Basaltic ring structures of the Columbia Plateau

摘要: Circular structures, defined by arcuate, concentric ridges and scarps that surround hills, mesas, or crater-like depressions, are localized in an exceptionally thick section of the Roza Member Yakima Basalt on Columbia Plateau. Autointrusive dikes conspicuous most ridge segments. Palagonite discovered a few central hills appears genetically related to ring structures suggests origin involving interaction water lava. The intruded crust about 30 m thick, however, so substantial cooling must have occurred prior formation rings. A plausible explanation for these unusual features may hinge disruption drainage as result voluminous basalt extrusion. If, after partial cooling, molten interior this ponded flow were intersected rising ground-water table, rapidly accumulating volatiles could caused explosive venting well doming cracking crust, with concurrent emplacement granulated sideromelane (later palagonitized) tephra craters fractures. Subsidence initial permitted intrusion melt into fractures predominantly focus pressure release. Subsequent catastrophic erosion Missoula floods effectively removed surficial clues original structure morphology, leaving roots enigmatic partly exposed.