Eruptions on Maunaloa Start At The Summit Before The Rift Zones (Part 2)

What To Know Before Maunaloa Erupts... Written with Philip Ong. Virtually all eruptions of Maunaloa historically will start at the Moku’aweoweo caldera, and it’s our expectation that the next eruption of Maunaloa will continue that trend. In the last 750 years, no eruption that originated within the Moku’aweoweo caldera has reached areas now densely populated. Rather, a summit eruption represents a strong warning from the volcano for residents to brace for the next potential phase of activity, a potentially more voluminous eruption on one of the rift zones. It’s possible that the next summit eruption would transition into the rift zone within a few hours from the onset of an eruption at the summit, as many eruptions have in the past [1]. However, it’s also possible that the next eruption will be a summit-only eruption, and not descend into the rift zones at all. In determining the likelihood of next eruption being entirely confined to the caldera or making its way to the rift zones, there are several factors to examine. The historical record contains summit-only eruptions, summit-to-rift eruptions with rapid transitions (and one that was delayed), and what we will refer to as extended summit-summit-rift sequences. The summit-only type eruptions are those that occur surrounding Moku’aweoweo caldera and do not transition into the lower part of either rift zone, with these eruptions lasting anywhere between a couple of hours and multiple years. It doesn’t have to be wholly confined to the caldera walls, encompassing fissures that extend out of the caldera for roughly a mile into both rift zones and still be considered a summit eruption. The eruption may go on for some time and produce lava flows several miles long, but still well away from people. The summit-to-rift type of eruption also begins with a summit phase but transitions to a lower rift zone phase within hours to days. These are the eruptions with the highest potential to impact island communities. Historically, once the eruption moves into one of the rift zones activity does not migrate to the other rift zone, which will be important factor in any crisis response as each rift displays historically different eruption rates and impacts, as detailed in a later article. If we look back at the eruptions since 1832, nearly half have migrated outside of the summit region, with 8 eruptions on the Northeast Rift Zone, 7 on the Southwest Rift Zone, and 2 radial eruptions on the north flank of the volcano [2]. While the odds of an eruption transitioning into the rift zone without any further consideration is nearly a coin-flip, over 90% of eruptions in the historical record that move into the lower rift zones do so within 5 days of the onset of eruptive activity at the summit. If this time period is elapsed and no lower rift zone eruption has occurred, then the odds of that eruption source migrating to the rift go down considerably. However, it is conceivable that a summit-only eruption makes it easier for the following eruption to occur as a summit-to-rift [3]. This is loosely suggested by the most recent historical record of summit-only eruptions, followed by a repose period lasting from months to years, and then a summit-to-rift eruption. The 1949 eruption of Maunaloa was a summit-only eruption, the 1950 eruption then began at the summit and quickly moved into the Southwest Rift Zone. The next eruption was a summit-only eruption in 1975, which was followed by a summit-to-rift eruption into the Northeast Rift Zone in 1984. These 4 eruptions showcase this extended summit-summit-rift sequence, which is also noted in the two previous pairings of eruptions, first in 1933 and 1935-36, then in 1940 and 1942. If this tentative pattern were to continue then the next eruption of Maunaloa would be a summit-only eruption, but the eruption after that would ultimately transition into one of the rift zones. There are all kinds of complexities that make this pattern questionable, for one the duration between the 1984 eruption and today of 37 years is largely without precedent in the historical record of Maunaloa. The previous longest repose period between eruptions was 25 years between the eruption in 1950 and the summit-only eruption of 1975, making most of the historical record prior to 1950 of frequent Maunaloa eruptions inconsistent with the last 71 years. There are also considerations on the magma supply rates to Maunaloa, as well as the interplay with Kilauea, which will be addressed in a later article. There was also the eruption of 1880-1881, which threatened the then small town Hilo. That eruption started at the summit with a six days of fountaining that illuminated the night’s sky before going dark once more. Six months later the Northeast Rift Zone erupted without revisiting the summit, creating one instance in the historical record that breaks from either the quick-transition or summit-first historical trend of activity, out of 34 total eruptions. Whatever the next eruption of Maunaloa holds in store for us, we at least know that it will start at the summit with a high degree of confidence, but determining if the eruption will move into the rift zones is another question entirely, for which the first five days will be carefully watched. [1] Gordon A. Macdonald, (1983), Volcanoes in the Sea: The Geology of Hawaii, https://books.google.com/books?id=IuADTBNksO0C&pg=PA72&lpg=PA72&dq=mauna+loa+%22summit+flank+sequence%22&source=bl&ots=3pp6qAZkwa&sig=ACfU3U0SrKz2m_8n-wISndfsUjWPF1B4IQ&hl=en&sa=X&ved=2ahUKEwjczam7mLvxAhWGuJ4KHWjtBRwQ6AEwDXoECAIQAw#v=onepage&q=1950&f=false [2] John P. Lockwood, (1987), Holocene Eruptive History of Mauna Loa Volcano, Professional Paper 1350 - Volumes I and II, Chapter 18:509-535pg. https://www.researchgate.net/publication/262223101_Holocene_eruptive_history_of_Mauna_Loa_Volcano_Hawaii/link/00b495371379331de2000000/download [3] Fred Klein, (1982), Patterns of historical eruptions at Hawaiian volcanoes, Journal of Volcanology and Geothermal Research https://www.researchgate.net/publication/222854572_Patterns_of_historical_eruptions_at_Hawaiian_volcanoes Image credit: Decker, Robert W.; Wright, Thomas L.; Stauffer, Peter H. (1987). "Volcanism in Hawaii". "Aerial view of lava cascades into Lua Hou, upper southwest rift zone. Cascade height about 90 m."