LAVA FLOW-FIELD EMPLACEMENT AT ROCK CORRAL BUTTE, EASTERN SNAKE RIVER PLAIN, IDAHO: A NEW MECHANISM FOR “TUMULI” FORMATION

Rock Corral Butte (RCB) is a monogenetic basaltic (46 wt.% SiO₂) shield in the eastern Snake River Plains that erupted ~55,000 years ago. It is surrounded by a hummocky flow field that covers <200 km². At first glance, these hummocks appear similar in shape and dimension to tumuli observed at the Mauna Ulu, Hawaii flow field. Closer investigation, however, reveals that the apparent “tumuli” actually reflect individual flow lobes that were emplaced, inflated, and then burst to feed the next set of lobes downstream. Locally, proximal lobes are collapsed and drained to fill adjacent downstream lobes. This proposed mechanism is similar to that for submarine pillows or individual subaerial pahoehoe toes, but with a significant difference in scale. The inflated tumuli-like lobes at RCB are as long as tens of meters and are 2 – 4 m high. Remote sensing data allows tumuli-like lobes to be mapped and lava pathways to be traced from the summit to distal reaches of the flow field (<7 km). Thus, an aligned sequence of tumuli-like lobes represents a crusted-over lava pathway that is neither a lava tube (sensu strictu) nor a pahoehoe sheet flow. It is possible that, if the eruption had been longer lived or more steadily supplied (similar to the current Pu'u O'o eruption at Kilauea volcano, Hawaii), a true lava tube system may have developed. The tumuli-like lobes at RCB are probably the result of emplacement on initially flat-lying terrain, and an unsteady lava supply rate. This provides an interesting contrast to the tumuli fields observed at Mauna Ulu, and presents a new hypothesis for the emplacement mechanisms of similar small basaltic shields in regions on Mars where lava-plains volcanism is apparent.