MAPPING WITHIN THE INFERNO CONE QUADRANGLE, CRATERS OF THE MOON NATIONAL MONUMENT AND PRESERVE: A DETAILED LOOK AT THE BROKEN TOP CINDER CONE AND ITS ASSOCIATED ERUPTIVE SEQUENCE AND FEATURES

Craters of the Moon National Monument and Preserve, along Idaho’s eastern Snake River Plain, is one of the nation’s premiere research areas for basaltic volcanism. In summer of 2004 detailed mapping was conducted, centering on the most widely used area of the Monument for educational purposes, and generated a new large scale map (1:4,000). Mapping on this larger scale allows for the inclusion of many features unmapped before and the refinement of volcanic events associated with the Broken Top cinder cone. The Craters of the Moon (COM) lava field straddles the 85 km long Great Rift, and consists of eruptive and noneruptive fissures and at least 60 flows and 25 cinder cones. The COM field is the largest dominantly Holocene basaltic lava field in the Continental U.S. and ranges in age between about 15,000 to 2,000 years old. Mapping was done by field study and use of aerial photographs within a selected area of the COM lava field focusing on the Broken Top cinder cone and encompassing all the flows associated with the cone. This area includes two of the COM lava field’s youngest flows, the Blue Dragon and Broken Top (~2.1 ka and younger, respectively). The investigation adds new details such as the location and extent of inflation pits, rafted blocks and kipukas, or ‘islands’ of older lava flows surrounded by younger flows. Near-vent features mapped were areas of inflation, blisters and hollows, shelly pahoehoe lava and younger breakout lava toes. The project also refines the sequence of flow events that occurred along the north flank of the Broken Top cone. Contacts were discerned between the Blue Dragon and Broken Top flows from older flank eruptions to evaluate the flanks’ uplift from more recent injections of magma. Eruptive and non-eruptive fissures were mapped and investigated in the Broken Top and Trench Mortar Flat areas; these fissures, upon further investigation, may possibly correlate with the trend of other fissures of the Great Rift to the southeast. This small mapping area is readily accessible by trail; exhibits a diverse and nearly complete range of classic features of Hawaiian basaltic volcanism, including spatter ramparts, cinder cones, lava tubes, pressure plateaus, tumuli, shelly, spiny and slabby pahoehoe, tree molds and lava bombs; and is an appropriate area for future geologic/ educational exercises.