4) Explosive Eruptions

The rare explosive eruptions in Hawai'i generally are caused by the interaction of magma and ground water. The magnitude of the resulting steam explosion varies from harmless to catastrophic. Small steam-blast explosions occurred during the 1960 Kapoho eruption when the magma beneath the vents, which were near sea level, encountered saltwater trapped in the surrounding rocks. These steam blasts ejected black clouds of pulverised rock fragments but
were of little hazard except to scientists working close to the vents.

The 1924 eruption of Kilauea mainly affected the immediate vicinity in Halemaumau crater.

A much larger steam-blast eruption occurred at the summit of Kilauea in 1924, when ground water apparently flowed into the heater rocks beneath the Halemaumau vent, which had been erupting nearly continuously for over a century. The explosions continued at intervals for 2 weeks, carpeting the area around Halemaumau crater with large rocks and a thin layer of ash. Boulders weighing several tons were thrown as far as 3,000 feet from the crater. The greatest hazard
posed by this type of activity is that it may start abruptly and endanger unwary onlookers. The 1924 eruption claimed one fatality - a man who ventured too close to the vent between
explosions to take photographs and was struck by a rock when the activity suddenly returned.

The largest explosive eruption on Hawai'i within historical time occurred in 1790. This eruption produced pyroclastic surges that originated at Kilauea's summit and flowed several miles to the Southwest. Pyroclastic surges are ***extreme***ly dangerous because they move at speeds of 30 to
200 mph, and humans and animals caught in their path are killed by either asphyxiation or heat.
A band of Hawai'ian warriors traveling from Hilo to the Ka'u district to battle with Chief Makehameha were overtaken by one of the 1790 pyroclastic surges, and about 80 of them
were killed. The 1790 eruption left deposits of rock fragments and ash up to 30 feet thick
on the rim of Kilauea's summit caldera.

The thick deposits of ash exposed at many sites on the island indicate that even larger explosive eruptions occurred in prehistoric times and probably originated from Mauna Kea as well as from Kilauea. Explosive eruptions of any size take place infrequently in Hawai'i, but the possibility of one occurring in our lifetime should not be totally discounted. Such eruptions are unlikely to begin without some warning. The most widespread hazard from an explosive eruption would be windborne ash, which could damage structures, machinery and agricultural crops.

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