ERUPTION CYCLES AND EXPLOSIVE HAZARDS AT KīLAUEA VOLCANO, HAWAIʻI

For at least the past 2500 years, Kīlauea’s explosive eruptions have come in cycles. Magmatic explosive activity (Hawaiian-style lava fountains and spattering) accompanies eruption of lava flows during periods of high magma supply rate to the volcano. The more powerful and hazardous phreatomagmatic and phreatic explosive eruptions (volumes up to 10⁷ m³, a low VEI 3) generally take place during periods of low magma supply, when a deep caldera exists that enables groundwater to interact with spurts of magma and hot conduit wall rock. The alternating periods of high and low magma supply rate last for centuries; the most recent period of low supply rate, a deep caldera, and dominantly explosive eruptive activity was ca. 1500–1800 CE. The cause of the changes in supply rate is uncertain, potentially involving variations in rate of melting at the source or temporary storage along the transport path from mantle to volcano. Whatever their cause, the cycles pose a practical problem for hazard assessment. Hazards in a period of high magma supply, such as now, must emphasize lava flows and volcanic gas, especially SO₂. Explosive hazards are relatively small during such periods; an exception was the mild phreatic activity in 1924, which killed one person when lava drained from Halemaʻumaʻu Crater and groundwater entered the conduit system. During periods of low magma supply, however, the dominant hazard is caused by phreatomagmatic and phreatic explosive eruptions, with life-threatening pyroclastic density currents and ballistic showers in proximal areas and ash fall in distal areas. Lava flows and volcanic gas are of less importance, because eruption of lava is relatively infrequent and of low volume. Hazard assessments for both types of activity must be developed in parallel, for it is not yet possible to foresee when the current period of high magma supply will end, a new caldera develop, and phreatic and phreatomagmatic eruptions again dominate Kīlauea. In an exceptionally innovative paper, Clague and Dixon (2000) showed how extrinsic controls influence Hawaiian volcanoes. This is nowhere more evident than in the impact of groundwater on the explosive nature and hazards of Kīlauea.

Clague, D.A., and Dixon, J.E., 2000, Extrinsic controls on the evolution of Hawaiian ocean island volcanoes: G-cubed, v. 1, doi:10.1029/1999GC000023.