What causes a volcano to erupt?

A volcanic eruption is a means of releasing heat and pressure from beneath Earth’s crust. In some cases, that happens when magma rises through boundaries between tectonic plates, such as around the “Ring of Fire,” a region prone to earthquakes and volcanic eruptions that lines the Pacific. In the case of Mauna Loa and Hawaii’s other volcanoes, eruptions are the product of what’s known as a hot spot — one where Earth’s mantle is abnormally hot and molten rock rises toward the surface because it’s less dense and therefore more buoyant than solid rock.

What type of volcano is Mauna Loa?

Mauna Loa is what’s known as a shield volcano, formed by repeated fluid lava flows that tend to give the volcano a gently sloping, rounded profile like that of a shield. That is where Mauna Loa gets its name — it means “long mountain” in Hawaiian. That is in contrast to composite volcanoes like Mount Hood in Oregon, Mount Rainier in Washington or Mount Fuji in Japan that have what to many is a more classic volcano shape, with tall, steep sides created by repeating layers of lava flows, volcanic ash and cinders, according to the U.S. Geological Survey .

Is Mauna Loa dangerous?

Eruptions from Mauna Loa can be dangerous, especially if they send lava spewing down the volcano’s western slope, volcanologists said. That side of the mountain is steep enough that lava flows can reach populated areas within hours. For the ongoing eruption, lava is flowing out of the northeast side of the volcano, where the slopes are more gradual and populated areas are not in imminent danger. If the current Mauna Loa eruption continues for long enough, and if lava flows are voluminous enough, the current eruption could still prove dangerous. Lava could reach the outskirts of Hilo, a populous area on the eastern side of Hawaii Island, within perhaps a week, said Ken Hon, scientist in charge of the Hawaiian Volcano Observatory. “Hopefully it will have relatively minor effects on residents and visitors to the island,” Hon said. Explosive eruptions are rare at Mauna Loa, but not unheard of. There hasn’t been one since 1843, according to USGS, but geologists have found evidence of at least four explosive eruptions over the past millennium. Eruptions can also be associated with earthquakes and tsunamis. In 1868, a tsunami and landslides associated with an eruption killed 77 people, according to the National Park Service. Hawaii officials said they don’t believe there is an elevated risk of earthquakes tied to Sunday’s eruption.

How many active volcanoes are there?

Mauna Loa is one of six active volcanoes in Hawaii that rose from the Pacific Ocean floor over the past 70 million years. Of these, one is an underwater volcano yet to rise above the ocean surface. It is among about 160 active or potentially active volcanoes USGS monitors around the United States , mostly along the West Coast and the southern Alaskan coast. And around the globe, there are about 1,350 active or potentially active volcanoes on land, about 500 of which have erupted during the past century, according to USGS.

What is the largest volcano in the world?

Mauna Loa is the largest active volcano on Earth, at 13,680 feet above sea level and more than 56,000 feet from its peak to its underwater base, according to USGS. Its volume is estimated to be some 18,000 cubic miles. Tamu Massif, an underwater structure discovered in the Pacific east of Japan in 2013, was thought to be Earth’s largest volcano by far — it covers 120,000 square miles, compared with Mauna Loa’s 2,000-square-mile footprint, scientists said in announcing its discovery . But research published in 2019 said Tamu Massif isn’t actually a volcano , as scientists previously thought, but instead an unusual and exceptional buildup of oceanic crust at the junction of three ocean ridges.

How often do volcanoes erupt?

There is no rule of thumb to dictate when it’s time for lava to start flowing. While Mauna Loa had not erupted since 1984, its close neighbor on the Big Island, Kilauea, has been erupting routinely over the same period, with its current eruption ongoing since September 2021. A volcano erupts somewhere on Earth about once a week, on average, according to USGS. At Mauna Loa, there have been signs that flow into the volcano’s magma chamber has been increasing over the past 20 years, said Falk Amelung, a professor at the University of Miami who has studied the volcano. And a spate of small earthquakes that began in September indicated that magma began pushing its way through rock. By Sunday night, it had nowhere else to go, Amelung suggested: “The pressure got so much, it ruptured the magma chamber and came to the surface.”

Mauna Loa, the world’s largest volcano, is erupting. Here’s why and what will happen next.

By Scott Dance

November 29, 2022 at 10:03 a.m. EST

Video taken Nov. 28 shows the extent of lava flow from Hawaii's Mauna Loa after it erupted one day earlier. (Video: The Washington Post)

5 min

For the first time in a generation, lava began flowing from Mauna Loa at the heart of Hawaii’s Big Island late Sunday. No one knows when it will stop, or what, if any, damage it might cause.

But there is a lot we do know about the more than 700,000-year-old volcano, and others like it.

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Hawaii’s Mauna Loa volcano erupts for first time in 38 years

Here are some answers to common questions about the geologic marvel.