Mauna Loa is the largest volcano on earth and one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean. It is an active shield volcano, with a volume estimated at approximately 18,000 cubic miles (75,000 km³), although its peak is about lower than that of its neighbor, Mauna Kea. The Hawaiian name "Mauna Loa" means "Long Mountain". Lava eruptions from Mauna Loa are silica-poor, thus very fluid: and as a result eruptions tend to be non-explosive and the volcano has extremely shallow slopes.
The volcano has probably been erupting for at least 700,000 years and may have emerged above sea level about 400,000 years ago, although the oldest-known dated rocks do not extend beyond 200,000 years. Its magma comes from the Hawaii hotspot, which has been responsible for the creation of the Hawaiian island chain for tens of millions of years. The slow drift of the Pacific Plate will eventually carry the volcano away from the hotspot, and the volcano will thus become extinct within 500,000 to one million years from now.
Mauna Loa's most recent eruption occurred from March 24, 1984, to April 15, 1984. No recent eruptions of the volcano have caused fatalities, but eruptions in 1926 and 1950 destroyed villages, and the city of Hilo is partly built on lava flows from the late nineteenth century. In view of the hazards it poses to population centers, Mauna Loa is part of the Decade Volcanoes program, which encourages studies of the most dangerous volcanoes. Mauna Loa has been intensively monitored by the Hawaiian Volcano Observatory (HVO) since 1912. Observations of the atmosphere are undertaken at the Mauna Loa Observatory, and of the Sun at the Mauna Loa Solar Observatory, both located near its summit. Hawaii Volcanoes National Park covers the summit and the southeastern flank of the volcano, including a separate volcano, Kīlauea.
Mauna Loa is the world's largest shield volcano. Mauna Loa is shaped like a shield, because its lava is extremely fluid (it has low viscosity), and its slopes are not steep. Eruptions are rarely violent, and the most common form is in the Hawaiian style, which involves lava fountains feeding lava flows. Typically, at the start of an eruption, a rift up to several kilometers long opens, with lava fountains occurring along its length in a so-called "curtain of fire." After a few days, activity normally becomes concentrated at one vent.
Eruptions generally occur in three regions on the mountain: at the summit and in two rift zones extending northeast and southwest of the summit. About 38% of eruptions in the last two hundred years have occurred at the summit, 31% in the northeast rift zone, and 25% in the southwest rift zone. The remaining 6% have occurred from vents to the northwest of the summit, away from the rift zones. Its summit caldera is called Mokuaweoweo; it is 1.75 to 3 miles (3–5 km) in diameter. The caldera probably formed 1,000–1,500 years ago when a very large eruption from the northeast rift zone emptied out the shallow magma chamber beneath the summit, which then collapsed.
Seismic data can reveal the locations of the magma chambers beneath the volcano which feed activity. Some types of seismic waves, known as "S-waves," cannot travel through liquid rock, so magma chambers cast 'shadows' in seismic data. Seismic shadows reveal a magma chamber about 1.75 miles (3 km) beneath the summit and smaller magma bodies beneath the rift zones.
Trade winds blow from east to west across the Hawaiian islands, and the presence of Mauna Loa strongly affects the local climate. At low elevations, the eastern (windward) side of the volcano receives heavy rain, and the city of Hilo is the wettest in the United States. The rainfall supports extensive forestation. The western (leeward) side has a much drier climate. At higher elevations, the amount of precipitation decreases, and skies are very often clear. Very low temperatures mean that precipitation often occurs in the form of snow, and the summit of Mauna Loa is described as a periglacial region, where freezing and thawing play a significant role in shaping the landscape.
The hotspot has existed for at least 80 million years, and the Emperor Seamounts chain of old volcanoes stretches almost away from the hotspot. Currently, the hotspot feeds activity at five volcanoes: Mauna Loa, Kīlauea, and Hualālai on the Big Island, Haleakalā on Maui, and Loihi, a submarine volcano south of the Big Island and the youngest Hawaiian volcano. Mauna Loa is the largest of these, although Kīlauea is currently the site of the most intense volcanic activity.
Prehistoric eruptions of Mauna Loa have been extensively analyzed by carrying out radiocarbon dating on fragments of charcoal found beneath lava flows. The mountain's prehistoric activity is probably the best known of any volcano. Studies have shown that a cycle occurs in which volcanic activity at the summit is dominant for several hundred years, after which activity shifts to the rift zones for several more centuries, and then back to the summit again. Two cycles have been clearly identified, each lasting 1,500–2,000 years. This cyclical behavior is unique to Mauna Loa among the Hawaiian volcanoes.
Records show that between about 7,000 and 6,000 years ago Mauna Loa was largely inactive. The cause of this cessation in activity is not known, and no known similar hiatus has been found at other Hawaiian volcanoes except for those currently in the post-shield stage. Between 11,000 and 8,000 years ago, activity was more intense than it is today. However, Mauna Loa's overall rate of growth has probably begun to slow over the last 100,000 years, and the volcano may in fact be nearing the end of its tholeiitic basalt shield-building phase.
