18 MAGGIO 1980, ORE 8:32, MT ST HELENS: UN’ERUZIONE CHE HA CAMBIATOLA STORIA DELLA VULCANOLOGIA

Roberto Scandone Former Professor of Volcanism Physics University of Roma Tre Research Associate, INGV Vesuvius Observatory On May 18, 1980, at 8:32 a.m., a magnitude 5+ earthquake accompanied the landslide of the northern flank of St. Helens volcano in the state of Washington, USA. This began a violent explosive eruption that would change our way of studying and understanding the functioning of active volcanoes. The first signs of the volcano's renewed activity occurred on March 20, with a first magnitude 4 Volcano-Tectonic (VT) earthquake, occurring at a shallow depth (less than 5 km) beneath the volcano. The subsequent sequence initiated a period of sustained seismicity with VT earthquakes, subsequently accompanied by long-period (LP) earthquakes and tremor events. On March 27th, the first of a series of phreatomagmatic explosions occurred, widening a crater formed in the glacier at the volcano's summit. In April, macroscopic deformation was noted affecting the volcano's north flank, with displacement rates in the order of meters per day. A pause in eruptive activity between late April and early May seemed to indicate a slowdown in the volcanic crisis. On May 18th, at 8:32 a.m., a magnitude 5+ earthquake accompanied the landslide on the northern flank of St Helens and triggered a directional explosion that devastated a 180° sector north of the volcano, covering an area of ​​approximately 550 km2. The explosion decapitated the volcano by approximately 1000 m, and a Plinian eruption column rose in the new crater to a height of approximately 30 km. Around 1:00 PM, new pyroclastic flows poured into the area already devastated by the first directional explosion. The eruption ended around 5:00 PM. New explosive eruptions occurred on May 25, and in the following months, a lava dome formed at the top of the new crater, which would grow through discrete eruptive episodes until 1986. A new eruption with the extrusion of a new lava dome occurred between 2004 and 2008. Bibliography Cashman K. V., R. P. Hoblitt, Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA, Geology 32, 141-144 (2004) Lipman P. W., and Mullineaux D. R. (editors), , The 1980 eruptions of Mt St Helens. USGS Professional paper 1250, 844 pp (1981) Scandone R., S. D. Malone, Magma Supply, Magma Discharge and Readjustment of the feeding system of Mount St. Helens during 1980, J. Volcanol. Geoth. Res. 23, 239-252 (1985) Scandone R., Cashman K., Malone S.D., Magma Supply, Magma Ascent and the Style of Volcanic Eruptions, Ea. Plane. Sci. Lett. 253, 513-529, doi: 10.1016/j.epsl.2006.11.016, (2007)