Scientists with the USGS Hawaiian Volcano Observatory (HVO) are intrigued by three observations from the past several months that underscore that the world’s largest volcano is still an active, restless one that will erupt at some time in the future. While an eruption of Mauna Loa is not imminent, HVO says “now is the time to revisit personal eruption plans. Similar to preparing for hurricane season, having an eruption plan in advance helps during an emergency.”
Mauna Loa is considered the largest active volcano on Earth, rising to 13,681 feet above sea level on the Big Island of Hawaii. Mauna Loa rises up from the ocean floor of the Central Pacific at a depth of about 3 miles. Because of the volcano’s significant mass, the ocean floor directly beneath Mauna Loa is depressed by another 5 miles. According to USGS, this places Mauna Loa’s summit about 56,000 feet above its base; the enormous volcano covers half of the island of Hawaii, also known simply as the “Big Island of Hawaii.”
Mauna Loa eruptions tend to produce voluminous, fast-moving lava flows that can impact communities on the east and west sides of the Big Island from Kona to Hilo. Since the 1850s, Hilo in eastern Hawaii has been threatened by 7 Mauna Loa lava flows. On the south and west sides of the island, Mauna Loa lava flows have reached the coast there 8 times: in 1859, 1868, 1887, 1926, 1919, and three times in 1950.
According to USGS, while Mauna Loa is not erupting right now , rates of deformation and seismicity remain elevated above long-term background levels. GPS measurements continue to show slow, long-term summit inflation consistent with magma supply to the volcano’s shallow storage system. A slight increase in the rate of inflation that began in January continues.
HVO research geophysicist Ingrid Johnson penned an update describing three observations that have been particularly interesting to scientists studying Mauna Loa. Recent deformation, caldera subsidence, and changes of tilt have caught their eye.
According to Johnson, the direction of deformation at Mauna Loa’s summit has reversed twice since October 2020, switching back and forth from an outward, expansive direction to an inward, contractional one. The most recent contractional episode shortened the distance across Mauna Loa’s summit caldera, known as Mokuʻāweoweo, by only about 0.6 inches. This is small compared to amount of expansion that has taken place, but Johnson adds the sudden reversal is new behavior that hasn’t been observed at Mauna Loa with modern instruments.
The second observation was the occurrence of a small, shallow magnitude-3.2 earthquake in Mokuʻāweoweo on March 6. Using satellite radar, HVO was able to see that the earthquake caused nearly 4 inches of subsidence in a small portion of the caldera. Johnson writes, “This is very unusual for a magnitude-3.2 earthquake and indicates that the fault may have slipped all the way up to the ground surface.”
Johnson says the third observation is the smallest but may be the most meaningful. “Beginning on March 23, a tiltmeter near Mauna Loa’s summit recorded a change in tilt of about 5 microradians. It is not unusual to see this much tilt at Kīlauea, but there has never before been a tilt signal that could be attributed to changes in Mauna Loa’s volcanic system.” According to HVO, changes in tilt at Mauna Loa have been observed due to daily or seasonal temperature changes and/or because of precipitation , but never before due to volcanic deformation.
With another eruption on Mauna Loa inevitable, although the timing is not yet defined, the USGS is urging people on Hawaii to have a personal response plan, prepare a “go bag”, and determine what one would do in the event of an eruption at different times of the day or week.
“The most important thing you can do is to have a personal response plan,” says the USGS. Document what you’d do when a volcano erupts and make sure your family and friends are aware of what that plan is.
USGS suggests getting a “go bag” in order. “Nowadays, people pack “go” bags containing essential items in case you have to leave your house under an evacuation order. You may want to include important documents, like your birth certificate, deeds, legal papers, and medications.”
USGS says people in Hawaii should develop plans that factor in different types of days and time of day: if family members are at work or school at specific times, the plan should address what people should do and how they should communicate if an eruption occurs when people aren’t home. USGS says, “It is useful to also have a communication plan, so you can be in touch with those you care about.”
Mauna Loa is one of 5 volcanoes that make up Hawaii’s Big Island. The oldest volcano on Hawaii Island is Kohala, which is more than one million years old. Kilauea is the youngest, at an estimated 300,000-600,000 years old. Mauna Loa is the second youngest volcano on the island, estimated to be about 700,000 years old.
Mauna Loa is a shield volcano with long, broad slopes that fall off into the ocean. Lava eruptions from Mauna Loa are very fluid and are typically non-explosive. Magma for Mauna Loa and Kilauea, the latter which has been erupting since December, is sourced from a hot spot in the middle of the Pacific Plate. Because the plate is slowly drifting, Mauna Loa will eventually move away from the hotspot, become an extinct volcano in the next 500,000-1,000,000 years.
Mauna Loa’s most recent eruption occured from March 24 to April 15 in 1984. Lava from the 1984 flow spilled over near the summit while additional fissures drove flows in the general direction of Hilo. The flows stopped roughly 4 miles outside of Hilo’s city limits. No lava has flowed from Mauna Loa since.
While scientists are certain Mauna Loa will erupt, they aren’t exactly sure when yet or where lava will flow from the next eruption. Previous flows impacted the communities of Puako and Waikoloa Beach on the northwest coast, Kailua-Kona and Captain Cook on the west coast, Milolii and Ocean View on the southwest coast, and Hilo on the east coast. Because of the wide range of impacts in past eruption events, the USGS is encouraging people island-wide to prepare for the possibility of volcanic activity. Beyond the hazards of fast moving lava flows, there could be toxic volcanic gasses and a volcanic haze known as vog, as well as fall-out of volcanic debris such as ash or volcanic glass.
Hawaii also doesn’t have a monopoly on volcanic threats in the United States. While USGS rated Kilauea as the biggest volcanic threat in the U.S. in its updated 2018 list of volcanic dangers, Mauna Loa is only 16 on the list. Washington’s Mount St. Helens and Mount Rainer, Alaska’s Redoubt, California’s Mount Shasta, and Oregon’s Mount Hood and Three Sisters are considered to be even more dangerous.