The relation of eruptive intensity with magma composition and viscosity has been investigated for Mount St. Helens, Washington, where eruptive activity has ranged from basaltic lava flows to dacitic Plinian eruptions. The Plinian eruptions have varied in eruptive intensity from 106 to 108 kg/s, yet all erupted dacitic magma. These dacites, however, differ greatly in temperature, water content, and crystallinity, and thus magma viscosity varies by two orders of magnitude. The variation in viscosity is correlated inversely with intensity, demonstrating the control of composition on intensity. In addition, more mafic magmas erupted at lower intensities, showing that the wide range in eruptive behavior is linked to magma composition. Changes in composition result mainly from mixing of basaltic and dacitic magmas and occur in cycles. The rate of change during a cycle depends upon the length of the preceding repose period and the flux of basalt to the reservoir, because the supply of dacite has been relatively constant. When the flux of basalt is low, cycles progress from dacite to andesite over extended periods, whereas a higher flux leads to more rapid changes. Because composition controls eruptive intensity, the eruptive behavior of the volcano also varies through a cycle.