A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
August 6, 2014
If it happened yesterday, it can happen tomorrowWe've learned a lot about Kīlauea's explosive history in the last 15 years. Once thought to be rare, explosive eruptions from the volcano's summit are instead frequent and clustered into periods lasting several centuries. For example, between 1500 and the late 1700s, Kīlauea's eruptions were almost always explosive. We can be thankful that Kīlauea is in a quiet period now, but we shouldn't have an ostrich mentality about the future. If it happened yesterday, it can happen tomorrow.
Early residents of the island experienced only lava flows and accompanying lava fountains during a period of eruptive activity similar to today's. Some archaeologists now say that the island was first populated in the early 1200s, 300400 years later than previously thought. This is controversial, but it is true that, until about 1500, people on the island probably knew only about lava flows. The Pele concept may have arrived in the late 1300s, just before a giant lava flowthe largest erupted on the island since people have been herelasted for about 60 years and destroyed much of Puna's forest.
Things changed dramatically in about 1500, when the summit of Kīlauea collapsed to form the caldera that, with some changes (mostly filling), remains today. The depression was so deep that it apparently intersected the water table, ushering in about 300 years of mostly steam-driven explosive eruptions. Imagine the impression that such a change, from quiet lava flows to violent explosive eruptions, must have had on island residents!
At least two elements of oral tradition tell of events during this time. The first describes when Hiʻiaka, digging for the body of Lohiʻau, who was killed by Pele, was warned to stop digging or water would come in to drown Pele. This may record the dropping of the caldera down to the water table, so that standing water could be seen in places on the caldera floor. We now know that this happened in about 1500.
The second well known story tells of Kamapuaʻa trying to drown Pele during a lover's tiff. Pele almost succumbed but eventually mustered all her strength, threw rocks at Kamapuaʻa, and chased him to the ocean. This is a colorful metaphor for a steam-driven explosive eruption.
The only eruption before 1790 strong enough to throw fine gravel into the ocean took place in about 1650. We determined this date by studying and dating several deposits left by explosive eruptions. One explosive eruption, whose carbon-14 charcoal age was 1650, dropped such rocks about where the Chain of Craters Road meets the sea.
These events may seem minor, inconsequential curiosities in comparison to occurrences in today's society. But imagine yourself caught in an explosive eruption at Kīlauea's summit. Major or minor doesn't matter when ash or blocks are falling around you, the air is hot and choking, and hot ash-air-steam mixtures are surging across the ground surface at hurricane velocities.
That is just what happened in November 1790, when several hundred people were killed, likely between Nāmakanipaio and Jaggar Museum, during the deadliest eruption known to have occurred at a volcano now in the United States. The unfortunate victims were apparently caught by a hot, ash-rich surge that both suffocated and burned them to death.
Today, most island residents and visitors are blithe to what happened in 1790, much less the preceding 300 years of violent but generally smaller explosive eruptions. Reading the geologic past, however, we think it almost certain that Kīlauea will eventually return to a long period of mostly explosive activity, just as it did in about 1500. The explosive period will probably be initiated by collapse of the caldera floor down to the water table, which today is about 615 m (2015 ft) below the high point on the caldera rim. For comparison, the caldera today is about 120 m (400 ft) deep. During the next explosive period, which could last several centuries, the summit of Kīlauea will o