A recent study by a University of Rhode Island doctoral student and colleagues at the University of Hawaii at M?noa changes the scientific understanding of how the Hawaiian Islands were formed. Their results were published in Geophysical Research Letters in July. Lead author Ashton Flinders, a student at the URI Graduate School of Oceanography, found that it is the eruptions of lava on the surface, called extrusion, that grows Hawaii’s volcanoes rather than internal emplacement of magma, as was previously thought.

Before this work, most scientists thought that Hawaiian volcanoes grew primarily internally – by magma intruding into rock and solidifying before it reaches the surface. According to Flinders, this type of growth does occur — along Kilauea’s East Rift Zone, for example – but it does not appear to be representative of the overall history of how the Hawaiian Islands formed. Previous estimates of the internal-to-extrusive ratios (internally emplaced magma versus extrusive lava flow) were based on observations over a very short time frame, in the geologic sense.

Flinders compiled historical land-based gravity surveys with more recent surveys on the Big Island of Hawaii and Kauai, along with marine surveys from the National Geophysical Data Center and from the University of Hawaii’s R/V Kilo Moana. These types of data sets allow scientists to infer processes that have taken place over longer time periods.

“The discrepancy we see between our estimate and these past estimates emphasizes that the short-term processes we currently see in Hawaii (which tend to be more intrusive) do not represent the predominant character of their volcanic activity,” said Flinders.

“This could imply that over the long-term, Kilauea’s East Rift Zone will see less seismic activity and more eruptive activity than previously thought. The three-decade-old eruption along Kilauea’s ERZ could last for many, many more decades to come,” said Garrett Ito, professor of geology and geophysics at UHM and co-author.

“I think one of the more interesting possible implications is how the intrusive-to-extrusive ratio impacts the stability of the volcano’s flank. Collapses occur over a range of scales from as large as the whole flank of a volcano, to bench collapses on the south coast of Big Island, to small rock falls,” said Flinders.

Intrusive magma is more dense and structurally stronger than lava flows. “If the bulk of the islands are made from these weak extrusive flows then this would account for some of the collapses that have been documented, but this is mainly just speculation as of now,” Flinders added.

The authors hope this new density model can be used as a starting point for further crustal studies in the Hawaiian Islands.

The image is a 3D perspective view of the topography of the Hawaiian islands (gray shaded) and seafloor relief viewed from just south of the Hawaii’s big island, NNW up the Hawaiian Chain. The colors show residual gravity anomaly, measured on land and along ship tracks: red-cyan representing an excess pull of gravity, blue representing a small deficit in the pull of gravity. The highest excesses are seen over the volcano summits. Also shown are illuminated surfaces (in red) surrounding the densest crust beneath these gravity highs, as determined by inverting the residual gravity anomaly for subsurface density structure. These dense bodies are what create the excess gravity over them and are interpreted as cumulate-rich volumes of crust to where magma has been focused before erupting: the magmatic centers of the volcanoes.