Mauna Loa is massive. Geologists call it the largest volcano on earth. It covers half of the surface area of the Big Island. Besides it size, Mauna Loa is one of the best-studied mountains on earth. It is also one of the most active volcanoes on the planet. While Kilauea continues to pump out the magma and capture the headlines, the giant Mauna Loa quietly inflates and readies itself for another show. With its mass, history of monitoring and study, and its very active eruptive rate, Mauna Loa is the superlative Hawaiian volcano.
Mauna Loa’s immensity is difficult to comprehend. Though it rises 13,680 feet above sea level, Mauna Loa begins its rise from the ocean floor some 16,400 feet down. So the mountain is over 30,000 feet high, taller than Mt. Everest. For sheer mass it is unequalled. The volcano covers an area roughly the size of all the other Hawaiian Islands combined. I’ve heard one geologist call it “the largest projected landmass between Mars and the Sun.” Another oft-quoted statement is that the entire Sierra Nevada mountain range in California could fit within Mauna Loa. Or how about the fact that Manhattan Island could fit within its summit caldera Mokuaweoweo.
Recently, the staff of Hawaii Volcano Observatory printed in the weekly Volcano Watch a fantastic description of Mauna Loa’s true vertical relief. One reason the ocean is so deep here in Hawaii is because of the downward flexing of the Pacific Plate. The weight of Hawaii’s volcanoes is pushing down the oceanic crust. With new technologies geologists have been able to view with an almost X-ray vision inside the bottom of Mauna Loa. As the volcano has built up, it has also depressed a mirror image of itself down into the sea floor. This depression extends 26,000 feet down from the ocean floor. Therefore, the complete height of Mauna Loa from its true base to the summit is 56,000 feet!
Scientists have been studying and monitoring Mauna Loa since 1843. The new images of Mauna Loa’s great depth are an example of remarkable technology and research. Just as remarkable are the studies that have mapped the surface of the volcano. With nearly a million acres of lava fields, some barren, but many covered with thick vegetation, geologists such as John Lockwood have spent years identifying and dating the different lava fields that cover the mountain. Their efforts, encompassing very nearly the entire surface of the mountain, have led to the best-documented chronology of eruptive activity of any volcano on earth. From their studies they’ve calculated that every 1000 years Mauna Loa covers 40% of her surface area with new lava. The first flow from Mauna Loa that was documented historically was in 1843. Since then the mountain has erupted 33 times. This eruptive record is one of the most active on earth.
Mauna Loa last erupted in 1984. The previous flow to that was 1975. Before both the 1975 and the 1984 eruptions, geologists measured an inflation of Mauna Loa. Magma charges the mountain swelling it like a balloon and with varying technologies this deformation can be measured. They also recorded increased seismic activity two years prior to the eruptions associated with the magma movement. Since the 1984 flow the volcano has inflated from magma storage underground. Mauna Loa has inflated 20 inches since then. Geologists believe that more magma has charged the mountain since the ’84 eruption, than did previous to it. The ’84 flow was not a small eruption. It produced “220 million cubic meters of lava, the third largest volume this century.” Geologists tell us it was putting out enough lava every hour to build a sidewalk from Honolulu to New York City. That flow made it within a few miles of Hilo. Though the mountain continues to swell, currently there is no increased earthquake activity suggesting an imminent eruption. But you can be sure Mauna Loa will erupt again.
When the eruption occurs it will add more mass to the big mountain. And it will offer the students and researchers another chance to observe and learn. Mauna Loa’s lessons extend beyond geology of course. It is not just a mountain of volcanic rock. It contains a wide variety of terrain and ecosystems all organized very nicely among differing ages of well cataloged and dated substrates. This marriage of earth and life sciences makes Mauna Loa a model system for ecological research. As the noted biologist Peter Vitousek says, “To biologists as well [as geologists], Mauna Loa is a place of global significance-not for rocks, but for the organisms and ecosystems that occupy it’s surface.” Mauna Loa is swelling with magma, sinking with weight, and offering us glimpses into some of the life’s lessons.