Explosive eruptions and pyroclastic avalanches from Ngauruhoe in February 1975

Abstract

Pyroclastic eruptions of Ngauruhoe volcano on February 19, 1975, were observed closely. A 1.5-hour period of voluminous gas-streaming emission was followed by a series of violently explosive cannon-like eruptions, which threw blocks at least 2.8 km from the vent. Initial ejecta velocities of 400 m s −1 indicate high explosion gas pressures, possibly caused by magmatic intrusions rapidly heating confined meteoric water, the explosions being driven largely by phreatic steam rather than magmatic gases. Pyroclastic avalanches were generated by both the gas-streaming and cannon-like eruptions; in the latter they were formed by collapse of dense eruption columns onto the summit of the volcano. Downslope transport appears to have been by inertial grain flow mechanisms; erosion of chutes and short runout distances indicate that the avalanches were neither highly fluidised nor of air-layer lubricated type. A minimum bulk volume of 3.4 × 10 6 m 3 of pyroclastic material was erupted. Although of smaller scale, the Ngauruhoe explosions appear to have analogies to the 1968 Arenal and Mayon eruptions.