flanks of this earlier volcano.

Shield volcanoes are broad, low-profile volcanoes. Their base diameters can range from just over a mile to more than 60 miles, and their heights are approximately 1/20th of their widths, according to San Diego State University's Department of Geology.

Scientists assumed that Wai'anae Volcano was exceptionally large, prior to recognizing Ka'ena Volcano. They also assumed that it had formed an unusually long distance from its next oldest neighbor, Kaua'i. "Both of these assumptions can now be revised: Wai'anae is not as large as previously thought and Ka'ena Volcano formed in the region between Kauai and Wai'anae," noted John Sinton, Emeritus Professor of Geology and Geophysics at the UHM School of Ocean and Earth Science and Technology (SOEST).

A research team in 2010 found enigmatic chemistry of some unusual lavas of Wai'anae. "We previously knew that they formed by partial melting of the crust beneath Wai'anae, 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 Ka'ena Volcano."

It has taken a long time, and a lot of effort, to make this discovery. One key ingredient was the acquisition of high-quality bathymetric data of the seafloor in the region. UHM's acquisition of the Research Vessel Kilo Moana, equipped with high-resolution mapping equipment, accelerated this mapping process. The map showed that the Ka'ena Ridge had very unusual morphology, quite unlike the underwater rift zone extensions of on-land volcanoes. This inspired the research team to begin collecting samples from both Ka'ena and Wai'anae submarine Ridges. The samples allowed the team to confirm Ka'ena was not part of Wai'anae using geochemical and age data, along with geological observations and geophysical data. Instead, they found that Ka'ena is an older volcanic edifice, and Wai'anae must have been built on its flanks.

"What is particularly interesting is that Ka'ena 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 what we know of the Hawaiian volcanoes is gleaned from the ones that rise high above sea level. Almost all of these high-profile volcanoes formed on the flanks of earlier ones. That makes Ka'ena a unique opportunity for researchers to study a Hawaiian volcano that formed in isolation on the deep ocean floor.

The Research Vessel Kilo Moana has made four cruises and collected almost 100 samples of rock, yet the research team says they have only just begun to study Ka'ena. The team has returned to Ka'ena Ridge with the newest remotely operated vehicle at UHM, the ROV Lu'ukai, to collect new samples, which they hope will allow them to estimate the timing of the most recent volcanism on Ka'ena.

The findings were published in a recent issue of Geological Society of America Bulletin.

Image 2 (below): Map showing schematically the distribution of the three volcanoes now thought to have made up the region of O'ahu, Hawai'i. From oldest to youngest these are the Ka'ena, Wai'anae, and Ko'olau Volcanoes. Upper panel: bold dashed lines delineate possible rift zones of the three volcanoes; also shown are the major landslide deposits around O'ahu. The lower panel shows how the three volcanic edifices overlap. Credit: J. Sinton, et al., UH SOEST