Mighty Mauna Loa

Mauna Loa, the world’s largest active volcano, has erupted many times — most recently three decades ago with lava coming within just miles of Hilo — and it will erupt again, posing a significant risk to those who call Hawaii Island home, a Hawaii Volcano Observatory geologist said Wednesday.


Mauna Loa, the world’s largest active volcano, has erupted many times — most recently three decades ago with lava coming within just miles of Hilo — and it will erupt again, posing a significant risk to those who call Hawaii Island home, a Hawaii Volcano Observatory geologist said Wednesday.

But, the million dollar question remains: When is Mauna Loa going to erupt again?

“When the Puu Oo eruption stops on the east rift zone of Kilauea,” said Frank A. Trusdell, of the U.S. Geological Survey, who has studied Mauna Loa for the past two decades. Historically, eruptive periods at Kilauea and Mauna Loa volcanoes appear to be inversely correlated. In the past, when Mauna Loa was exceptionally active, Kilauea was in sustained lava lake activity, he said.

“If the flanks become stable and buttress Mauna Loa’s flank, then Mauna Loa erupts, but if there is an eruption along the rift zones of Kilauea and that mobilizes, in turn, the south flank, the (Kilauea) side of the volcano can’t buttress Mauna Loa, so, Mauna Loa’s flank keeps moving,” Trusdell explained. “It keeps creating more and more space, so, the magma supply has to fill up all that excess space and it reduces the pressure.”

Currently, Mauna Loa’s flank is moving toward Kilauea, reducing pressure on Mauna Loa thus lessening the likelihood of an eruption there, he said.

Nevertheless, while Mauna Loa may not appear as though it’s ready to erupt today, Trusdell stressed the importance of being prepared because time can be limited depending on where you live. According to data presented by Trusdell, scientists estimate, depending on the location of the actual eruption on Mauna Loa, it would take the lava about 29 hours to reach the shoreline below the Hawaiian Ocean View Estates subdivision of Ka‘u, as little as three hours to reach the South Kona shoreline, eight days to reach Kiholo Bay and some 280 days to reach Hilo Bay.

The USGS suggests all people who live on or near Mauna Loa learn about and keep up to date on the volcano’s hazards and develop a family emergency plan in the event of an eruption.

“Mauna Loa puts out lots of lava and has a steep slope, and, if you don’t heed the warning you’re going to end up in trouble,” Trusdell said.

Dozens of people packed the Ocean View Community Center in Ka‘u for Trusdell’s presentation titled “Mauna Loa: How Well Do You Know the Volcano in Your Backyard?” It was the first USGS presentation of its kind in the community, which is located on Mauna Loa’s southwest flank.

The talk was also part of Hawaiian Volcano Observatory’s Volcano Awareness Month, held annually in January for the past five years to increase awareness of the volcanoes on which we live. On Jan. 22, HVO Scientist-in-Charge Jim Kauahikaua and geologist Janet Babb will recount the volcanic history, stories and impacts of volcanic features flanking Highways 11 and 190 in the Ka‘u and Kona districts during a public presentation titled “Stories of lava flows and volcanic landscapes from Ka‘u to North Kona.” The presentation is slated for 5:30 p.m. at the Natural Energy Laboratory of Hawaii Authority’s Gateway Visitor Center in Kailua-Kona.

Hawaii Island is comprised of five volcanoes, Kohala, Mauna Kea, Mauna Loa, Hualalai and Kilauea — the latter three remain active today. Kohala last erupted 60,000 years ago and Mauna Kea 4,500 years ago, he said. Mauna Loa last erupted in 1984 and Hualalai in 1800-01 with a failed eruption, which included earthquakes, flames and gas but no lava, in 1929. Kilauea has been erupting since 1983.

