Pyroclastic edifices record vigorous lava fountains during the emplacement of a flood basalt flow field, Roza Member, Columbia River Basalt Province, USA

Abstract

The 1300 km3 tholeiitic lava flow field of the 14.98 Ma Roza Member of the Columbia River Basalt Province has the best-preserved vent system of any known continental flood basalt. Detailed geological mapping and logging of the exposed pyroclastic rocks along the >180-km-long vent system enable the reconstruction of pyroclastic edifices (partial cones) built around vents. The pyroclastic cones differ from those constructed during typical basaltic effusive eruptions and are characterized by low to moderate slope angles (<20°), have minimum heights up to ∼160 m, and are composed of dominantly coarse-grained, moderately to densely agglutinated and welded spatter and scoria that extend up to 750 m away from the vent. Thick, well-sorted fall deposits composed of moderately to highly vesicular scoria lapilli extend away from some vents and exhibit some characteristics comparable to the proximal deposits of violent Strombolian or subplinian eruptions. The recorded volcanic activity does not fit with common eruption styles of basaltic magmas, and the evidence indicates that the Roza eruption was punctuated by eruptive activity of unusually high intensity that was characterized by vigorous lava fountains. The extensive agglutinated deposits accumulated around the vents as a result of fallout from high (>1 km) fountains enhanced by fallout from the lower parts of convective plumes that rose above the fountains.