An especially big eruption in 1935 was large enough that its lava flows threatened Hilo, leading to an unusual employment of air power. Five bombers of the 23d and 72d Bombardment Squadrons of the United States Air Force dropped bombs ahead of the lava in order to divert it away from Hilo.
Until 1950, eruptions occurred roughly every 3–4 years, but since then the repose periods have dramatically lengthened with the only eruptions occurring in 1975 and 1984. The two most recent eruptions have been the most extensively studied. The 1975 eruption lasted only two days and occurred at the summit of the mountain. The 1984 eruption saw fissures open up to the northwest and southeast, from the summit down to above sea level. Flows from this eruption headed rapidly towards Hilo again, but stopped about from the outskirts when the eruption ended after three weeks.
The volcano has been dormant since 1984. Seismic activity remained low until 2002, when there was a sudden onset of inflation, and the caldera walls started to move apart at a rate of per year. This is thought to indicate that magma is filling a reservoir about beneath the summit. The inflation has been intermittent, sometimes slowing, and sometimes stopping for several weeks. Thus far, though, it has always restarted, and this is likely to indicate an increased probability of an eruption in the next few years.
The inflation has been accompanied by increased seismic activity. A swarm of deep earthquakes began in July 2004, and continued until the end of the year. Earthquakes were detected at a rate of one per day for the first three weeks, increasing steadily over subsequent months to 15 or so per day by the end of the year. The swarm ended in December 2004, and earthquake levels have been only moderately elevated since then.
Kīlauea lies on the southern flank of Mauna Loa and was originally thought to be a satellite vent of Mauna Loa. However, chemical differences between the lavas from the two volcanoes show that they have separate shallow magma chambers. They are now considered separate volcanoes. Nevertheless, activity patterns at the two volcanoes do appear to be correlated.
The most apparent relation between the two mountains is that, generally, periods of frequent activity at one volcano coincide with periods of low activity at the other. For example, between 1934 and 1952, Kīlauea was dormant and only Mauna Loa was active, while from 1952 to 1974, only Kīlauea was active while Mauna Loa lay dormant.
The 1984 eruption of Mauna Loa started during an eruption at Kīlauea, but had no discernible effect on the Kīlauea eruption. Occasionally, though, eruptions at one volcano do seem to influence activity at the other. The recent inflation of Mauna Loa's summit began on the same day as a new large lava flow broke out at Kīlauea's Puu Ōō crater. Geologists have suggested that a "pulse" of magma entering Mauna Loa's deep plumbing system could have increased pressure inside Kīlauea and triggered the eruption.
The main volcanic hazard at Mauna Loa is lava flows. Most flows advance at about walking pace and present little danger to human life, but eruptions at Mauna Loa can be more intense than those at Kīlauea; for example, the 1984 eruption emitted as much lava in three weeks as Kīlauea's current eruption produces in three years. Such high emission rates can generate comparatively fast-moving flows.
Two eruptions of Mauna Loa have destroyed villages. In 1926, the village of Hoōpūloa Makai was overrun by lava flows. In 1950, the most voluminous eruption ever seen at Mauna Loa sent lava flows racing towards the sea. The village of Hookena Mauka was destroyed on June 2, 1950 by the advancing flows. Hilo is partly built on lava from an 1880 eruption and is at risk from further lava flows. The brief but intense 1984 eruption saw lava flow towards Hilo, but it had not reached any buildings when the eruption stopped.
A recent example of the risks associated with slumps occurred in 1975, when the Hilina Slump suddenly moved forward by several yards. A magnitude-7.2 earthquake resulted which triggered a small tsunami with a wave height of a few yards.
Mauna Loa is an intensively monitored volcano. The Hawaiian Volcano Observatory (HVO) was established in 1912 to observe the Hawaiian volcanoes, and the HVO has developed many techniques to help predict when eruptions at Mauna Loa and other volcanoes are imminent.
One of the most important tools is seismometry. More than 60 seismometers around the Big Island enable scientists to measure the intensities and locations of hundreds of small earthquakes every week. Earthquakes can begin to increase years before an eruption actually starts: The 1975 and 1984 eruptions were both preceded by one to two years of increased seismic activity at depths of less than .
Another type of seismic activity occurs in the hours preceding an eruption. So-called harmonic tremor is a continuous "rumble" which contrasts with the normal seismic activity of sudden shocks and is believed to be caused by the rapid movement of magma underground. Volcanic tremor normally indicates an imminent eruption, although it may also be caused by shallow intrusions of magma which do not reach the surface.
Another important indicator of what is happening underground is the shape of the mountain. Tiltmeters measure very small changes in the profile of the mountain, and sensitive equipment measures distances between points on the mountain. As magma fills the shallow reservoirs below the summit and rift zones, the mountain inflates. A survey line across the caldera measured a 3-inch (76 mm) increase in its width over the year preceding the 1975 eruption and a similar increase before the 1984 eruption.