A major explosive eruption occurred 15 May 1981 at Mount Pagan Volcano, the larger of two historic eruptive centers on Pagan Island, Mariana Islands. The eruption was preceded by increased numbers of locally felt earthquakes beginning in late March or early April and by new ground cracks, new sublimates, and increased gas emissions. A swarm of felt earthquakes began at 0745h (local time = UCT+10 hours) 15 May, and at 0915 h, closely following a loud sonic boom, a strong plinian column issued from the volcano. The high-altitude ash cloud (at least 13.5 km) travelled south-southeast, but ash and scoria deposits were thickest (> 2 m) in the NW sector of the island because of the prevailing low-altitude southeasterly winds. The early activity of 15 May probably involved magmatic eruption along a fissure system oriented about N10??E. However, the eruption became hydromagmatic, possibly within minutes, and was largely restricted to three long-lived vents. The northernmost of these built a substantial new scoria-ash cinder cone. Flows and air-fall deposits, consisting almost entirely of juvenile material, exceeded 105 ?? 106 m3 in volume (75 ?? 106 m3 of magma) on land and at least 70-100 ?? 606 m3 at sea. An unknown volume was carried away by stratospheric winds. Lithic blocks and juvenile bombs as large as 1 m in diameter were thrown more than 2 km from the summit, and evidence for base-surge was observed in restricted corridors as low as 200 m elevation on the north and south slopes of the volcano. Neither of these events resulted in serious injuries to the 54 residents of the island, nor did the eruption produce serious chemical hazards in their water supply. Weak eruptions occurred during the ensuing month, and some of these were monitored by ground observations, seismic monitoring, and deformation studies. Precursory seismicity and possibly deformation occurred with some of the observed eruptions. More vigorous eruptions were reported by visiting residents in late 1981 and early 1982, but these were of lesser magnitude than the 15 May 1981 event. The 15 May lava is predominantly aa and ranges from 3 to > 30 m in thickness. In composition, it is a high-alumina basalt with small (< 1 mm long) phenocrysts of plagioclase and clinopyroxene (7%) that is more or less typical of basalt of the northern Marianas volcanoes. It contains slightly more SiO2 (52%), K2O, TiO2, and less Al2O3 and CaO than does the basalt of the last eruptive event of Mount Pagan Volcano in 1925. Gas analyses indicate that a large portion of air was introduced into the vent system through the porous volcanic edifice and that the carbon gases were not in equilibrium with the magma or each other. ?? 1984.
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The eruption of Mount Pagan volcano, Mariana Islands, 15 May 1981