Geologists have long known the Hawaiian Islands formed as the Pacific plate moved over a magma hot spot that feeds the volcanoes.
But the youngest in the chain still offers a bit of mystery.
Why, for instance, do the volcanoes of the Big Island follow two parallel paths — known as the Loa and Kea tracks — with their own distinct chemical composition? After all, they are fed by the same hot spot deep below the ocean floor.
A study recently published in the journal Nature offers a new answer, and it starts about 3 million years ago with a change in the movement of the Pacific plate.
Previously, the plate was moving northwest, pulling a magma plume along with it.
But, according to the report, a slight northerly bend in the plate’s path exposed two separate magma zones in the hot spot that would have otherwise been stacked on top of each other.
With the plate moving at a different angle to the plume, these zones — one deep, the other shallow — begin to supply separate volcanoes merely miles apart. That would explain the volcanoes’ parallel paths as well as a difference in their chemical makeup, the report’s authors say.
The Loa line includes Mahukona, Hualalai, Mauna Loa and Loihi. The Kea group involves Kohala, Mauna Kea and Kilauea.
The same theory applies to the volcanoes that made the islands of Maui, Molokai, Lanai and Kahoolawe, which also follow the Loa and Kea pattern. According to the report, that pattern began after Oahu formed.
Garrett Ito, a University of Hawaii at Manoa geology professor, said he found the report, authored by an international group of researchers, intriguing and that it could lead to follow-up research.
“What I like about this hypothesis is it explains both the geology, the separation of volcanoes, as well as the difference in composition,” he said. “It’s attributing that separation to a recent change in plate motion.”
But it also relies on a controversial prediction that the Loa magma is shallower, Ito said.
“The most conventional wisdom is Loa material probably, in many situations, is expected to melt deeper,” he said.
Sorting that out matters since it helps explain the planet’s geological history.
“The Loa material is thought to have subducted basaltic crust as a component,” Ito said.
“… Another test would be to see if we can better characterize that Loa material and test whether in fact it’s consistently the type of material that melts shallower or not.”
Eventually, the magma plume is expected to readjust, merging the Loa and Kea tracks once again, he said. But that could take a few more million years.
Email Tom Callis at tcallis@hawaiitribune-herald.com.