Chapter 5: Mauna Loa

The boffins started community meetings about this volcano’s restlessness as I was revising the DV book chapter on it. Thought it would be timely to share this updated version (with a few added videos and images)/edited January 17, 2023.

Several islands in the State of Hawaii have spectacular mountain scenery, with canyons, lace-like waterfalls, lush walls of tropical foliage, and massive black to rusty red volcanic cliffs.

The Big Island — which is also called Hawaii — is beautiful, too, but in a different way. There are more broad fields and rolling open terrain here than canyons and other knife-edge landscapes.

Why? Because this island, closest to the Hawaiian hotspot, is still young and rambunctious, buoyed up by Earth’s inner heat and growing faster than water, wind, and plants can wear it down.

Whenever these natural forces start to carve a canyon by eroding out a small gully or slump, sooner or later a lava flow comes along and not only fills that in but also builds up the land even more.

For over half a million years (Lockwood and Lipman), much of that lava has been coming from what we know today as a Decade Volcano: Mauna Loa.

Most people recognize the name, but few of us outside Hawaii have a clear idea of what Mauna Loa actually looks like, for two reasons:

There aren’t many images of it in the media, as there are of the photogenic giants Etna and Teide. No single ground-based camera can take in something this big: the volcano’s dry-land part is about 30 miles wide and more than 70 miles long; its caldera-scarred summit rises 2-1/2 miles above the Pacific Ocean. When the US Geological Survey did an overflight of the restless volcano in late October 2022, this involved so much air travel time that they had to speed up the highlights 2x before sharing them online!

What we are able to see of Mauna Loa — Hawaiian for “Long Mountain” — in a photograph isn’t very exciting. In the title image, it is that (yes) long mountain in the distance, behind Kilauea Volcano’s collapsed 2018 summit caldera: pretty, but not as dramatic as the big hole in the ground.

Once we know what we are looking at, though, Mauna Loa is stunning.

This single volcano makes up 51% of the Big Island, per the Hawaiian Volcano Observatory (HVO).

Clinging to the coasts around it are four more hotspot volcanoes: extinct Kohala and active Mauna Kea, Hualalai, and Kilauea (which is a special case since it’s much younger than Mauna Loa — the others are older; oddball Kilauea also formed on Mauna Loa’s south flank and yet in many ways acts like a separate volcano). (Lipman et al.; Lockwood and Lipman; Miklius and Cervelli)

Offshore submerged volcanoes belonging to this group include ancient Mahukona, long dead, to the northwest and young Kamaehuakanaloa, formerly known as Loihi, which is still thousands of feet below the ocean surface, just off Hawaii’s southeast coast, building up what will become, over the next 200,000 years, the next island in this chain. (HVO, 1995)

Despite these impressive hangers-on, Mauna Loa owns the Big Island.

What is Mauna Loa?

This is one of those simple-sounding questions that’s awfully hard to answer.

The problem is that Mauna Loa isn’t “open” the way Mount Etna is; its rocky archives are thoroughly sealed.

However, while geologists can’t peek inside it, this Decade Volcano’s basics are clearly seen from the outside.

Mauna Loa is a basalt shield volcano, as are all Hawaiian volcanoes at some point during their existence.

It has the typical Hawaiian volcano appearance, too, with vents at the summit and along rift zones extending out from the summit in opposite directions. (Kauahikaua and Tilling)

However, Mauna Loa also has erupted in a few places on its north and west flanks, including an underwater eruption in Kealakekua Bay during the late 19th century. (HVO, 2022a)

There very well could be the remains of earlier small volcanoes inside Mauna Loa, just as there are in massive Etna, but everything is covered by lava flows, with the youngest on top, burying all that came before. Too, this giant volcano’s lower half hasn’t been fully explored yet, since it is underwater. (Lockwood and Lipman)

Submersible technology has made it possible to start mapping the lower flanks (Lipman et al.), but there’s still a long way to go.

You see, Mauna Loa is the largest active volcano on Earth. (HVO, 1998; USGS)

Like Teide, it hides much of its great bulk beneath the ocean’s waves.

The summit rises roughly 13,680 feet above sea level. That’s high enough for snowfall in winter, and both Mauna Loa and Mauna Kea sported glaciers during the last ice age!

Mauna Loa’s flanks extend down another 16,000 feet or so below the ocean surface.

