Growth of the lava dome at Mount St. Helens, Washington, (USA), 1981–1983

Nine dominantly nonexplosive episodes of dome growth at Mount St. Helens during 1981–83 added about 40 × 10 6m 3of dacitic lava to the active composite dome in the volcano’s 1980 crater. Endogenous and exogenous growth, the latter mostly in the form of stubby lava flows that accumulated on the dome, combined to build an edifice 880 m long, 830 m wide, and 224 m high by December 1983; the total volume (1980–1983) was about 44 × 10 6m 3. Every 1–5 months during 1981–82, periods of increasing seismicity and ground deformation lasting 1–3 weeks culminated in extrusions lasting a few days. Endogenous growth became increasingly important during this interval, and in February 1983, the style of activity changed from episodic to essentially continuous endogenous and exogenous growth. In March 1982 and February 1983, extrusions were preceded by lateral explosions that triggered snow avalanches from the crater wall and mudflows down the volcano’s north flank. Collapse of part of the north face of the dome during rapid endogenous growth in April 1982 produced a hot rock avalanche and small mudflow. Ejections of gas and comminuted dome rock from the top of the dome were frequent throughout 1981–83, and averaged several per day in 1983. As the dome continues to grow, its capacity to accommodate newly supplied magma without rupturing, and hence the ratio of endogenous to exogenous growth, will probably continue to increase. Endogenous growth will be especially favored when magma supply is continuous and slow; extrusive activity will be favored if the short-term supply rate becomes significantly higher than the current rate of about 0.9 × 10 6m 3/month. Future explosive activity is possible and perhaps even likely in view of Mount St. Helens’ history and the histories of similar contemporary domes at other volcanoes, but at this time we can foresee no change from the pattern of mostly nonexplosive dome growth that characterized activity during 1981–83.