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A closer look at Mahukona: Lesser-known volcano is submerged some 30 miles west of Kohala

Updated: 
December 28, 2015 - 3:33pm

During the past weeks, West Hawaii Today has been taking a closer look at each of the volcanoes on and around Hawaii Island. This week, the final edition of the series: Mahukona.

Most have never heard of Mahukona, one of the smallest volcanoes in the Hawaiian Island chain. That’s probably because, even though Mahukona’s summit stands at about 9,500 feet — making it taller than the highest prominence in the Canadian Rockies — it has probably only ever come within a few hundred meters of the surface of the Pacific.

The volcano is submerged some 30 miles west of Kohala and named for the Hawaii Island ahupuaa (land division) off which it sits.

If you were to dive down and explore Mahukona, you would find a sloped shield volcano with a cinder-cone pocked surface — its eastern edge supporting five distinct shelves of coral reef. In 2009, University of Hawaii at Manoa volcanology professor Mike Garcia, along with a team of researchers, took the research vessel Kilo Moana to the waters just above Mahukona to better understand its shape. They conducted a series of bathymetric and sonar surveys and mapped 63 cinder cones spread diffusely across the volcanoe’s surface. Because there seemed to be no logic behind where the cinder cones were located, scientists concluded Mahukona lacked a distinct magma chamber or obvious rift zones.

“There was no preferred zone of weakness, so, lava just plopped out wherever it could,” Garcia said.

Mahukona is the most recently discovered shield volcano in the Hawaiian Islands — it was identified and named in 1987. But it wasn’t until Garcia and his team dredged lava from the surfaces of Mahukona in 1990 that scientists knew definitively that they were looking at a distinct volcano.

He says it only made sense for Mahukona to be there — the volcano filled a gap in a sequence of paired volcanoes that run from Hawaii Island as far north as Oahu. He named his scientific paper describing Mahukona’s legitimacy as a shield volcano, “Mahukona: The missing Hawaiian volcano.”

“Mahukona is younger than other seamounts that are on the seafloor around it,” said Garcia. “It’s not just an old volcano (by Hawaii Island standards), it’s actually a much younger one in the Hawaiian Island chain — its age fits in with the sequence we see heading northwest throughout much of the main islands.”

The submarine volcano follows a trend: Volcanoes tend to get older heading northwest through the Hawaiian Island chain, as they migrate away from the hotspot that created them all. The last stages of volcanism on Mahukona are believed to have occurred between 350,000 and 400,000 years ago. That’s significantly older than the last stages of volcanism on the next closest Hawaiian volcano to the southeast, Kohala, which last erupted some 65,000 years ago. Mahukona may have become an extinct volcano while Kohala was nearing the end of its eruptive prime.

“As Kohala grew, its lavas entered the sea and covered parts of Mahukona,” Garcia said.

Scientists and observant cartographers since the mid-1800s recognized that volcanoes in the lower portion of the Hawaiian-Emperor chain tend to occur along two parallel lines, both running northwest through the main Hawaiian Islands. This idea was revisited in the 1970s and theorized that volcanoes such as Mauna Loa and Kilauea, Hualalai and Mauna Kea were paired — the trend continued with volcanoes on Maui, Kahoolawe, Lanai, Molokai and Oahu, but Kohala volcano was missing its match. Geochemical analysis has since confirmed unique markers in the lavas of volcanoes along these trend lines, but interestingly, both Loa and Kea chemical signatures were found in lava samples from Mahukona.

The origin of trend lines, called Loa and Kea, do seem to have distinct conduits to the Hawaiian hotspot, but volcanologists are still resolving the details on how this works, said Garcia.

“We can recognize similar paired volcanoes in other oceanic chains, such as in the Galapagos and Samoa, Hawaii is not unique in that respect. But the odd thing is, if you go north of Kauai, the paired volcanoes break down and become singular. Up toward Midway, however, there could be paired volcanoes again — it’s harder to tell there which volcanoes are new and Hawaiian, and which ones were there before.”

The presence of Mahukona helps suggest how the trend might continue.

“Now we can predict that the next Hawaiian volcano will form on the ocean floor east of the newest Hawaiian volcano, Loihi Seamount, and south of the Big Island,” said Garcia.

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