Mauna Loa, meaning “Long Mountain” in Hawaiian, is the largest active volcano on Earth, covering over half of the Island of Hawaiʻi. This massive shield volcano extends 30,000 feet from its base on the ocean floor, making it taller than Mount Everest when measured from bottom to top. Its immense size and proximity to populated areas like Hilo and Kailua-Kona make its eruptions a serious hazard for the island’s residents and infrastructure. Since 1843, Mauna Loa has erupted 34 times. The Hawaiian Volcano Observatory (HVO) constantly monitors the volcano, as 90 percent of its surface has been covered by lava flows within the last 4,000 years, confirming its persistent activity.
The Immediate Threat of Lava Flows
The primary consequence of a Mauna Loa eruption is the massive volume of fast-moving basaltic lava flows it produces. Its low-viscosity lava is exceptionally fluid, allowing it to travel long distances quickly, permanently altering the landscape and destroying everything in its path. The steep slopes of the volcano’s flanks, particularly on the western side, accelerate these flows toward vulnerable coastal communities.
An eruption originating from the Southwest Rift Zone (SWRZ) presents the most immediate danger due to the steep gradient. Historically, flows from the SWRZ have reached the ocean in a matter of hours. During the 1950 eruption, the lava traversed the distance from the vent to the coast in as little as three hours, leaving very little time for evacuation or response.
Lava flows from the Northeast Rift Zone (NERZ), while generally slower, pose a significant threat to Hilo, the island’s largest population center. The 1984 eruption sent lava flows within four miles of the city limits, and Hilo has been threatened by seven Mauna Loa flows since the mid-19th century. The lead time for a NERZ flow to reach populated areas is measured in days to weeks, offering a longer window for action.
The sheer volume of erupted material is staggering; the 2022 eruption produced more than 8.8 billion cubic feet of lava, extending the flow front over 12 miles. This high effusion rate means that large areas of land are rapidly covered by molten rock, which can be tens of feet thick. The resulting new land surface is uninhabitable and impossible to traverse, fundamentally changing the geography of the island.
Disruption to Critical Infrastructure
Beyond the immediate destruction caused by molten rock, an eruption would cripple essential services across the Island of Hawaiʻi by severing lifelines. The Daniel K. Inouye Highway, also known as Saddle Road, is especially vulnerable, as NERZ lava flows directly cross the saddle region between Mauna Loa and Mauna Kea. The loss of this single highway would isolate the east side of the island (Hilo) from the west side (Kona), preventing the movement of essential goods, emergency services, and personnel.
The physical destruction of linear infrastructure is a foregone conclusion in the flow path. Power lines are easily taken out by the advancing lava, leading to widespread utility outages. Communication networks, including fiber optic cables and microwave transmission sites, would also be severed, disrupting telephone and internet services.
Water supply systems, particularly pipelines and pumping stations, are also at risk of being cut or damaged by lava and associated seismic activity. The isolation resulting from the simultaneous failure of transportation, power, and communication would complicate sustained response efforts. Communities would become islands of limited resources, reliant on air and sea transport until new infrastructure could be built over the cooled lava, a process that can take many months or years.
Atmospheric and Health Hazards
A Mauna Loa eruption releases massive quantities of sulfur dioxide (\(\text{SO}_2\)) gas, which poses a significant airborne health hazard. This gas reacts chemically with sunlight, oxygen, moisture, and dust particles to create Volcanic Smog, or VOG. VOG is a hazy mixture of gas and tiny sulfate aerosol particles carried far downwind from the eruption site.
The primary health concern from VOG is respiratory irritation, which can be severe for sensitive populations. Individuals with pre-existing conditions are particularly susceptible to the effects of \(\text{SO}_2\) exposure, which can cause narrowing of the airways and lead to increased hospital admissions. Health officials advise reducing outdoor activities and staying indoors with windows closed when VOG levels are high.
VOG severely impacts agriculture, as the sulfuric acid droplets in the smog have corrosive properties. This acid can cause chemical burns on plant leaves, leading to significant crop damage. Additionally, the acid rain created by VOG can leach toxic metals from roofing materials into rainwater catchment systems, which many residents use for drinking water.
While Mauna Loa is primarily an effusive volcano, fine ashfall is possible, especially near the vent, impacting air quality and visibility. Significant ash plumes can ground local air traffic and pose a threat to aircraft engines, requiring the Federal Aviation Administration to issue aviation warnings. These airborne hazards affect the entire island, not just those in the path of the lava.
Preparedness and Response Actions
The most important factor in managing a Mauna Loa eruption is the high probability of advance warning due to intensive monitoring efforts. The USGS Hawaiian Volcano Observatory (HVO) maintains a sophisticated network of instruments that track seismic activity, ground deformation, and gas emissions. These data provide scientists and emergency managers with days to weeks of lead time before an eruption is likely.
Upon confirmation of a likely flank eruption, the County of Hawaiʻi Civil Defense Agency coordinates the official response, including mandatory evacuation plans for at-risk zones. Residents living downhill of the rift zones are instructed to evacuate early, especially those on the steeper western slopes where flow speeds are fastest. The mobilization of emergency services, including the National Guard and federal agencies, focuses on securing evacuation routes and assisting displaced residents.
The preparedness framework emphasizes that residents must develop personal emergency plans, including “go” bags with supplies and documentation, and heed all guidance from civil defense officials. The ability to communicate rapidly changing hazards and the community’s response to official guidance are paramount in mitigating the threat.