BLOCKY RIMMED PITS: A CRATER-LIKE LANDFORM PRODUCED BY COLLAPSE OF BASALT INFLATION PLATEAUS

Blocky rimmed pits, previously identified as explosion craters, in basalt flows of the Aden Crater, New Mexico, are re-accessed in light of improved imaging, field studies, and comparison with Hawaiian volcanic landforms. The pits range from 50-250 meters across and possess a raised rim, four to five meters high, of fragmented basalt blocks. Previous observers assumed that the blocky material was the result of explosive venting beneath cooled lava flows. Current observations note that the “craters” are nor circular; the floors are relatively smooth; the rim material overlies floor material; and lava channels radiate away from the pits. Field studies indicate a progression of land forms from rootless shields, blocky rimmed pits, partially collapsed inflation plateaus; and fully developed inflation plateaus. Inflation plateaus form when the crust on lava flows becomes strong enough to resist flow, and the interior inflates as more lava moves into the thickening flow. The upper crust is lifted to form a steep edged plateau. Blocky rimmed pits are viewed as failed inflation plateaus at which interior lavas escaped by breakouts around the flow margins and allowed the plateau crust to deflate leaving a raised rim of blocky lava.