Kilauea— An explosive volcano in Hawai‘i

ïlauea Volcano on the Island of Hawai‘i, though best known for its frequent quiet eruptions of lava flows, has erupted explosively many times in its history—most recently in 2011. At least six such eruptions in the past 1,500 years sent ash into the jet stream, at the cruising altitudes for today’s aircraft. The eruption of 1790 remains the most lethal eruption known from a U.S. volcano. However, the tendency of Kïlauea’s 2 million annual visitors is to forget this dangerous potential. Cooperative research by scientists of the U.S. Geological Survey, Smithsonian Institution, and University of Hawai‘i is improving our understanding of Kïlauea’s explosive past and its potential for future violent eruptions.

Kilauea…has not always been the tame creature of today.

Hitchcock, C.H., 1909, Hawaii and its Volcanoes, p. 167

At 2:58 a.m. on March 19, 2008, a gaping, glowing hole, visible from the U.S. Geological Survey’s (USGS) Hawaiian Volcano Observatory (HVO) 1.8 km (1.1 mile) away, opened in the floor of Halema‘uma‘u Crater almost beneath the visitor overlook. A scientist working all night at HVO heard and felt nothing. Just after dawn, three geologists drove to investigate. Thick, sulfurous fume blowing across the road 500 m (yards) downwind from the hole obscured visibility. The driver, knowing the road was straight, tightly gripped the steering wheel and crept ahead. Crunch, bump—oh no, must have driven off the road! Then came the sudden realization that the car was still on the road but was lurching over rocks blown out of the crater. This was how the first explosive eruption at the summit of Kïlauea since 1924 was discovered.

Later in 2008 and then in 2011 eight more similar events took place. All were tiny by comparison to explosive eruptions in Kïlauea’s past, but they jarred the public into realizing that Kïlauea could be explosive. Long considered a “milquetoast” of a volcano, erupting only photogenic lava fountains and flows, Kïlauea has the capability of quickly turning violent. Its eruption of 1790 killed a large number of people (estimates, all made decades afterwards, range from about 80 to more than 5,000) within a few hundred meters of where the Jaggar Museum and the adjacent Hawaiian Volcano Observatory stand today. And, in 1924, a too-adventurous accountant was killed while photographing the largest in a 17-day series of explosive eruptions at the volcano's summit.

Kïlauea Volcano, on the Island of Hawai‘i, is best known for its frequent sustained eruptions of lava flows and lava fountains, which are gentle enough that they can be observed and enjoyed from close range. Occasionally, however, Kïlauea erupts explosively, hurling out rocks and plumes of fine pulverized material. Several small explosions occurred in 2008 and 2011, like this one on October 12, 2008, but much larger explosive eruptions have oc- curred in past centuries, sometimes with deadly consequences. Inset shows a large block thrown out of Halema‘uma‘u Crater during an explosive eruption on March 19, 2008.

Although they are not its usual style, Kïlauea produces explosive eruptions about as often as does Mount St. Helens in Washington State—several periods of multiple explosive events every millennium. Those at Kïlauea are generally smaller in volume and in area affected, but their risks are substantial. More than 5,000 people a day visit Kïlauea’s summit, where most of the explosive eruptions take place, and hundreds live nearby. A busy highway now passes through the area where so many died in 1790—a tragedy that, by itself, makes Kïlauea historically the most lethal of all volcanoes in the United States.

What Is the Nature of Explosive Eruptions at Kïlauea and How Large Can They Be?

Technically, even lava fountains are explosive eruptions. For the purpose of this fact sheet, however, an eruption at Kïlauea is considered to be explosive if fragments of broken rock from the vent walls (called “wall rocks”) are ejected, generally but not always accompanied by fresh shreds and droplets of magma (molten rock). Lava fountains at Kïlauea eject only fresh magma particles and so are not considered explosive; in contrast, the explosive eruption of March 19, 2008, threw out only wall rocks.

Most explosive eruptions at Kïlauea are relatively small, but they can be hazardous to people and infrastructure. The pulverized wall rocks (and magma droplets) fall as volcanic ash around the vent and downwind. During heavy ash fall, breathing is difficult, and the weight of ash, especially when wet, can cause roofs of buildings to collapse. Such hazards are mainly of concern to people near the summit area, but more distant effects can also be substantial. In 1924, ash falling 40 km (24 miles) from the summit shut down a railroad, and a layer of ash amounting to 4.9-7.4 tonnes/hectare (2-3 tons/acre) accumulated 30 km (18 miles) away, contaminating grass used for livestock feed. The 2008 explosive eruptions were much smaller than those of 1924, creating a hazard only directly adjacent to the vent. Such tiny eruptions could have happened many times in the past but their record may be overlooked today because they left such limited, transient deposits.

Much more powerful eruptions occurred in the more distant past. One large explosive eruption at Kïlauea’s summit 1,000-1,200 years ago sent golf-ball-size dense wall rocks to the south coast of the island, 18 km (11 miles) away. One block 24 cm (9.5 inches) across and weighing 4.43 kg (9.7 lbs) fell 5 km (3 miles) from the vent. Several other large explosive eruptions, including the deadly one in 1790, produced volcanic ash that blew southeast, east, and northeast of the summit, as recognized from the distribution of the ash deposits. Dispersal of ash in those directions could not have resulted from the normal trade wind, which blows from the northeast, or the less common south wind. Instead, such easterly dispersal implies that the eruption columns penetrated high into, or even above, the west winds of the subtropical jetstream, which in Hawai‘i blow year-round at altitudes of 4-17 km (13,000-56,000 feet). Volcanic ash drifting at these altitudes poses a clear hazard to modern aircraft—a chilling prospect never dreamed of a few years ago for Hawaiian volcanoes.

