Abstract

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 of the Yakima Basalt on the Columbia Plateau. Autointrusive dikes are conspicuous in most ridge segments. Palagonite discovered in a few central hills appears genetically related to the ring structures and suggests an origin involving interaction of water and lava. The dikes intruded a crust about 30 m thick, however, so that substantial cooling must have occurred prior to formation of the rings.

A plausible explanation for these unusual features may hinge on the disruption of drainage that occurred as a result of the voluminous basalt extrusion. If, after partial cooling, the molten interior of this thick ponded Roza flow were intersected by a rising ground-water table, rapidly accumulating volatiles could have caused explosive venting as well as doming and cracking of the crust, with concurrent emplacement of granulated sideromelane (later palagonitized) and tephra in craters and fractures. Subsidence of the crust after initial venting could have permitted intrusion of melt into fractures predominantly concentric to the focus of pressure release. Subsequent catastrophic erosion by the Missoula floods effectively removed most of the surficial clues to original structure and morphology, leaving the roots of these enigmatic features partly exposed.

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