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The Great Rift and the Evolution of the Craters of the Moon Lava Field, Idaho

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4.23 The Great Rift and the Evolution of the Craters of the Moon Lava Field, Idaho Mel A. Kuntzl, Duane E. Champion2, Elliott C. Spiker3, Richard H. Lefebvrelsd, and Lisa A. McBroomes ABSTRACT The Snake River Plain of southern Idaho is a region of late Cenozoic and Quaternary basalt vol- canism and many volcanic rift zones. The most spectacular volcanic rift zone on the plain is the Great Rift, an 85-kilometer-long and 2-to 8-kilometer-wide belt of cinder cones, shield volcanoes, eruptive fis- sures, and open cracks. The Holocene and latest Pleistocene Craters of the Moon lava field is alined along a 45-kilometer segment of the northern part of the Great Rift. The lava field is a composite of more than forty lava flows that have erupted from more than twenty-five vents. It covers an area of about 1,650 square kilometers, contains about 30 cubic kilometers of lava, and is the largest dominantly Holocene basalt lava field in the conterminous United States. Radiocarbon and paleomagnetic data reveal that lava flows of the Craters of the Moon lava field were emplaced in at least eight eruptive periods that began about 15,000 years ago and ended about 2,000 years ago. The lava flows of the Craters of the Moon field were erupted from cinder cones and fissures, most of which are in the Craters of the Moon National Monument. Two other Holocene lava fields on the Great Rift, the Kings Bowl and Wapi fields, represent a small-volume (about 0.005 cubic kilo- meter) fissure eruption and a large-volume (about 6 cubic kilometers) shield eruption, respectively. INTRODUCTION The Snake River Plain of southern Idaho is a region of geologically young basalt volcanism. As ‘U. S. Geolog~al Survey, Denver, Colorado 80225. W. S. Geological Survey, Menlo Park, California 94025. W. S. Geological Survey, Reston, Virginia 22092. “Department of Geology, Grand Valley State Colleges, Allendale, Michigan 49401. SDepartment of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822. many as eight lava fields in the area are known or believed to be younger than 20,000 years old: they include the Craters of the Moon, Kings Bowl, Wapi, Hells Half Acre, Cerro Grande, North Robbers, South Robbers, and Shoshone lava fields (Figure 1). The Craters of the Moon, Wapi, and Kings Bowl lava fields lie along the Great Rift, an 85-kilometer- long and 2- to 8-kilometer-wide belt of shield vol- canoes, cinder cones, eruptive fissures, associated lava flows, and noneruptive fissures. In this report, we describe the largest of these lava fields-the Craters of the Moon field. This lava field, the largest accumulation of dominantly Holocene lava in the conterminous United States, covers an area of 1,650 square kilometers and contains approximately 30 cubic kilometers of lava. The Wapi and Kings Bowl lava fields to the southeast have been described and mapped by Kuntz and others (1980, 1981), King (1977), Covington (1977), Champion (1973), and Champion and Greeley (1977); they are described in only minor detail in this report. REGIONAL SETTING The Snake River Plain is an arcuate topographic depression, 50 to 100 kilometers wide, that extends from near Payette, Idaho, on the west for about 250 kilometers southeastward to Twin Falls and then for about 300 kilometers northeastward to near Ashton, Idaho (Figure 1). It is bounded on the north by Mesozoic and early Tertiary granitic rocks of the Idaho batholith and by Tertiary and Quaternary basin-range, block-faulted mountains. The southeast side of the plain is also bounded by basin-range, block-faulted mountains. The southwest side of the plain is bounded by Tertiary rhyolitic and basaltic rocks of the Owyhee Plateau. Upper Tertiary and Quaternary rhyolitic and basaltic rocks of the Yellow- stone Plateau occur at the northeast end of the plain. Geologists and geophysicists have traditionally di- vided the Snake River Plain into eastern, central, and western parts based on different geological histories424 Cenozoic Geology of Idaho and geophysical features in each part (Mabey, 1976, 1978, and 1982 this volume). GENERAL FEATURES OF THE CENTRAL AND EASTERN SNAKE RIVER PLAIN The central and eastern Snake River Plain is a broad, flat lava plain consisting, at the surface, of basalt lava flows and thin, discontinuous, interbedded loess, eolian sand, and alluvial fan deposits that together have a total thickness of about 1 to 2 kilo- meters near the area described in this report (Zohdy and Stanley, 1973; Stanley and others, 1977; Doherty and others, 1979). Lava flows of the central and eastern Snake River Plain were erupted from low volcanic vents that are generally alined with and parallel to volcanic rift zones that trend mainly at 117ow 116OW ’ / /’ /’ right angles to the long axis of the eastern Snake River Plain. We define a volcanic rift zone as a narrow belt of faults, grabens, noneruptive fissures, eruptive fissures, spatter cones, spatter ramparts, cinder cones, lava cones, pit craters, and shield volcanoes. Most eruptive vents in volcanic rift zones are elongated and are believed to overlie eruptive fissures (Wentworth and Macdonald, 1953; Kuntz, 1977a, 1977b). Well-defined volcanic rift zones of the Snake River Plain are as much as 10 kilometers wide and 120 kilometers long. The Great Rift is the best example of a volcanic rift zone in the Snake River Plain. In the central and eastern Snake River Plain, the basaltic volcanism represents the latest phase of a complex and poorly understood tectonic and volcanic history that also includes an earlier phase of rhyolitic volcanism. Available geological, geophysical, radio- metric, and drilling data suggest that the rhyolitic 114”W 113ow 112ow


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