Then the weight of some 10,000 cubic miles of dense basalt rock depresses the Pacific seafloor a further 26,200 feet, making this volcano almost 56,000 feet (10.6 miles) tall from base to summit, dwarfing Mount Everest’s height of 29,032 feet. (HVO, 1995, 1998)

Mauna Kea does rise a little higher above the Pacific than Mauna Loa does, but its submerged part doesn’t go as far down. (HVO, 2002)

This Decade Volcano is like something we might find on another planet!

How did it get so big? Ah, that’s one of the tough questions.

Much of Mauna Loa’s individual history is unknown. But with a prolific hotspot in the neighborhood, there is little doubt about the general outlines of this basalt giant’s story — and its ultimate fate.

The hotspot “assembly line”

That is how volcanologists (HVO, 1995) explain the existence of the Hawaiian Islands (which continue underwater as the Hawaiian Ridge and then as the Emperor Seamounts all the way to Kamchatka!).

According to this HVO source, Hawaiian volcanoes typically go through the same stages:

1. Submarine pre-shield. At some point in Pleistocene times that was more than 500 millennia before the present, what we know today as Mauna Loa was a bubbling seamount, just as Kamaehuakanaloa/Loihi is today. The way this Hawaiian hotspot works, Baby Mauna Loa might have taken some 200,000 years to boost lava production enough for the next step.

2. Shield. This actually describes both Kilauea and Mauna Loa now, with geologically young Kilauea, about 100 millennia old, being in the early explosive phase and Mauna Loa — at least 400,000 years older — possibly on its way out. It takes Hawaiian shield volcanoes about 500,000 years to build up their volume, so Mauna Loa may be as big right now as it’s ever going to get.

3. Post-shield. Not all volcanoes go through this stage. Mauna Kea is one that did, transitioning out of its shield period as much as 250,000 years ago. In this stage, a volcano’s lava chemistry changes, producing tall fountains that pile up lava around vents and eventually fill in the old shield’s caldera. After half a million to one million years of this, all eruptions cease (for a while).

4. Erosion and subsidence. It’s canyon time! Wind, water, and plants have their way, cutting deeply into the volcano. No longer buoyed up by the Earth’s internal heat, the cold, dense rocky edifice also sinks a bit down toward the sea. But its story is not over yet.

5. Rejuvenated/renewed. Even though plate tectonics has now carried the volcano away from the hotspot, occasional eruptions occur; this intermittent activity can go on for millions of years. For example, it’s how some features over on Oahu have formed, including Diamond Head.

After that stage, though, it’s all downhill — literally, with erosion and subsidence first reducing the now extinct volcano to a coral atoll and finally a guyot (submerged mound of basalt).

I know. It’s hard to imagine mighty Mauna Loa turning into Kure atoll, but such indeed has been the fate of all these hotspot volcanoes since dinosaur times — this process has been going on for at least 81 million years (the age of the oldest known formation in the Hawaii/Emperor Seamounts chain, up near Kamchatka).

The general outline of this awesome Hawaiian geologic history was quite well understood back in the International Decade.

What was less clear then, and of more concern, were the hazards that gigantic Mauna Loa posed for people and their lifelines.

Common Hazards at Mauna Loa

Lava, erupted quickly and in huge quantities, and other phenomena associated with an eruption, such as increased volcano-related seismicity (usually less than magnitude 3), toxic gases, and tsunami — these are the most common hazards at Mauna Loa. (HVO, 2022a; Kauahikaua and Tilling; Lipman; Varugu and Amelung)

Hardened lava from its 1984 eruption sits less than five miles from Hilo. During the last two centuries, seven of the nine flows that have erupted out of the volcano’s Northeast Rift Zone headed for that city, one of them getting about a mile from Hilo Bay in 1881. And serving as the foundation for Hilo’s airport, as well as for much of the city’s beachfront, is a single Mauna Loa lava flow that dropped in for a visit some 1,500 years ago. (HVO, 1997, 2019; Kauahikaua et al.;Trusdell and Lockwood)

That’s just the Northeast Rift Zone.
On the other side of the Big Island is the volcano’s Southwest Rift Zone, where construction began in the 1950s on what is now the largest residential development in the United States.

It was built on fissures that had drained lava in 1887 — recently in geological terms but ancient history, over and done with, to most of us.

That project wasn’t the only important event to happen here during the Fifties.

At 9:04 p.m. on June 1, 1950, new fissures opened up at Mauna Loa’s summit and in its Southwest Rift Zone — above and, I think, a little west of the Ocean View development as it exists today — and one of the most spectacular eruptions ever recorded in the Hawaiian Islands began.

