The bimodal Pleistocene part of the Coso volcanic field has erupted rhyolite and basalt at constant long-term rates during the past ∼0.5 m.y. Both basalt and high-silica rhyolite were erupted in several independent, geologically brief episodes. The interval between eruptions of rhyolite was proportional to the volume of the preceding eruption. Basaltic eruptions appear to have followed a similar pattern. These time-predictable relations would be expected if (1) extensional strain accumulates in roof rocks at a constant rate, (2) the accumulated strain is relieved by near-vertical fractures, which serve as conduits for eruptions, and (3) the volume of erupted material is proportional to the sum of the conduit (dike) widths. The long-term eruption rate of rhyolite is about 5.4 km3/m.y.; that of basalt is about 2.8 km3/m.y. These rates are less than those of magma supply inferred from heat-flow and petrologic arguments by factors of between 100 and 200.

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