Tiny gas bubbles reveal secrets of Hawaiian volcanoes

Geologists have long proposed that, as the Hawaiian Islands slowly drift northwest with the Pacific Plate, they move away from a deep, heat-rich plume rising from near Earth's core. Young volcanoes like Kilauea -- positioned directly above the hotspot on Hawaii's main island -- receive a steady flow of magma. Far less is known about older volcanoes like Haleakala -- located northwest on the island of Maui -- where magma flow has significantly diminished.

The new research finds that as volcanoes move off the hotspot, their magma flow not only shrinks, but shifts deeper underground, reshaping assumptions about how Hawaii's volcanic "pluming system" has evolved.

"This challenges the old idea that eruptions are fueled by magma stored in the Earth's crust and suggests a new possibility," said lead author Esteban Gazel, "that magma is stored and matures in the Earth's mantle, and eruptions are fueled from this deep mantle reservoir."

By analyzing fluid inclusions -- tiny gas bubbles trapped inside crystals formed in magma -- the researchers calculated the pressure, and therefore the depth, at which the inclusions were trapped before an explosive eruption ejects them to the surface.

"The technology allows us to measure pressure from depths with an uncertainty as small as just hundreds of meters, which is very, very precise for depths that are tens of kilometers below the surface," Gazel said. "Before this, measuring magma storage was much more difficult, with uncertainties that could span kilometers."

To achieve such level of precision, researchers optimized a custom gas chamber that fits under a laser-based Raman spectrometer.

"Our contribution to significantly increase accuracy was to get the thermocouple inside the chamber and precisely control and measure temperature and pressure," Gazel said. By analyzing carbon dioxide behavior, researchers can determine its density and calculate the original depth of magma storage, he added.

The method was applied to samples from three Hawaiian volcanoes representing different evolutionary stages:

• Kilauea, an active "shield" volcano, showed magma storage at shallow depths of 1-2 kilometers, consistent with previous findings;
• Haleakala, in the post-shield stage, revealed dual storage zones: one shallow at approximately 2 kilometers and one deep at 20-27 kilometers in the Earth's mantle; and
• Diamond Head, a rejuvenation-stage volcanic vent on the island of O'ahu, showed magma stored around 22-30 kilometers deep, all within the Earth's mantle.

"Knowing these depths precisely matters, because to understand the drivers of eruptions, one of the most important constraints is where magma is stored," Gazel said. "That is fundamental for physical models that will explain eruptive processes and is required for volcanic risk assessment."