Study of the chemical composition of lavas suggests that there are two parallel lines of Island volcanics, which researchers call the Loa trend and the Kea trend. They get their names from their biggest mountains, Mauna Loa being of one line and Mauna Kea the other.
(Image: Much of the work discussed in these papers involves study of the chemistry of lavas. Here, the robot arm on the JASON2 submarine, operating 10,000 feet below sea level, collects a lava sample from Mauna Loa. Credit: University of Hawai`i.)
One paper in June in the journal Nature Geoscience, was written by Maxim Ballmer and Garrett Ito of the University of Hawai`i School of Ocean and Earth Sciences and Technology, Jeroen van Hunen of Durham University in the UK and Paul Tackley of the Swiss Institute of Geophysics in Zurich. It is entitled, “Spatial and temporal variability in Hawaiian hotspot volcanism inducted by small-scale convection.”
The other paper, published in Nature Geoscience in November, is by Dominique Weis, Mark Jellinek and James Scoates of the University of British Columbia, Michael Garcia of the University of Hawai`i's Department of Geology and Geophysicsand Michael Rhodes of the University of Massachusetts. It is entitled “Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume.” You can find the University of Hawai`i's press release on this paper here.
The traditional theory about how the Hawaiian archipelago was formed involves a molten “hot spot” which pushes magma up from the Earth's mantle, popping periodically through the ocean floor as the Pacific tectonic plate grinds slowly to the northwest.
But there are problems with that theory, including the parallel lines of volcanoes, as well as what's called the rejuvenated stage or secondary volcanism—which involves why features like Diamond Head and Punchbowl develop a couple of million years after most of the islands' mass has been erupted.
Ballmer and his associates proposed a new model, in which asymmetric melting in the mantle, uneven heat transfer, and a washboard model of the underside of the Earth's crust help explain what's seen on the surface.
It suggests that the rising plume of magma divides in two, feeding the Loa line and the Kea line separately, which explains why Loa lavas tend to be chemically different from Kea lavas. In part that's because the magma feeding the Kea side is hotter, they say.
“Lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years,” writes the Weis team. They argue that the differences in the composition of the lavas may be because the different sides of the magma plume are remelting different kinds of rock as they rise toward the surface.
The Kea line includes Kilauea, Mauna Kea, Kohala, Haleakala, West Maui and both sides of Moloka`i. The longer Loa line includes Lo`ihi, Hualalai, Kaho`olawe, Lana`i, Ko`olau, West Ka`ena and Kaua`i.
Mauna Loa is so darn big that while its caldera is on the Loa line, its slopes extend all the way to the Kea line, which is why Kea-fed Kilauea appears to lie on the slope of Mauna Loa.
Issue two: Why isn't the Hawaiian archipelago one long continuous ridge rather than a series of islands separated by deep channels? Perhaps because of the washboard effect on the bottom of the crust. The volcanoes are able to pour out large amounts of lava where the crust is thin, but not where it's thick.
Issue three: Ballmer and his associates argue that secondary volcanism is associated with a melting zone under older islands that drags nearly 200 miles downstream of the main hot spot activity. That explains why small eruptions at cinder cones like Diamond Head occurred a few hundred thousand years ago, far from the main activity at that time at Hawai`i Island.
A side note: A few decades ago, one of the fun questions for volcano freaks who chase eruptions was this: Is it technically possible for Mauna Loa and Kilauea to erupt at the same time. The theory then was that each was fed by the same plume, so maybe only one could erupt at a time.
But in 1984, Mauna Loa erupted during a Kilauea eruption, setting the question to rest. Now there's a good theory on why that's possible. One is a Loa and one is a Kea.
© Jan TenBruggencate 2011