Multiple loci of volcanoes are intertwined to form one island chain.

One locus at the southeast end of the chain includes the volcanoes of Kilauea, Mauna Kea, Kohala, Haleakala (East Maui), West Maui, and East Moloka`i. This locus is called the Kea locus.

The other locus includes Lo`ihi, Mauna Loa, Hualalai, Mahukona (a submerged volcano centered about 60 km (35 miles) west-northwest of Mahukona, Kohala), Kaho`olawe, Lana`i, and West Moloka`i. This locus is called the Loa locus.

The pattern becomes harder to see farther up the chain. Some workers place Ko`olau on the Loa line and Wai`anae on another line that includes the two volcanoes on Kaua`i, with Ni`ihau on a separate line. Others include Ko`olau, Wai`anae, and Kaua`i on the Loa line and Ni`ihau through Nihoa on another one.

Regardless of details and ambiguities, though, it is possible to place all of the volcanoes in the Hawaiian-Emperor chain along a small number of loci.

In 1849, the renowned pioneering geologist J.D. Dana noted these geographic patterns for the southeastern part of the island chain and coined the terms Loa and Kea series for the two most prominent loci. No one had much interest in such an empirical observation until the late 1960s, when, during development of the concepts of plate tectonics and hot spots, prominent geologists E.D. Jackson, Eli Silver, and G.B. Dalrymple revived and extended Dana's observations up the entire Hawaiian-Emperor chain.

The loci are not parallel to the island chain but instead are rotated clockwise from the Hawaiian chain as much as 30 degrees or so. Thus each locus is relatively short and gives way to another locus that brings the volcanoes back on trend. Each locus tends to start 25-75 km (15-45 miles) north or east of the average centerline of the chain and swings south or west for about the same distance before giving way to the next locus.

In general the order of shield development zigzagged back and forth between the Kea and Loa lines, in other words, from Kohala to Hualalai to Mauna Kea to Mauna Loa to Kilauea.

Despite these and other intriguing consistencies, some scientists doubt that the volcanoes are truly aligned along curving paths, in part because there is no unique way to connect all the different volcanoes, and in part because there is no easy way to explain the parallel loci. These doubts are lessening, however, as more evidence is found for differences between the Kea and Loa lines.

Exceptionally careful geochemical work has been done over the past decade in Hawai`i. One group of workers from Caltech, Wisconsin, and Germany has found that "the isotopic distinctions between Loa and Kea trend volcanoes imply a systematic difference in the magma supply and plumbing systems of volcanoes on these two trends."

Another group, from Cal-Berkeley and USGS, reported isotopic and chemical differences that are consistent "between volcanoes along the western 'Loa' and eastern 'Kea' trends and reflect large-scale variations in source composition and melting environment." These geochemists suggest that the two loci tap different parts of the plume that rises from the hot spot and becomes modified by incorporating different material from the mantle.

All well and good, but why don't volcanoes occur randomly rather than along two lines? P.D. Ihinger of Yale recently suggested that a strong current in the upper mantle beneath Hawai`i flows in a direction opposite, and about 30 degrees oblique, to that of the overriding Pacific plate. This current splits the rising hot-spot plume into "plumelets," which erupt along separate parallel lines.

This is all pretty difficult stuff but is one early step along the way to developing an understanding for one of the most puzzling aspects of Hawaiian volcanism.

(Editor's Note: This article was written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory. UniSci passes it along to our readers as an example of good science writing, and a sample of the kind of thing Hawaii residents get on a regular basis.)


[Contact: Carolyn C. Bell]

05-Nov-2001