Mauna Loa: a deep dive into Earth's largest active volcano
Mauna Loa is the largest active volcano on Earth by volume — a vast, gently sloping shield that takes up half of Hawaii's Big Island and rises 4,170 metres above sea level. Measured from its base on the ocean floor, the edifice is more than 9 kilometres tall and roughly 75,000 cubic kilometres in volume. It is the kind of thing one only really understands by flying over it.
A shield built drop by drop
Like Kīlauea, Mauna Loa is a hot-spot shield, but it has been constructed by a much higher long-term lava output. Fluid basaltic flows have piled up over hundreds of thousands of years to make a dome so wide that it depresses the oceanic crust beneath it. The summit caldera, Mokuʻāweoweo, is roughly 6 kilometres long.
The Northeast and Southwest Rift Zones
Most large Mauna Loa eruptions break out not at the summit but along two long rift zones running northeast and southwest from it. These zones are where fresh lava reaches inhabited country fastest, and they are why historical flows have approached Hilo on one side and South Kona on the other.
The 1984 eruption and its long silence
The previous big event was a three-week fissure eruption in 1984 whose flows came within about 7 kilometres of Hilo before stalling. After that, Mauna Loa entered the longest documented quiet period in its recorded history — 38 years.
The 2022 reawakening
On 27 November 2022, fissures opened along the Northeast Rift Zone and fed a fast-moving lava channel that crossed several kilometres of high plateau and threatened the Daniel K. Inouye Highway. The eruption ended after about two weeks without reaching populated areas, but it reminded everyone that Mauna Loa is still very much active.
The summit observatory
The summit of Mauna Loa hosts NOAA's Mauna Loa Observatory, one of the world's most important atmospheric monitoring stations. The Keeling Curve — the long record of atmospheric CO₂ — has been measured here since 1958. The 2022 eruption briefly cut power and access to the site.
Climbing the mountain
Mauna Loa is a serious mountaineering objective. The standard route is a long, exposed walk via the Mauna Loa Cabin at 3,400 metres and the summit cabin at 4,000 metres. Altitude sickness is common; weather can turn from sun to blizzard in minutes. Most visitors instead drive the Saddle Road to the trailhead and turn back.
Hazards to the future
USGS scenarios place much of South Kona, Kaʻū and the Hilo side within the long-term lava-hazard zones of Mauna Loa. The 1950 eruption sent a flow to the sea in three hours. The 2022 event was modest by comparison — but a worst-case Mauna Loa eruption is a much bigger problem than a worst-case Kīlauea one.
Why Mauna Loa matters
Mauna Loa is the model for what a really large hot-spot volcano looks like. It is the volcano whose silent decades flatter our sense of control over the place. It is also, simply, the largest active volcano on the planet — a fact that the gentle slope on the horizon does not quite communicate.
On the map
Open the map and find Mauna Loa filling the middle of Hawaii's Big Island. Kīlauea sits on its southeast flank; Mauna Kea, dormant but slightly taller, rises to the north.