THE JOYCE RANCH VOLCANO: A SUBLACUSTRINE-TO-EMERGENT PHREATOMAGMATIC VOLCANO WITH A WIDE VARIETY OF PRODUCTS, WESTERN SNAKE RIVER PLAIN, SOUTHWESTERN IDAHO

The Western Snake River Plain graben hosts one of the world's most diverse and productive basaltic phreatomagmatic systems, with >100 volcanoes. Almost all the volcanism occurred either between 9 and 7 Ma (small fields of basalt erupted from both sublacustrine and subaerial vents) or between 2.2 and 0.4 Ma (tuff cones, tuff rings, maars, bedded red cinder deposits and low shields), as basaltic magmas that rose to the surface during episodes of graben extension and encountered the waters and wet sediments of Lake Idaho, a large tectonically impounded freshwater lake. The Joyce Ranch Volcano, shown on the Sinker Butte 7.5-minute topographic quadrangle map as Hill 3337, is the only volcano yet studied that erupted between the older and younger groups, both in time (between 4.8 and 4.1 Ma) and in space (along a NW-trending line separating outcrops of Miocene Chalk Hills and Poison Creek sediments from outcrops of Pliocene Glenns Ferry sediments). Its products include Surtseyan tuffs (all facies), peperites (near-vent facies), water-affected shelly pahoehoe (near-vent facies), columnar basalt (medial facies), abundant pillow lavas (medial to distal facies), and highly energetic and erosive debris flows (distal facies). The debris flows may have been produced by collapse of pillow delta margins and of sectors of the growing cone. These flows rushed across the lake bottom, scouring and incorporating decimeter-scale tightly folded rip-ups of fine-grained sediments and thin-bedded basaltic ashes. Component analysis of the debris flows suggests a large juvenile component.