March 25 is the 30th anniversary of the most recent eruption of Mauna Loa, Hawaii Island’s largest volcano. According to historical records, Mauna Loa has erupted 33 times since 1843, an average rate of one eruption every five years. When we look at the past 3,000 years of Mauna Loa’s eruptive history, we see it has erupted about once every six 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 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.
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 one event every six years for the past 3,000, and one every five years for the past 170 years, it hardly seems likely that the volcano is on its deathbed.
Some people might think the ongoing eruptions at Kilauea’s summit and east rift zone are robbing Mauna Loa of magma. We know that Kilauea 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 Kilauea 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.
Looking at the past 170 years, researchers have noted that eruptive periods at Kilauea 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 Kilauea’s pressing against Mauna Loa would have minimal effect on the larger edifice, but Mauna Loa’s pushing against Kilauea 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 because of 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 Kilauea 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 Hawaii Island can rest assured that we are diligently monitoring Mauna Loa and are better equipped to inform you if activity at the volcano changes.
Kilauea activity update
A lava lake within Halemaumau produced nighttime glow visible via HVO’s webcam during the past week. The lava level fluctuated between 125 and 190 feet below the rim of the overlook crater.
On Kilauea’s east rift zone, the Kahaualea 2 flow continued to be active northeast of Puu Oo. The active flow front was 5.1 miles northeast of the vent on Puu Oo on Wednesday. Webcam images indicate that small, lava-sparked forest fires continue to burn.
Two earthquakes were reported felt on Hawaii Island in the past week. At 1:25 p.m. March 13, a magnitude 3.5 earthquake occurred 5 miles southwest of Kawaihae at a depth of 7 miles. At 11:08 a.m. Wednesday, a magnitude 2.7 earthquake occurred 1 mile southwest of Paauilo at a depth of 10 miles.
Visit hvo.wr.usgs.gov for Volcano Awareness Month events and Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.