Ballistic blocks around Kīlauea Caldera: Their vent locations and number of eruptions in the late 18th century

Thousands of ballistic blocks occur around Kīlauea Caldera and record part of the latest major period of explosive activity on the volcano, in late 1790 or within a few years thereafter. The sizes of the blocks – the largest of which is more than 2 m in nominal diameter – and differences in rock types allow the definition of at least 6 dispersal lobes of mostly undetermined relative age. The orientations of the lobes help approximate the locations of vents or explosion sources on the floor of the caldera, now deeply buried by younger lava flows. The vents may have been distributed northward for about 2 km from near the site of the modern Halema'uma'u Crater and were apparently confined to the western half of the caldera. The blocks are entirely lithic except for those in one dispersal lobe, which contains cored bombs and blocks as well as juvenile lapilli. Eruption parameters calculated from EJECT! suggest that the phreatic and phreatomagmatic explosions could have been generated at the water table, about 600 m below the high point on the caldera rim.

► Ballistic blocks as large as 2 m in diameter occur around the west and south sides of Kīlauea Caldera.
► These blocks were erupted in the late 18th century, when the caldera floor was at the water table.
► The blocks were erupted in several different events that dispersed ejecta in different directions.
► Calculations of eruption dynamics are consistent with explosion sources about 600 m below the caldera rim and at the water table.
► About 5000 people a day visit this area and would be subject to severe hazards if such eruptions should occur announced.