By 10:30 p.m., the rift zone had unzipped down to around 8,000 feet, unleashing an 11-mile-long line of huge lava fountains whose floods traveled quickly down the volcano’s steep southwestern flank, crossing the South Kona coast in just three hours. No one died during the rapidly unfolding nighttime event, but they lost property; lava also cut the island’s main highway and its phone lines before reaching the sea. (HVO, 2016; 2022a)

It was a terrifying experience but also the sort of thing that Hawaii’s residents know Madam Pele may throw at them from time to time.

Various forms of lava diversion have been tried at Mauna Loa and Kilauea since at least the 1880s. These range from building barriers to military bombing runs on lava feeding channels during Mauna Loa’s eruptions in 1935 and 1942.

Some attempts failed and other, at most, had equivocal success (since the eruption in these cases ended at around the same time). (Kauahikaua and Tilling)

In 1935, Hawaiians protested the bombing. Cultural traditions are still so strong that even today discussion of any form of lava diversion is controversial. (Kauahikaua et al.)

And volcanologist George Macdonald succinctly described relevant secular concerns while looking into the possibility of protecting Hilo with earthen barriers:

The question of whether a barrier should be built involves complex considerations of relative values of the area to be protected, income to be expected from the area, effects of loss of the area upon the economy of surrounding areas, effects of displacement of population as a result of loss of the area and influence on adjacent areas, cost of construction of the barrier plus interest on the cost, the ability of the community (either locally or at large) to pay this cost, and no doubt other factors. There are also the legal questions arising from diversion of lava onto land that otherwise might not have been covered during that eruption. These questions fall outside the province of the volcanologist and must be decided by economists, sociologists, and lawyers.

The net result: No diversion attempts were made in late 2022 when Mauna Loa’s lava threatened to cut the Big Island’s only central transportation route (Saddle Road).

Reportedly, Hawaii County was considering the option, but the eruption stopped before lava reached this important highway.

Rare But Serious Hazards at Mauna Loa

Strong tectonic earthquakes occasionally hit the Hawaiian Islands. (magnitude 6 or higher).

They can happen when a volcanic edifice slips along the decollement fault that runs between the volcano’s base and the underlying Pacific Ocean floor. (HVO, 2022a; Kauahikaua and Tilling; Lipman; Varugu and Amelung)

This occurred in 2018, for example, when a magnitude 6.9 temblor rocked the island during Kilauea’s East Rift eruption.

Mauna Loa, which is much bigger than Kilauea, can have correspondingly larger earthquakes.

In April 1868, all of the Big Island south and east of Mauna Loa’s summit, where a quiet VEI 2 eruption was in progress, dropped several yards down toward the sea during the Great Kau Earthquake, which was as strong as the shock that devastated San Francisco in 1906. Seventy-eight people died in the associated mudflow and tsunami waves. (Brown et al.; HVO, 2018; National Park Service)

Even more rare, and potentially much more deadly, are flank collapses, which tend to happen in the Hawaiian Islands every 350,000 years or so. (HVO, 2008; Lipman)

The last of these giant landslides happened at Mauna Loa about 100,000 years ago. Gorgeous Kealakekua Bay, located along the Big Island’s western coast, formed when the volcano’s flank gave way there, spreading debris more than 60 miles out across the Pacific seafloor and possibly causing a megatsunami. (HVO, 2008; Kauahikaua and Tilling)

This worst-case event, while something to be aware of, is nothing to obsess over: giant landslides are devastating, but they don’t happen very often. Eruptions and earthquakes aren’t that catastrophic, and those are the hazards at Mauna Loa and elsewhere in Hawaii that people can expect to face during their lifetimes. (Kauahikaua and Tilling; Varugu and Amelung)

In case you are still worried — volcanologists were concerned enough to check it out in the 1990s — just remember that, as we noted at Mount Etna, it’s a little scary but normal for such massive volcanoes to shift around a little bit.

They now have measured “typical” south flank movements at Mauna Loa and Kilauea — respectively, averaging about half an inch per year and 2-3 inches per year. (Lipman et al.) Close monitoring should be able to pick up warning of an impending slide, such as accelerating rate of slip, early enough to take appropriate steps. (Kauahikaua and Tilling)

Such detailed surveillance, although still not perfect, wasn’t possible back when the International Decade began. Yet everyone was aware that multiple hazards existed at Mauna Loa and that the Big Island’s population was soaring.

How could this active Hawaiian giant NOT have become a Decade Volcano?

