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.