“Seeing” inside Mauna Loa for the first time in decades

USGS: “Aerial photo taken during a flyover of the Northeast Rift Zone eruption at Mauna Loa at approximately 5-6:30 PM HST on November 28, 2022. This image looks at the line of fissure openings erupting above 10,000 feet elevation on the Northeast Rift Zone of Mauna Loa.” (Image courtesy of Civil Air Patrol)

(BIVN) – Following up on the latest eruption of Mauna Loa volcano, USGS Hawaiian Volcano Observatory geologist Kendra Lynn writes about what was learned from lava samples this week Volcano Watch Article:

Understanding volcanic eruptions requires learning about where the erupted lavas come from deep within the volcano. But how do scientists get information about magma storage when they can’t visit these regions?

Analyzing lava samples can help us “see” inside a volcano, and results from the 2022 Mauna Loa eruption gave us a window into the volcano’s plumbing system for the first time in nearly 40 years.

Mauna Loa began erupting around 23:21 HST, November 27, 2022, for the first time since 1984. The eruption began in the summit caldera, Mokuʻāweoweo, and subsequently a 500 m long (1,640 ft) fissure propagated to the southwest, but remained mostly within the summit.

By early morning the following day, eruptive activity had migrated from the summit into the Northeast Rift Zone (NERZ) at four fissures between 3,755 and 3,365 m (12,320 and 11,040 ft) above sea level, localized to a vent on 2 December. A network of lava channels fed by ‘a’ā flows that extended 19 km (12 mi) down the volcano’s north flank before the eruption ended on 10 December.

The Hawaiian Volcano Observatory’s (HVO) network of monitoring instruments recorded data that was analyzed in real time to better understand the eruption. HVO field crews also made direct observations and measurements that helped assess lava flow advance rates and characterize hazards. One facet of HVO’s monitoring efforts included collecting molten and solidified lava samples almost daily for near-real-time analysis in our laboratories, as was done during Kīlauea’s 2018 Lower East Rift Zone (LERZ) eruption.

Since the eruption of Mauna Loa in 1984, the fields of petrology (the study of rocks and the conditions under which they form) and geochemistry have made great strides. New instruments and techniques are available now, which meant we could learn a lot more about this eruption much faster than in 1984.

Energy-dispersive X-ray fluorescence (ED-XRF) analyzes performed in near real time with our partners at the University of Hawai’i at Hilo revealed the composition of the erupted magma and where it came from. These analyses, performed within 24 hours of sample collection, were later followed by secondary electron microanalysis (SEM) and electron probe microanalysis (EPMA), which allow us to measure compositions of minerals and glasses at very small scales (a few microns). or about 0.00004 inch). This type of rapid analysis was not possible in 1984 and is an example of how chemical information about samples was acquired much faster for the 2022 eruption.

We learned that the erupted lavas were similar to other Mauna Loa compositions that have erupted since 1843. The average MgO (magnesium oxide) content of the lava samples was 6.2% by weight (weight percent), slightly lower than any other Mauna Loa eruption in over the last 200 several years. This data can be used to calculate the temperature at which the lavas erupted and extinguished, which was about 1,155 degrees Celsius (2,111 degrees Fahrenheit).

USGS: “Lava samples collected near Mauna Loa’s Fissure 3 vent (shown in this Dec. 7 flyover image) are glassy and contain bubbles and some very small (200 microns or 0.008 inches long) minerals such as plagioclase and pyroxene, as shown in the gray-scale microscope image inset.” (USGS photos by Kendra J. Lynn)

Samples collected at the vent(s) have no crystals visible to the naked eye, although minerals such as plagioclase, clinopyroxene, olivine, and oxides (all common at Mauna Loa) increase in abundance and size with distance from the vent as the lava flows cool and crystallized downward slope.

All the lava produced during the nearly two-week eruption and from all vents spanning 17 km (10.6 miles) across the summit and upper NERZ has the same composition. This tells us that the entire eruption was fed by a homogeneous magma, and that this almost crystal-free, low-MgO eruption was not affected by rift-stored magma left from 1984.

This is different from the Kīlauea 2018 eruption, which initially produced lavas mixed with cooler stored magma from the LERZ. Instead, the composition of the 2022 Mauna Loa eruption reflects a new intrusion of magma, consistent with earthquake activity that HVO monitored 2–4 km (1.2–2.5 miles) below the summit in the months before the eruption.

Each eruption provides clues to the inner workings of our volcanoes here on the island of Hawai’i. As technology advances, we will improve and expand the ways in which we can study the erupted lavas. We continue to monitor Mauna Loa for future signs of unrest, and future eruptions will provide more insight into Mauna Loa’s inner workings.

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