Mauna Kea erupts, November 30, 2022. Image: USGS

Mauna Loa’s eruption as of November 30, 2022. (Credit: USGS) Hawaii’s Mokuʻāweoweo, or Mauna Loa, hadn’t seen any external activity for nearly four decades when it erupted in November. Lava flowed from the volcano’s caldera for almost two weeks; ash drifted into downhill communities. Ever since the eruption stopped in early January, however, researchers have hoped to study Mauna Loa’s geological makeup and inner workings.

Now the US Geological Survey (USGS) is “seeing” into the volcano for the first time—not with binoculars or any other visual equipment, but with lava samples. Since its warning earthquakes began, geologists with the USGS Hawaiian Volcano Observatory (HVO) have been eyeing Mauna Loa using direct observations and real-time monitoring equipment. Thanks to lava sample collection efforts, they now have more information about Mauna Loa’s plumbing than ever.

HVO obtained several molten and solidified lava samples, beginning with the volcano’s eruption and ending with its subsequent rest period. HVO partnered with the University of Hawaii at Hilo to analyze the samples using energy-dispersive X-ray fluorescence (ED-XRF) within 24 hours of collection. This technique, combined with secondary electron micro-analysis (SEM) and electron probe micro-analysis (EPMA), unveiled the magma’s composition.

The researchers found that the volcano’s erupting lavas contained a magnesium oxide content of 6.2 weight percent—lower than Mauna Loa lava’s typical concentration. This allowed HVO to determine the temperature at which the volcano erupted: 2,111 degrees Fahrenheit (1,155 degrees Celsius). Solidified lava samples collected from Mauna Loa’s vents contained minerals common to the volcano, like plagioclase, olivine, and clinopyroxene. These minerals were more abundant in samples found a short distance from the vents due to the lava’s natural inclination to cool and crystallize as it made its way downslope. Though concentration varied, these minerals were found in all of the lava samples collected during and after Mauna Loa’s eruption, meaning the eruption was fed by one homogenous magma.

Mauna Loa’s last eruption was in 1984, when ED-XRF, SEM, and EPMA weren’t available methods of rapid analysis. Since there hadn’t been an eruption between then and last year, researchers lacked a way to study the volcano’s plumbing in real-time; instead, they had to use older samples from as far back as 1843. Because the samples from 2022 lack crystals and contain less magnesium oxide than ever, HVO knows Mauna Loa’s most recent eruption wasn’t influenced by residual rift-stored magma from 1984.

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