Hawaiian volcanoes have produced explosive eruptions ranging in size and vigor from relatively small lava fountains to large eruptions. Larger events can generate pyroclastic surges—hot, ground-hugging, and fast-moving currents of rocks and gas—and send a column of rocks, ash, and gas high into the atmosphere. During large explosive eruptions, wind disperses the finest rock particles (ash) and gases tens to hundreds of kilometers (miles) from an erupting vent and can result in heavy ash fall downwind. Volcanic ash can make breathing difficult and can cause buildings and structures to collapse (especially when the ash is wet), damage agricultural crops, and contaminate grass used for livestock feed. The downwind ash plume from such eruptions also poses a hazard to aircraft.The USGS sponsored Volcanic Ashfall Impacts Working Group offers resources and guidance for ashfall preparedness and impact.
The small-scale explosive activity of lava fountains shreds and blasts molten rock into the air, forming a variety of lava fragments that solidify as tephra and fall back to the ground. Most fragments land within a few hundred meters from the vent, typically forming a spatter cone or rampart. Finer windborne fragments, however, can be carried many tens of kilometers (miles) downwind of the vent.
For example, in 1984-1986 during the episodic lava fountains at Pu‘u ‘Ō‘ō, tephra reached the town of Nā‘ālehu about 63 km (39 mi) from the vent. The tephra that fell on inhabited areas was not harmful to most people, but it was a source of irritation to those with respiratory problems and an inconvenience to the many residents with rain-water-catchment systems. Following some of the lava fountain episodes, Hawai‘i County Civil Defense recommended that people disconnect and clean their rain-water-catchment systems to prevent the particles from washing into their water supply.
Eruption of lava flows is the dominant style of activity at Kīlauea today, but layers of fragmented rocky deposits on the volcano record many explosive eruptions during the past 2,500 years. For example, between about 1500 and 1800, at least a dozen pyroclastic surges swept across the summit area. One surge erupted in 1790 killed a large number of Hawaiians traversing the summit area near the current location of HVO—these deposits can be found as far as 3.5 km (2.1 mi) beyond the summit caldera. Other explosive events resulted in ash fall at the summit and over broad areas far downwind.
Unlike lava fountains, large explosive eruptions eject tephra that consists mostly of broken rock from the sides of the vent and little magma (molten rock). The largest of these eruptions are known to have resulted in rocks about 3 cm (1 in) in diameter falling on the south shore of Kīlauea and beyond into the sea. Tephra in the eruption column reached high into the westerly jet stream wind currents and were blown downwind.
The long-term hazards will be severe when a repeat of similar explosive eruptions occur over a period of several centuries. The early part of the next explosive period will likely include a major collapse of the summit caldera that lasts days to weeks. Such a dramatic shift will be apparent to everyone, because there will be many large-magnitude earthquakes that accompany collapse of the next caldera. After caldera formation, Kīlauea will not likely erupt continuously. There will likely be periods of years to decades between explosive eruptions. The change from the current effusive activity to intermittent explosive activity will challenge nearly everyone on the Island of Hawai‘i, but there is no way to forecast when the next explosive period may begin.