Washington:
The sprawling chain of Hawaiian volcanoes just got bigger!
The Hawaiian island of O'ahu has been actually formed by three massive shield volcanoes and not just two as previously thought, a new study has found.
Researchers from the University of Hawaii at Manoa (UHM), and colleagues recently discovered that O'ahu actually consists of three major Hawaiian shield volcanoes, not two, as believed earlier.
The island of O'ahu, as we know it today, is the remnants of two volcanoes, Wai'anae and Ko'olau. But extending almost 100 km from Ka'ena Point, the western tip of the island of O'ahu, is a large region of shallow bathymetry, called the submarine Ka'ena Ridge.
It is that region that has now been recognised to represent a precursor volcano to the island of O'ahu, and on whose flanks the Wai'anae and Ko'olau Volcanoes later formed.
Prior to the recognition of Ka'ena Volcano, Wai'anae Volcano was assumed to have been exceptionally large and to have formed an unusually large distance from its next oldest neighbour - Kauai.
"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," said John Sinton, lead author of the study.
"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 realise that the deep crust that melted under Waianae is actually part of the earlier Ka'ena Volcano," Sinton said.
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.
The new data showed that Ka'ena Ridge had an unusual morphology, unlike that of submarine rift zone extensions of on-land volcanoes. Researchers then began collecting samples from Ka'ena and Wai'alu submarine Ridges.
The geochemical and age data, along with geological observations and geophysical data confirmed that Ka'ena was not part of Waianae, but rather was an earlier volcanic edifice; Wai'anae must have been built on the flanks of Ka'ena, researchers said.
"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.
