VENT GEOLOGY OF LOW-SHIELD VOLCANOES OF THE CENTRAL SNAKE RIVER PLAIN, IDAHO: LESSONS FOR MARS AND THE MOON

To better understand basaltic volcanism on the terrestrial planets, we have studied five basaltic volcanoes (Shale, Rocky, Crater, Kimama, and Black Buttes) of the central Snake River Plain of southern Idaho. Like most volcanoes on the Snake River Plain, they are monogenetic low-shield volcanoes composed of lavas, minor spatter, and rare scoria. Topographic shields range from 1.5 to 11 km in diameter, from 50 to 180 m in height, and erupted volumes range from 2 to 30 km³. The initial eruptions at each vent appear to have been the most voluminous and to have proceeded at the highest effusion rates forming large lava fields (compound lava flows) or long (over 50 km) lava flows. Calculations based on the dimensions of fissures suggest that eruption rates of approximately 10⁶ kg/s may be associated with the initial eruptive phase. As eruption rates declined during the later stages, three types of activity were typical: 1) plug-like rise and overflow of lava lakes; 2) Hawaiian-style lava fountaining from stagnant lava lakes, and 3) strombolian explosions yielding small accumulations of scoria. These processes built small topographic shields atop the lava fields at each vent. Apparently, degassing of nearly stagnant magma columns occurred within the conduits driving these small, late eruptions. Gas exsolution and bubble rise exceeded the rise speed of the magma as it slowed or stagnated within the conduits during the waning stages of the eruption. Craters are common, but not universal on low shield volcanoes of the Snake River Plain. A crater may form by foundering of the walls into the vent during lava lake activity, but the crater does not widen substantially after the solidification of the lava lake. The similarity of these low shields to those on Mars and the Moon suggests that similar styles of eruption also prevailed on these planets. A highly fractured lithosphere with access to the staging areas for volatile-poor, fluid magmas may promote monogenetic character, dominance of lava or scoria, and wide distribution of low shields (which contrasts with multiple eruptions from large Hawaiian-type shields). The larger craters on lunar low shields may result from the volatile-poor nature of lunar basalts which could prolong the existence of stagnant lava lakes and increase the foundering of the walls into the vent.