If Mauna Loa erupted, the most immediate consequence would be fast-moving lava flows threatening communities on the Big Island of Hawaii, followed by volcanic air pollution spreading across the island chain and significant disruption to tourism and agriculture. Mauna Loa is the world’s largest active volcano, and its most recent eruption in 2022 produced 150 million cubic meters of lava covering roughly 14 square miles in just 13 days. That eruption stayed within the summit and upper rift zones, away from populated areas. A larger or differently directed eruption could be far more consequential.
Lava Flows and Who’s in the Path
Mauna Loa’s primary threat is lava, not explosive blasts. The volcano produces fluid basaltic lava that can travel long distances downslope, and the direction it flows depends entirely on where the eruption vent opens. The USGS divides the Big Island into lava flow hazard zones, with Zone 1 covering the summit and rift zones where vents have repeatedly opened in recorded history, and Zone 2 covering areas directly adjacent to and downslope of those rift zones. Communities in Zone 2 sit in the natural drainage paths for lava.
On Mauna Loa’s northeast flank, the city of Hilo (population around 45,000) has been threatened by lava flows multiple times, most notably in 1935 and 1984. Flows from a sustained northeast rift zone eruption could reach Hilo’s outskirts in a matter of days to weeks, depending on eruption rate and terrain. On the western side, the Kona coast’s resort communities and coffee-growing regions face a similar risk from flows traveling down the volcano’s western slopes. The 2022 eruption sent flows toward the Daniel K. Inouye Highway (Saddle Road), the only direct route connecting the east and west sides of the island, before stopping about two miles short.
Lava flows rarely kill people because they move slowly enough to evacuate, but they destroy everything in their path permanently. Homes, roads, farms, and infrastructure buried under lava are unrecoverable. During Kilauea’s 2018 lower East Rift Zone eruption (a smaller volcano on the same island), entire subdivisions were consumed.
Air Quality and Vog
A major Mauna Loa eruption would release enormous quantities of sulfur dioxide into the atmosphere. During the 2022 eruption, emission rates hit approximately 180,000 tonnes per day, creating thick volcanic smog, known locally as “vog,” that spread across the island. Vog is a persistent haze of sulfur dioxide and fine sulfate particles that causes eye irritation, skin irritation, and respiratory problems, particularly for people with asthma or other lung conditions. It also damages crops, harms livestock, and corrodes metal structures.
Where the vog goes depends on wind patterns. Under normal trade wind conditions, volcanic emissions drift to the island’s leeward (western) side, blanketing the Kona coast and sometimes reaching neighboring islands. When winds shift to “Kona winds” blowing from the south or southwest, vog pushes north and east toward Hilo and can spread across the entire island chain. During Kilauea’s prolonged 2018 eruption, vog affected air quality on Oahu and Maui for months. A large Mauna Loa eruption, with its higher elevation vents and potentially greater emission rates, could produce even more widespread air quality problems.
Where active lava meets rain, a secondary hazard forms: acidic steam fog near the ground that severely limits visibility and can make breathing difficult. Near the ocean, lava entering seawater creates “laze,” a corrosive plume of hydrochloric acid and glass particles that is dangerous to anyone nearby.
Ashfall and Flight Disruptions
Hawaiian eruptions are not typically explosive, but Mauna Loa is capable of producing ash under certain conditions, particularly if magma interacts with groundwater or if a vent collapses. For context, the most recent significant ash-producing eruption in Hawaii (Kilauea in 1924) deposited measurable ash 25 miles from the vent.
Even modest ash production from Mauna Loa could disrupt air travel across the Pacific. Under normal trade winds, ash would be carried west of the Big Island, potentially affecting flight routes to the South Pacific and South America. If the ash column rises above roughly 20,000 feet, upper-level winds would carry it to the northeast. Kona wind conditions would push ash clouds north, directly into inter-island and mainland air traffic lanes. The Big Island’s two main airports, in Hilo and Kona, could face temporary closures. During the 2022 eruption, flight disruptions were minimal because ash production was negligible, but a more explosive phase would change that calculus quickly.
Economic Fallout
The best preview of the economic damage comes from Kilauea’s 2018 eruption, which combined with Hurricane Lane caused nearly $975 million in total economic impact on the Big Island. Tourism losses alone reached an estimated $415 million over the following year, with visitor arrivals dropping 20% against the pre-eruption growth trend and visitor spending falling 12.3% year over year. The damage rippled statewide, with an estimated $751 million reduction in total economic output across Hawaii and roughly $100 million in additional tourism losses on Maui alone, even though Maui is a separate island with no volcanic activity.
That drop happened largely because of perception. Potential visitors canceled trips to the entire state, not just the affected area. A Mauna Loa eruption threatening well-known Kona coast resorts or the Hilo area would likely trigger a similar or larger reaction. The Big Island’s coffee industry, centered on the Kona and Ka’u districts on Mauna Loa’s slopes, would face direct risks from lava inundation and indirect damage from prolonged vog exposure, which stunts plant growth and reduces yields.
Road closures compound the economic pain. Saddle Road, the island’s critical east-west corridor, runs directly between Mauna Loa and Mauna Kea. A lava flow crossing that highway would force all traffic onto much longer coastal routes, disrupting supply chains and daily commutes for tens of thousands of residents.
How Much Warning You’d Get
Mauna Loa is one of the most closely monitored volcanoes on Earth. The USGS Hawaiian Volcano Observatory tracks it continuously using seismometers, GPS stations, gas sensors, and satellite imagery. The agency uses a four-tier alert system: Normal, Advisory, Watch, and Warning. Before the 2022 eruption, Mauna Loa showed months of elevated earthquake activity and ground deformation, giving scientists and emergency managers time to prepare.
The 2022 eruption provided roughly two years of escalating warning signs before lava appeared. That pattern is typical for Mauna Loa. The volcano tends to telegraph its intentions through increasing earthquake swarms, measurable inflation of the summit (the ground literally swells as magma accumulates), and changes in gas emissions. This monitoring means a surprise eruption with zero warning is unlikely, though the exact timing and location of a vent opening remain difficult to predict precisely.
Once an eruption begins, the critical variable is vent location. Summit eruptions may stay contained within the caldera. Rift zone eruptions at higher elevations give communities more time as lava travels farther. But vents opening on the lower flanks, closer to populated areas, compress the evacuation window significantly. In past eruptions, lava from Mauna Loa has reached the coast in as little as a few hours from lower rift zone vents.