The Basics - What To Know Before Maunaloa Erupts (Part 1)

Maunaloa has been in the news more and more over recent weeks and months. We know Maunaloa will erupt again, but the when and where about the next eruption remain unknown for the planet’s largest active volcano. This multipart written series will examine some of the most important aspects of Maunaloa relevant for those living on her slopes before the next eruption. Part of preparation is simply knowing what can be expected based on the recorded eruptive history, and some of lessons learned from the 2018 eruption of Kilauea. At the time of writing there is no immediate threat from Maunaloa, rather these posts will be for the purposes of preparation. “The unseen enemy is always the most fearsome.” ― George R.R. Martin Maunaloa did not get to be the largest volcano on our island by not erupting, but in the last 70 years the volcano has been relatively quiet. There are two rift zones that descend from the summit caldera of Maunaloa that will be frequently referenced throughout this series, the Southwest Rift Zone (SWRZ) and the Northeast Rift Zone (NERZ). An large eruption with the right conditions on the NERZ could potentially reach Hilo, and has done so in the past, most recently the 1880-1881 eruption. An eruptive vent on south side of the NERZ can flow towards Piʻimauna and the Kīlauea Caldera. A SWRZ eruption could potentially impact anywhere between Honaunau and Nāʻālehu, with varying likelihood that will be discussed in a following post. The last eruption on Maunaloa was in 1984 (at the same time Kīlauea was erupting), 1975 before that, with the span between 1984 and the present day representing the longest period in recorded history without an eruption. The USGS-HVO alert level of Maunaloa is currently set at “Advisory”, the second out of the four possible ascending alert levels, indicating elevated activity over what has been determined to be background levels of activity by volcanologists. The idea that Maunaloa is “overdue” is not really valid, as the volcano does not care for our brief perceptions of time and even shorter documented history. Prior to the lull in activity following the 1950 eruption, Maunaloa was entirely different from most of our experience of the volcano. If we go back to the first recorded eruption in 1832 up to and including the 1950 eruption, the frequency of eruptions comes out to having one eruption on average every 3.6 years [1]. The eruption of 1950 was a cap on a relatively active period in modern history, when a rather large eruption on the SWRZ that destroyed many homes. An eruption similar to 1950 today would be significantly more destructive and impactful, as the population in the relevant areas has dramatically increased over the last 71 years, and that is without mentioning the impacts our modern tourism driven economy would take for as much as a near miss from Maunaloa. While the 1950 eruption of Mauna Loa was the most destructive eruption in Maunaloa’s modern history, Hilo has had its share of close calls. Five times in the last 170 years lava has flowed from NERZ fissures and came within a few miles of the most mauka part of Hilo [2]. None of those eruptions really show what Maunaloa is capable of however. The legendary Pana`ewa lava flow from over 1,000 years ago covered the coastline from Ha`ena (Shipman's Estate) near Hawaiian Paradise Park, all the way to the Hilo Bay Front. That millennia old eruption is estimated to be even more voluminous that the 2018 eruption of Kilaeua (1.5km^3 vs 1.72km^3) [3][4]. With all that said, the odds of a lava flow from Maunaloa running through a community is relatively low overall due to the volcano’s immense size. One way I describe the likelihood of Maunaloa flowing into heavily populated areas is by comparing it to a roulette wheel, where only landing on “green’ means lava flows into a semi-densely populated area. Kailua-Kona is sheltered by another volcano, Hualalai, where if an eruption were to happen the odds of it being catastrophic is significantly higher, more like a coin flip. Looking at modern history, there are gonna be many more spins of the Maunaloa roulette wheel than flips of the Hualalai coin. In the last 33 eruptions of Maunaloa there hasn’t been a pyroclastic eruption, but there exists evidence of explosive activity between 300 and 1,000 years ago, and those examples were related to summit activity, not on the rift zones, and are not suspected to be significantly large [5]. The likelihood of a destructive pyroclastic flow on Maunaloa is so low that this will be the last time it’s discussed. The rest of this series will look at the differences between the two rifts zones, the most common sequences for eruption, the Trusdell Inundation Zone Map and how to use it, radial eruptive history outside of the rift zones, and lessons learned from the 2018 eruption that could repeat and impact residents the next time lava enters a residential area. [1] Gordon A. Macdonald, (1983), Volcanoes in the Sea: The Geology of Hawaii, 1983 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, (1990), Implications of historical eruptive-vent migration on the northeast rift zone of Mauna Loa Volcano, Hawaii, https://doi.org/10.1130/0091-7613(1990)018<0611:IOHEVM>2.3.CO;2 [3] U.S. Geological Survey, (2020), Have Humans Influenced Volcanic Activity on the Lower East Rift Zone of Kīlauea Volcano? A Publication Review https://pubs.usgs.gov/of/2020/1017/ofr20201017.pdf [4] U.S. Geological Survey, (2001), Volcano Watch — When did Moku`aweoweo (the summit caldera of Mauna Loa) form? https://www.usgs.gov/center-news/volcano-watch-when-did-mokuaweoweo-summit-caldera-mauna-loa-form [5] U.S. Geological Survey, (retrieved 2021), Geology and History, Earth's Largest Active Volcano, https://www.usgs.gov/volcanoes/mauna-loa/geology-and-history Image of the 1975 Maunaloa eruption, credit to USGS. Edited by Philip Ong.