Résumé
The 40,000-year eruptive history of Mount St. Helens reveals an overall compositional trend from rhyodacite to andesite, with basalt at ~1.9 and ~1.6 ka. A cyclic eruption pattern is superimposed on this trend. Cycles comprised a repose interval, when compositional and thermal gradients developed in the underlying magma body, followed by an eruption interval in which progressive tapping of magma beheaded these gradients. Recovery of gradients varied with duration of the ensuing repose period. Eruption sequences follow the pattern: (1) eruptive progression from Plinian eruptions to dome growth accompanied by pyroclastic flows and tephra, followed (in some cases) by lava flows punctuated by pyroclastic outbursts; (2) a mineralogic progression from hydrous Fe-Mg phenocrysts (hb, cm, bi) toward pyroxenes; (3) a magmatic compositional progression from rhyodacite or dacite to andesite. Progressions 1 and 2 stem mainly from volatile gradients in the magma reservoir whereas progression 3 (and to some extent 2) reflects gradients of melt composition and crystal content. Three eruption cycles within the last 4,000 years follow this pattern. Earlier cycles are probable but only dimly perceived, mainly from the partial record of tephras and pyroclastic flows.