Mauna Loa is the world’s largest active volcano, said Trusdell, noting it extends 13,697 feet above sea level and about 3,100 feet below sea level. Its name meaning “long mountain” in Hawaiian. Mauna Loa covers more than 50 percent of Hawaii Island, extending into Hilo, most of the southern portion of the island and toward Kiholo Bay in North Kona.

It’s also 500 times greater in volume than Mt. Rainier, the picturesque volcano in Washington state.

“Mauna Loa is so huge that it actually causes the Pacific plate it is residing on to sag under the weight of the volcano,” said Trusdell, noting that like Kilauea, Mauna Loa remains in the shield stage, featuring a caldera at the summit some 3.7-by-1.9 miles — much larger than Kilauea’s 1.2-by-1.9 miles.

The volcano also has the capability to pump out a lot more lava than Kilauea, Trusdell said. Kilauea erupts 0.2 to 0.5 million cubic meters each day while Mauna Loa puts out 12 million cubic meters per day.

“What Kilauea puts out in one day, in Mauna Loa’s last eruption in 1984, Mauna Loa erupted in 20 minutes,” Trusdell said.

That 1984 eruption was the last to occur on Mauna Loa. The eruption, nearing its 30th anniversary, occurred between March 24 to April 15, 1984, according to the USGS. Lava flows came within 4.5 miles of Hilo. In 1950, lava erupted from a fissure on Mauna Loa’s southwest rift zone, at an elevation of almost 10,000 feet, crossing Highway 11 in three places and destroying about two dozen structures before reaching the South Kona shoreline about three hours later, according to the USGS.

In all, Mauna Loa has erupted 33 times since 1843, which is when written records of volcanic eruptions began post European contact, Trusdell said. Of those eruptions, about half stayed within the caldera, flowing at most 6.2 miles to 9.3 miles from the summit. Outside the caldera, an estimated 25 percent occurred along the northeast rift zone and 20 percent occurred along the southwest rift zone.

Eruptions from radial vents are less frequent, occurring 6 percent of the time, but are more hazardous because of the potential proximity to inhabited areas, he said. Radial vents, or eruptive fissures located outside the rift zone or summit, are found on the north and northwest flanks of Mauna Loa, including underwater.

Since 1843, three radial eruptions have been recorded including one in 1852 that propagated toward Pohakuloa, a submarine eruption in 1877 in Kealakekua Bay and an eruption at the 11,000-foot elevation on the northwest flank of Mauna Loa that sent lava all the way to Kiholo Bay — 31 miles — in just eight days in 1859.

“For Mauna Loa, the trump card for volcanic hazards are the radial vents,” Trusdell said. “If you live on a flank and a radial vent pops up in your backyard, then, well, your response times are very abbreviated.”

While only 170 years of written history exists for Mauna Loa, Trusdell said geologists have studied data and used radiocarbon dating to determine how many times Mauna Loa has erupted during the past 30,000 years. About 35 percent of all the lava on Mauna Loa has been dated, said Trusdell. He said 500 lava flows have been mapped and 300 flows radiocarbon dated.

Over the past 3,000 years, Trusdell said, Mauna Loa erupted once every six years and evidence of four explosive events have been found. Since written records began, the volcano has erupted about once every five years, he said.

“What the volcano did in the past is your best guide to what it will do in the future,” Trusdell said, noting that the data is compiled into maps used by emergency officials, as well as residents and planners, to determine the threat level.

The USGS monitors Mauna Loa using numerous seismic, GPS and tilt stations positioned on its flanks. When unrest is detected, usually a swarm of earthquakes greater than magnitude 1.8, scientists notify emergency officials and increase monitoring efforts.


“When we start issuing these notices, you should get your things in order,” Trusdell told attendees.

For more information, visit hvo.wr.usgs.gov, email askHVO@usgs.gov or call 967-7328. HVO scientists also compile a weekly column to keep residents up to date on Hawaii Island’s volcanoes. “Volcano Watch” is featured every week in the Sunday edition of West Hawaii Today.

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