A new study co-authored by University of Hawaii researchers challenges the conventional wisdom of how the Hawaiian Islands were formed.

Its peer-reviewed findings could also change how scientists understand the ongoing Kilauea eruption, its authors say. That flow has added about 500 acres to Hawaii island in the past 30-plus years.

For decades, geologists have held that the islands were mostly formed by intrusion, in which magma pushes up against rock from below the surface, said Garrett Apuzen-Ito, a professor of geology and geophysics at UH’s School of Ocean and Earth Science and Technology and the study’s co-author.

However, the study, which Apuzen-Ito completed with Ashton Flinders, a graduate student at the University of Rhode Island, found instead that most of the land mass making up the Hawaiian Islands was formed by external eruptions — the lava that poured out onto the surface.

The researchers concluded this, Apuzen-Ito said, by gathering data over several years measuring how dense the rock is on and around the islands.

Denser rock exists where the islands were formed from magma pressing below the surface, while the less-dense rock exists where lava flowed externally, Apuzen-Ito said.

The map showed that the islands’ higher elevations, such as the peaks of Mauna Loa, Mauna Kea on the Big Island and the Koolaus on Oahu, contained the denser rock. But most of Hawaii’s land mass is composed of the less-dense rock, the data showed.

To measure rock density, scientists use special instruments that can detect extremely small changes in gravity in the area, Apuzen-Ito said.

The discovery that the external lava flows created most of the Hawaiian Islands’ mass could mean that eruptions such as the one that continues at Kilauea aren’t very rare — and scientists might expect that flow to continue for years to come, he added.

Magma intrusions — the internal effects — are also occurring at the Kilauea East Rift Zone but they don’t represent how most of the islands were formed, Apuzen-Ito said.

Flinders first started gathering gravity data for the study on Kauai in 2008 when he was a UH master’s degree student, Apuzen-Ito said. The rest of the study took place during the past two years, and it was peer-reviewed by the international journal Geophysical Research Letters, he added. The study was publicized earlier this month.

The process to form each island lasted about 1 million years, Apuzen-Ito said.