Mauna Loa and the Decade Volcano program

Hawaii has many active, accessible volcanoes and a historical record that goes back to the 18th century — not surprisingly, one of the world’s first volcano observatories was established here in 1912.

Over the following century, observers monitored twelve Mauna Loa eruptions, but Kilauea had almost fifty over the same interval (Kauahikaua and Tilling), providing the scientists with much more data. This, combined with the fieldwork challenges imposed by Mauna Loa’s vast size, meant that Kilauea was by far the best studied of Hawaii’s two most active volcanoes as the International Decade began.

Volcanologists approached this US Decade Volcano just as they did enormous Teide, in the Canary Islands (Newhall, 1996): by building databases and developing software to systematically map various volcanic hazards for island population centers and lifelines, as well as to provide the most accurate, up-to-date crisis preparedness and emergency management information possible.

The first submersible studies of Mauna Loa’s lower flanks were begun, while geologists near the summit mapped and dated more than 180 lava flows to get an idea of how often eruptions occur. (Newhall, 1999)

1990 saw the establishment of a GPS network on Mauna Loa. This type of monitoring can pick up more subtle movements of the giant volcano’s flanks — giving earlier warning of an eruption or other changes than the older field surveying techniques could. (Lipman et al.)

In 1992, a new lava flow hazard map was published by a team of scientists and representatives from the County of Hawaii (which includes the whole island). It’s still in use today. (Kauahikaua and Tilling)

After “[c]ombined geologic mapping, petrologic and geochemical studies, geochronologic determinations, marine studies, and scientific drilling,” the US Geological Survey reported in 1995 that Mauna Loa appears to be nearing the end of its shield-building days, with fewer eruptions (although those that occur are still huge), probably a much lower rate of magma supply in recent millennia, and not as much land building on its lower flanks during the past 30,000 years. (Lipman, including quote)

They did look at the issue of giant landslides and found that Mauna Loa’s coastal flanks appear to have become more stable, for several reasons, over at least the last 30,000 years and possibly even longer. (Lipman)

Kilauea is still active and growing. Therefore, even though the island’s south flank is probably buttressed and otherwise reinforced (Lipman, Section 6), scientists still keep a careful eye on it. (Lipman et al.)

Much more work has been done on Mauna Loa since the 1990s, but most of it is far beyond the scope of this eBook.

However, the volcano started to get restless again early this century, and it had a brief eruption in November-December 2022 that began in the summit and then migrated into the Northeast Rift Zone.

Between this and Kilauea’s devastating 2018 East Rift eruption, emergency management procedures on the Big Island have been tested to the utmost recently, and the team of scientists and social/political decision-makets have gained experience that will help everyone stay safe when Madam Pele decides to light up the Long Mountain once again.


Location: 19.475° N, 155.608° W, Hawaii, United States. The GVP Volcano Number is 332028.

Nearby Population:

Per the Global Volcanism Program website:

  • Within 5 km (3 miles): 45.
  • Within 10 km (6 miles): 45.
  • Within 30 km (19 miles): 1,906.
  • Within 100 km (62 miles): 175,315.

Current Status:

Advisory/Aviation Code Yellow. Mauna Loa’s restlessness seems to have quieted down after rhe recent eruption, but volcanologists are keeping a close eye on it.

Biggest recorded event:

The one that volcanologists refer to with much respect happened in 1950, when a fissure high on the volcano let loose a lava flow that traveled 15 miles to the sea in under three hours.

Other flows were clocked at even higher speeds. Overall, the rate of lava discharge in 1950 was Mauna Loa’s personal best, at least during recorded history.

An 1859 eruption produced the same volume of lava as in 1950 (enough, per Hirji, to fill the Empire State Building more than 358 times), but took 10 times as long to do this.

Twice as much lava as either of these events was erupted from the summit in 1872 during an eruption that lasted about 1,200 days.

The Hawaiian Volcano Observatory notes on its website that these are the three largest eruptions at Mauna Loa — 1859, 1872, and 1950 — in the last two centuries.

The largest known lava flow at Mauna Loa is the 1500-year-old Panaewa flow that underlies Hilo’s airport and much of its beach front. (HVO, 1997; Kauahikaua and Tilling)


The Hawaiian Volcano Observatory.

They also have online webcams.

The Washington Volcanic Ash Advisory Center (VAAC) monitors Hawaiian volcanoes and issues aviation advisories as needed.

Featured image: National Park Service/J. Wei, public domain


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