CONSTRAINING THE EMPLACEMENT OF A UNIQUE LAVA FLOW FIELD AT ROCK CORRAL BUTTE, SNAKE RIVER PLAIN, IDAHO

Rock Corral Butte (43° 22.25'N, 113° 1.20'W) is a shield volcano composed of tholeiitic basalts within the eastern Snake River Plain. Rock Corral Butte is ~55,000 years old, and is surrounded by a large flow field (<230 km²; 29 km³) that is remarkable for its rolling, 2 to 4-m-high scale topography that is superposed on large, kilometer-scale horizontal topographic terraces. These 2 – 4 m tall hummocks within the flow are characterized by relatively steep margins, flat or depressed interiors and are ~5 to 10 m long and 3 to 10 m wide.  Although the hummocks are topographically similar to tumuli, they lack the typical axial cracks that define tumuli"]; instead, they display tensional cracks around their margins. The elongate hummocks tend to be aligned with each other:  each upslope hummock feeds into the next hummock immediately downslope. The broader topographic terraces are formed by several similar-sized hummocks located adjacent to each other at roughly the same radial distance from the summit. A combination of field work, sample analyses, and numerical modeling reveal a unique style of emplacement for this flow field, where individual lava lobes are emplaced, inflated, and then drained to feed the subsequent down-slope lobe.  This style of emplacement requires a steady effusion rate (~10 - 30 m³/s) of low-viscosity (~100 Pa s) lava on a gentle underlying slope.  Similar conditions may have existed on Mars, suggesting that some small shields on Mars may display flow fields with the same topographic expression and emplacement mechanisms. It is interesting to note that combined geochemical sample analyses, field surveys, and numerical flow modeling were required to determine the emplacement and eruption parameters for this flow field: lacking one (or more) of these produces inconclusive results.