(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

This past Wednesday marked the silver anniversary of Mauna Loa’s most recent eruption. The 22-day eruption, which began March 25, 1984, generated much excitement and anxiety. 

Its beautiful fountains and long aa flows sent lava to within 4 miles from the outskirts of Hilo. In reviewing the past, one can’t help but compare Mauna Loa, the largest volcano in the world, to its small, but highly admired neighbor Kilauea, now in its 27th year of nearly continuous eruption. 

At 13,680 feet, lone Mauna Loa towers above Kilauea — about 3 times as tall when measured from their respective submarine bases to their summits. 

After the roughly 3-week 1984 eruption was over, Mauna Loa had erupted more than 20 times the amount of lava that Kilauea would produce for a similar period. 

Will the much larger Mauna Loa also dwarf Kilauea in the creation of vog? 

The production of SO2 gas by Kilauea has created chronic air quality problems for downwind Hawaii Island communities, and vog-related losses have resulted in a U.S. Department of Agriculture-approved Secretarial Disaster Declaration for Hawaii Island farmers. 

An eruption of Mauna Loa will generate plenty of vog-forming volcanic gases and particles. The amount of SO2 gas released (the main culprit in the formation of vog) will be commensurate with the intensity of the eruption. 

During the 1984 eruption, a satellite-based instrument tracked the Mauna Loa SO2 plume for 15 days and measured between 130 and 190 million tons of SO2, an amount equivalent to about 50 days of SO2 emissions from Kilauea at the average 2008 rate.   

The effects of vog on populated areas will, however, be influenced by Mauna Loa’s great height. Much of the volcano soars above the trade-wind inversion, a layer of warm air that overlies the cooler air below it. This layer, which is present about 82 percent of the time, occurs at an average altitude of 7,400 feet around Hilo. The exact height of the inversion layer is controlled by time of day, season, precise location, and a variety of climatic effects. 

The inversion layer acts like a lid and keeps the cooler, denser air and lower local pollutants below it. When vents below the inversion layer erupt—such as the summit, east rift zone vents, and ocean entries on Kilauea—the emissions are trapped below and are likely to affect downwind populated areas. 

Emissions from eruptive activity above the trade-wind inversion—such as the summit and upper flanks of Mauna Loa—will likely stay aloft during the day but could be carried by downslope winds at night and impact lower-elevation communities. 

For example in 1984, satellite sensors tracked the sulfur dioxide eruption cloud as it first moved to the north and later to the southwest; it appeared to stay above the inversion layer. As far as we know, no one living below the inversion layer reported unusual gas levels during the eruption. 

The short duration of Mauna Loa eruptions in the past 160 years suggests that periods of eruption-produced poor air quality would persist for days to weeks, rather than years, as with the current Kilauea eruption. 

Because typical Mauna Loa eruption rates have historically been 6-24 times those of Kilauea, and because gas emissions are generally proportional to eruption rates, those limited periods of extremely poor air quality could be quite intense. 

As we wish the 1984 eruption a happy anniversary, let’s hope that the next Mauna Loa eruption is equally spectacular, equally brief, and somewhere above the inversion layer. 

More information on Mauna Loa’s anniversary activities, with her faithful companions in volcano monitoring and emergency management, can be found at http://hvo.wr.usgs.gov/.