A submarine volcano located just 20 miles off the south coast of Hawaii’s “Big Island” has erupted at least five times in the last 150 years.

This is the conclusion of researchers from the US, who were able to estimate the ages of the most recent lava flows from the Kamaʻehuakanaloa seamount.

The team were also able to date eight additional, older eruptions of the volcano going back some 2,000 years.

Named for their flat but broad profile, shield volcanoes like those found on and around Hawaii pass through a series of growth stages across their lifetimes.

Kamaʻehuakanaloa — formerly known as Lōʻihi — is currently in its earliest “pre-shield” stage, unlike its Big Island neighbor Kīlauea, which is in its main shield-building stage.

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The study was undertaken by geologist Professor Aaron Pietruszka of the University of Hawaiʻi at Mānoa and his colleagues.

Prof. Pietruszka said: “Kamaʻehu is the only active and exposed example of a pre-shield Hawaiian volcano.

“On the other Hawaiian volcanoes, this early part of the volcanic history is covered by the great outpouring of lava that occurs during the shield stage.

“Thus, there is great interest in learning about the growth and evolution of Kamaʻehu.”

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Until now, the only specifically identified eruption episode at Kamaʻehuakanaloa was one that occurred in 1996 — with the tremors associated with the volcanic activity being picked up by seismometers on the Big Island.

Prof. Pietruszka explained: “Seismometers can only be used to detect the ongoing active eruptions of submarine volcanoes because earthquakes are transient.

“In order to determine the ages of older eruptions at Kamaʻehu, we took a different approach.

“We used a mass spectrometer to measure tiny amounts of the isotope radium-226 in pieces of quenched glassy lava that were sampled from the seafloor outcrops of Kamaʻehu using a submersible.”

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Magma, the team explained, naturally contains the radioactive isotope radium-266, which decays at a predictable rate.

Given this, the concentration of radium-266 in a sample can be used to determine the rough amount of time that has passed since the rock erupted onto the seafloor as lava.

Prof. Pietruszka added: “The submersible dive images and videos provided independent confirmation of our estimates of eruption ages.

“The lavas with the freshest appearance also had the most radium-226, and vice versa for the lavas with the ‘older’ appearance — that is, fractured and broken, and/or covered with marine sediment.

“I was surprised to discover that Kama’ehu had erupted five times in the last [roughly] 150 years, which implies a frequency of [around] 30 years between eruptions at this volcano.

“This is much slower than at Kīlauea, which erupts almost continuously, with infrequent pauses of only a few years.”

Geologists know that the chemical makeup of the lava erupted from Hawaiian volcanoes evolves over time.

Combining the newly revealed history of eruptions with the analysis of the lava, the team have found that the volcanic chemistry at Kamaʻehuakanaloa changes over a timescale of around 1,200 years.

In contrast, the chemistry of Kīlauea’s eruptions changes over a timescale of a few years to decades — with a complete cycle of around 200 years.

Prof. Pietruszka said: “We think that the origin of this difference is related to the position of the two volcanoes over the Hawaiian hotspot.”

This, he explained, is “an area of Earth’s mantle that is rising toward the surface — a ‘mantle plume’ that ultimately melts to form the magma that supplies Hawaiian volcanoes.

“Models and other isotope data from thorium-230 suggest that the centre of a mantle plume should rise faster than its margin.

“Our results — specifically, the factor of six timescale of variation in lava chemistry at Kama’ehu — provides independent confirmation of this idea.”

The team hope that their findings will enable scientists to better understand the evolution of Hawaiian volcanoes — and the nature of the deep controls on their eruptions.

The full findings of the study were published in the journal Geology.

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