Hawaii Volcanoes National Park. 1984 eruption of Mauna Loa Volcano. Hawaiian Volcano Observatory geologists at lava fountains. Helicopters provided access to remote areas of the eruption and were essential for safety. Photo by R.B. Moore, 1984. Photo courtesy of USGS

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Hawaii Volcanoes National Park. 1984 eruption of Mauna Loa Volcano. Main vents and spatter rampart at 9200 feet elevation at about 10:45 a.m. Photo by J.D. Griggs, March 26, 1984. Courtesy of USGS

March 25 is the 30th anniversary of the most recent eruption of Mauna Loa, Hawai‘i island’s largest volcano. According to historical records, Mauna Loa has erupted 33 times since 1843, an average rate of 1 eruption every 5 years. If we look further back into Mauna Loa’s eruptive history, over the last 3,000 years, it has erupted about once every 6 years.

These statistics might lead one to believe that the world’s largest active volcano is overdue to erupt. We treat all volcanic hazards as random events; therefore, each event has no influence over the occurrence of the next event. Thus, even if a great deal of time has elapsed since the last eruption, it does not mean that Mauna Loa is due to erupt again soon. Furthermore, with random behavior, even if an eruption occurs this year, there is a chance that an eruption may still occur next year.

We know enough about Hawaiian volcanoes to recognize that the theorem is not 100 percent valid for Mauna Loa, because, once the volcano has erupted its stored magma, it needs time to fully replenish itself.

Hawaii Volcanoes National Park. 1984 eruption of Mauna Loa Volcano. Fountaining at en echelon fissures. Photo by J.D. Griggs, March 25, 1984.

What are some of the other reasons why Mauna Loa has not erupted for some time? Perhaps it is a dying volcano, moving away from the hot spot and nearing its demise. Given the long-term eruption rate of 1 event every 6 years for the past 3,000 years and 1 every 5 years for the last roughly 170 years, it hardly seems likely that the volcano is on its deathbed!

Some people might think that the ongoing eruptions at Kīlauea’s summit and East Rift Zone are robbing Mauna Loa of magma. We know that Kīlauea and Mauna Loa lavas have different abundances of chemical constituents and are fed from different regions of the hot spot. The amount of lava erupted by the volcanoes––a tiny fraction of the magma supplied by the hot spot––is more than enough to feed eruptions at both volcanoes simultaneously.

Another reason proposed for the dearth of eruptions is that the great exhalation of gas from KÄ«lauea somehow decreases the internal pressure within Mauna Loa. The gas hypothesis is analogous to the flawed lava-robbing hypothesis: the volcanoes have different plumbing systems, and degassing at one volcano has no effect on the other.

Hawaii Volcanoes National Park. 1940 eruption of Mauna Loa Volcano. Flows and fountain at Mokuaweoweo, the summit caldera. Photo by G.O. Fagerlund, April 9, 1940. Courtesy of USGS

Looking over the last 170 years, researchers have noted that eruptive periods at KÄ«lauea and Mauna Loa are inversely correlated. Using geologic mapping and C-14 dating, we can tell that, for the past 2,500 years, when activity at one volcano is elevated, the other is relatively quiescent.

What is the reason for this apparent inverse level of activity between the volcanoes? We already discounted decreased magma supply, magma piracy, and reduced pressure from degassing. Another hypothesis involves buttressing effects, when one volcano pushes against the other and causes pressure changes within the other. Researchers have concluded that Kīlauea’s pressing against Mauna Loa would have minimal effect on the larger edifice, but Mauna Loa’s pushing against Kīlauea could increase pressure within the smaller volcano. An analogous study also looked at changing stress in the magma source region and how it impacts magma supply.

A recent hypothesis proposes that eruptive activity at one volcano affects eruptions at the other due to factors that include magma supply, volcanic plumbing, magma pressure, and flank motion (deformation). It is based on the notion that the current east rift zone eruption of KÄ«lauea is driving the south flank seaward, leaving Mauna Loa’s southeast flank unbuttressed. Consequently, Mauna Loa’s flank would move eastward, resulting in the widening of the magma storage centers, decreased magma pressure, and, therefore, diminished ability to erupt.

With the recent unprecedented upgrades to our monitoring capabilities, including seismic equipment and the addition of GPS stations, tiltmeters, gas sensors, and Webcams, we can better investigate the complex interactions within and between the volcanoes. The improved capabilities will increase our understanding of how volcanoes work and enhance our ability to forecast eruptions. The residents of the Island of Hawai‘i can rest assured that we are diligently monitoring Mauna Loa and are better equipped to inform you if activity at the volcano changes.

Mokuʻāweoweo Caldera on Mauna Loa (3/13/14-3/20/14)