The deadly explosive eruption of Kïlauea in 1790 hurled numerous large blocks of rock—more than 1 meter (more than 3 feet) in diameter—out of the caldera, some as far as 2 kilometers (1.4 miles) beyond the rim.

Explosive eruptions of Kïlauea between 1500 and 1790 included the occurrence of at least a dozen fast-moving, ground-hugging clouds of ash, rock, and volcanic gas. Such “pyroclastic surges” are among the most dangerous of all volcanic phenomena— speeding at hurricane velocities and having temperatures of several hundred degrees Celsius. Ongoing research indicates that a surge probably caused the deaths in 1790. Surge deposits have been recognized as far as 3.5 km (2.1 miles) outside the summit caldera (the 4 km by 3 km volcanic depression within which lies the vent of Halema‘uma‘u Crater).

Although explosive eruptions at Kïlauea are infrequent in human terms, they are not rare geologically. Besides the eruption of 1924, significant explosive eruptions occurred repeatedly during a 300-year period between about 1500 and 1790, and also repeatedly between about 500 and 1000 C.E. Many deposits of older explosive eruptions are known but poorly understood. The eruptions appear to occur in clusters, during times when a deep caldera exists at Kïlauea’s summit. This is clear for the 1500-1790 and 500-1000 periods. The 1924 events, however, don’t fit this picture easily.

Not all of Kïlauea’s explosive eruptions take place at the volcano’s summit. Several hundred years ago, blocks of wall rock were ejected from ‘Alae Crater (now buried by recent lava flows) and from Pu‘ulena, both on the east rift zone (a zone of fractures and eruptive vents trending east from the summit area). Even Pu‘u‘Ö‘ö, site of an ongoing eruption on the east rift zone since 1983, had small explosive eruptions in 1987, 1991, and

In explosive eruptions, larger fragments of wall rock are thrown out shorter distances than smaller fragments, as shown here by samples of fragments ejected by the Kïlauea eruption of around 850-950 C.E. Small dots are observation sites, larger dots are sites of photographed fragments. Inferred extent of ash fall from the eruption is shaded.

ASH DISPERSION BY THE JETSTREAM

1996 and tiny bursts in 2008 and 2009; these eruptions were similar in size to those at the summit in 2008.

What Causes Kïlauea’s Explosive Eruptions?

Groundwater or surface water probably has played a major role in most of Kïlauea’s explosive eruptions. If water is heated under

WHAT CAUSES EXPLOSIVE ERUPTIONS AT KÏLAUEA?

pressure and the pressure then relieved, it can flash into steam, rapidly expanding and providing the propellant for explosive eruptions. Scientists theorize that, when the caldera is deep enough to be at or below the water table (the level below which rocks are saturated with water), water can seep into the vent and explosive eruptions can take place. The water table is currently about 500 m (1,640 ft) below the floor of the caldera. If the caldera collapsed to about that level—as it did around 1500 C.E., probably remaining there until 1790—explosive eruptions could likely occur. In 1924, geologists noted that lava drained from Halema‘ma‘u Crater to a depth apparently below the water table; rock falls from the collapsing walls of the crater dammed rising steam, pressure built up, and explosive eruptions occurred.

Not all of Kïlauea’s explosive eruptions may be related to groundwater, however. The summit explosive eruptions of 2008, and likely also those at Pu‘u‘Ö‘ö, were apparently propelled by gas carried by, and released from, magma independent of groundwater. The very powerful eruption around 800-1000 C.E. carried up fragments of almost solidified magma that was apparently crystallizing at a depth of 5-6 km (3-3.5 miles) when the eruption began. At such depths, carbon dioxide (CO2) is bubbling from the magma, and perhaps a buildup in CO2 pressure triggered that explosive eruption.

Can Explosive Eruptions Be Foreseen?

Could we forecast future explosive eruptions at Kïlauea? Whenever the bottom drops out of the caldera, or of a crater (like Halema‘ma‘u) within it, explosive eruptions become a realistic possibility. But explosive eruptions driven mainly or exclusively by magmatic gas may be harder to anticipate. The explosive event in March 2008, for example, was foreseen in only the most general way as a possible outcome of increased summit unrest (increased seismicity and gas output)–and everyone was surprised by the form it actually took. Deep-seated pressure buildup, such as might

Date (C.E.) 1790

occur before a CO2-driven explosive eruption, would be very difficult to detect with current monitoring capabilities.

As volcanoes go, Kïlauea is a pretty safe place, generally producing lava fountains and lava flows quietly when it erupts. Once in a while, however, it will erupt explosively. That is when the volcano becomes the most hazardous and when lives could be lost without prompt, proper warnings and actions. USGS scientists working out of HVO, and their collaborators from the Smithsonian Institution, the University of Hawai‘i, and other organizations, continue to closely monitor Kïlauea and study how the volcano works. Improved understanding of its workings will enable better protection of the public from the hazards of Kïlauea’s occasional explosive episodes.

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