Abstract

The Great Rift is an 85-km-long, 2- to 8-km-wide volcanic rift zone in the Snake River Plain, Idaho. Three latest Pleistocene to Holocene basaltic lava fields, Craters of the Moon, Kings Bowl, and Wapi, are located along the Great Rift. The Craters of the Moon lava field is a composite of more than 60 lava flows, 25 cinder cones, and at least 8 eruptive fissure systems. It covers 1,600 km2 and contains ∼30 km3 of lava flows and associated pyroclastic deposits. Field, radiocarbon, and paleomagnetic data show that the Craters of the Moon lava field formed in eight eruptive periods, each of which was typically several hundred years or less in duration and was separated from others before and after by intervals of several hundred to several thousand years. The first eruptive period began ∼15,000 yr B.P., and the latest eruptive period ended ∼2100 yr B.P. The small Kings Bowl lava field (3.3 km2, 0.01 km3) and the larger Wapi lava field (330 km2, 6 km3) both formed ∼2250 yr B.P.

Three magma types have fed flows along the Great Rift. The types are (1) a contaminated type that has a SiO2 range of ∼49%–64% and commonly shows petrographic evidence of contamination, (2) a fractionated type that has a SiO2 range of ∼44%–54% and shows no evidence of contamination and whose chemical and mineralogical variation can be accounted for mainly by crystal fractionation, and (3) a Snake River Plain type that has a SiO2 range of ∼45%–48%, shows little evidence of fractionation, and is represented by Kings Bowl–Wapi flows and olivine basalts of the Snake River Plain. The contaminated and fractionated magma types were erupted at the Craters of the Moon lava field, and the Snake River Plain magma type was erupted at the Kings Bowl and Wapi lava fields. These relations imply that the magma reservoirs are spatially isolated.

The magma output rate for the Craters of the Moon segment of the Great Rift was constant at ∼1.5 km3/1,000 yr for the period from 15,000 to 7000 yr B.P. The rate increased to ∼2.8 km3/1,000 yr from 7000 to 2000 yr B.P., mainly as a result of the addition of contaminated magma to the nearly constant output rate of fractionated magma. The Craters of the Moon segment of the Great Rift has experienced quasi-steady-state, volume-predictable volcanism for the last 15,000 yr. The recurrence interval of eruptive activity for the Craters of the Moon lava field ranges from several hundred to ∼3,000 yr. Because the present interval has lasted ∼2,100 yr, another eruptive period seems likely to occur within the next 1,000 yr. The steady-state, volume-predictable relationship suggests that 5–6 km3 of lava will be erupted in the next eruptive period.

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