Mauna Loa vs Mount St Helens: How the eruptions differ and what NW volcanoes are doing now

Washington state’s Mount St. Helens erupted catastrophically in 1980, killing 57 people. That eruption sent ash soaring over 80,000 feet and raining down as far as 250 miles away.

Mauna Loa is not exploding like Mount St. Helens did. The magma in Mount St. Helens tends to be stickier and traps more gas, making it much more likely to explode when it rises. It’s of a variety called composite volcanoes, which form concave cones.

Mauna Loa and Mount St. Helens erupt in different ways,” said Liz Westby, a geologist based at the US Geological Survey’s Cascades Volcano Observatory in Vancouver, Washington. “Mauna Loa trends toward eruptions of fluid-flowing lava (compositionally called basalt), which over time, has built its broad shield shape.”

In contrast, she said, “Mount St. Helens trends toward eruptions of thicker, more viscous lava (compositionally called dacite and andesite) that doesn’t flow very far and can pile up near the vent to form lava domes. Gases can be trapped in the thick and viscous magma as it waits in the shallow magma reservoir and the systems can become pressurized, sometimes leading to explosive eruptions.”

Westby added, “With lava flowing from fissures high on the flank of Mauna Loa, the chance of an explosive eruption is greatly diminished (gases are escaping).”

Mauna Loa’s hotter, drier, and more fluid magma allows the magma’s gas to escape and lava to flow down the side of the volcano the way it is starting to do now. Mauna Loa is called a shield volcano because the long, broad flanks built by repeated lava flows give it the appearance of a warrior’s shield.

On Friday, the United States Geological Service posted a routine update on Northwest volcanoes:

Activity Update: All volcanoes in the Cascade Range of Oregon and Washington are at normal background levels of activity. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State; and Mount Hood, Mount Jefferson, Three Sisters, Newberry, and Crater Lake in Oregon.

Recent Observations: Over the past week, earthquakes were located at Mount St. Helens, Mount Rainier, and Mount Hood, including a seismic swarm at Mount St. Helens from November 20–21 and a M 2.1 earthquake at Mount Rainier on November 21. These earthquakes are consistent with background levels of seismicity at each volcano. Activity at other Cascade volcanoes is also at background levels.

Westby noted Mauna Loa has historically had explosive eruptions, although not like Mount St. Helens’ 1980 lateral blast, and Mount St. Helens has had fluid flows of basalt. The Ape Cave lava tube on the south side of Mount St Helens is an example of a basalt flow, she said.

On the Big Island, the first eruption in 38 years of the world’s largest active volcano dredged up bad memories among some Hawaii residents who have been through harrowing volcanic experiences in the past.

It was just four years ago that Nicole Skilling fled her home near a community where more than 700 residences were destroyed by lava. She relocated to the South Kona area, only to find herself packing her car with food and supplies this week after Mauna Loa erupted late Sunday.

In 1989, Alaska’s Redoubt Volcano, another composite volcano, belched an 8-mile cloud of ash that clogged all four engines of a KLM Royal Dutch Airlines jet. The plane fell 13,000 feet before all engines restarted and the plane landed without injury to the 245 people aboard.

Mauna Loa released some ash this time but on a vastly smaller scale than these examples from composite volcanoes.

The eruption began Sunday night at its summit after a series of large earthquakes. It then spread to vents that formed in a rift zone where the mountain is splitting apart and it’s easier for magma to emerge.

These vents are on the mountain’s northeast side and lava emerging there could head toward Hilo, which is on the east side of the island.

Ken Hon, scientist-in-charge at the Hawaiian Volcano Observatory, said he doesn’t expect additional vents to form on the volcano’s southwest rift zone during this eruption. That means communities to the west would be spared lava flows this time.

Mauna Loa also erupted from the northeast in 1984. That time, lava headed toward Hilo but stopped a few miles short of the city.

Historically, each Mauna Loa eruption has lasted a few weeks. Hon expects the current eruption to follow this pattern.