Wednesday, December 12, 2012
Was Kaua`i formed by one or two volcanoes, and if it’s one, was its center was near the Alakai Swamp?
The newest and best answer seems to be one volcano—and the Alakai Swamp is not its center.
Researchers studying gravity anomalies appear to have answered a troubling geological question: how to make sense of the geological history of an island that’s been eroding for 5 million years, leaving a confusing array of ridges, cliffs, bogs and craters.
Three University of Hawai`i scientists, Ashton F. Flinders, Garrett Ito and Michael O. Garcia, all of the university’s Department of Geology and Geophysics, used gravity data to take another look.
Their 2010 paper in the Journal of Geophysical Research, which we came across recently, discusses the gravity fields in the Kaua`i-Ni`ihau area.
Since the dense rock of a volcanic core—a solidified magma reservoir—can create a higher gravity reading, they were able to identify what appear to be the centers of the ancient volcanoes of Kaua`i County.
The essence of their findings is this: Kaua`i was formed of a single volcano, whose center was under what’s currently known as the Lihue Basin. It is generally in the middle of an area bounded by the Hā`upu range to the south, the Kahili-Wai`ale`ale cliffs to the west, the Makaleha Mountains to the north and the Kalepa Ridge to the east. What happened to the old volcanic peak? “The summit of the volcano was likely removed by extensive mass wasting and/or erosion.”
That volcanic center is six miles east of the Alakai Swamp region, which many experts had assumed marked the center—the geologically mapped caldera. “This offset suggests that the mapped caldera is a collapsed feature later filled in with lava and not the long‐term center of Kaua’i shield volcanism.”
The data was collected using more than 300 land measurements, which were added to work performed on Kaua`i gravity anomalies in 1965.
In another bit of interesting data, they identified a second area of high gravity in the middle of the channel between Kaua`i and Ni`ihau, which they say is likely the heart of the older Ni`ihau volcano. Its center lies about 8 miles toward Kaua`i from Paniau, the highest point on remnant Ni`ihau.
The island of Ni`ihau is a remnant of that volcano, most of whose eastern side was covered by Kaua`i lavas. The underwater gravity survey was performed using University of Hawai’i’s R/V Kilo Moana.
The geology of the islands has been challenging, in part because of their age and degree of erosion, the authors say: “Extensive mass wasting, erosion, and subsidence have obscured the extent, shape, and centers of their original shield volcanoes.”
There remain plenty of questions for geologists to answer, particularly about the association of these two islands, and whether they are both part of a single volcanic rift that had two significant centers of volcanism—one forming Niihau and the second later forming Kaua`i.
“The overall trend of these features from eastern Ni’ihau, through the Kaulakahi Channel, and inland to western Kaua’i, combined with the elongate trend of the residual gravity, supports previous interpretations of a long volcanic rift zone, the Mana ridge, passing through the present locations of both islands. Yet, whether this feature is an extension of the Ni’ihau volcanic center... a rift zone radiating away from Ni’ihau similar to Lō’ihi’s rift system , a shared rift zone fed by either shield volcanoes or a separate and entirely unrelated feature, remains unresolved,” the authors say.
There are also multiple questions and theories about the formation of the vast plateau that forms the Alakai, which has been called the world’s highest-elevation swamp, and which is a center for biodiversity in the Islands.
One theory is that the Alakai , which is a part of features called the Olokele Volcanics, does represent the ancient Kaua`i caldera, and that it was fed at an angle by a volcanic center many miles to the east. But there is no Hawaiian volcano with so great an offset between the magma center and the caldera.
Another theory is that “the Olokele feature formed late in the shield stage and is not part of Kaua’i’s long‐term center of volcanism.”
As to the theory of whether there could have been two volcanoes for Kaua`i, one creating the Lihue basin and the other creating the Olokele features of West Kaua`i?
“We found no indications in either the surface gravity mapping or the inverted density structure to support the hypothesis that Kaua’i was formed by two sequentially buttressed shield volcanoes, each having a separate magma supply system.”
Another question is whether Kaua`i and Ni`ihau were ever connected above the surface of the sea. The authors say they were not—that Ni`ihau had already started a significant erosion trend before it encountered the Kaua`i lavas. And the summit of Ni`ihau may have been destroyed in a massive collapse of the island’s eastern flank.
Citation: Flinders, A. F., G. Ito, and M. O. Garcia (2010), Gravity anomalies of the Northern Hawaiian Islands: Implications on the shield evolutions of Kauai and Niihau, J. Geophys. Res., 115, B08412, doi:10.1029/2009JB006877
© Jan TenBruggencate 2012