DON SWANSON’S CONTRIBUTIONS TO UNDERSTANDING FLOOD BASALT EMPLACEMENT RATES AND VENTS

In the 1970s, Don Swanson co-authored two significant and prescient papers on flood basalts. The first of these, with H. Shaw in 1970, was the earliest study to tackle the question of the emplacement rates of flood basalt lava flows, based on observations made on Columbia River Basalt lavas. The study examined why these flows could be rapidly emplaced and proposed a range of flow rates and emplacement durations that spanned days to weeks. This work remained as the only model-based study of flood basalt emplacement rates for the best part of 30 years and had a long-lasting impact on thinking on this topic. This topic is still being debated today. In the other study, published in 1975 with T. Wright and R. Helz, the vent systems of two or three Columbia River lava flow fields were described. Most important in this work was the remarkable vent-system for the Roza flow, shown by Swanson and co-workers to be > 150-km long. They described dike segments, small shield-like constructs, and proximal scoria and spatter deposits arranged along an extremely lengthy, NNW-trending, fissure. Although some flood basalt eruptions were perceived to be fissure-fed by earlier workers, this was the first proof of a flood-basalt fissure system with surface exposures of deposits. The work formed one basis for a follow-up study by B. Martin some 14 years later, who determined compositionally that the Roza event consisted of up to 6 separate lava flows and linked each flow to one or more fissure segments. Apart from more recent work on the pyroclastic deposits along the Roza vent system fissure (and extension of the fissure by ~ 30 km), this early work on the Roza fissure still stands as the sole example of a complete flood-basalt vent system. Details and discussion of these two early and important papers by Swanson and colleagues on flood basalt vents and emplacement will form the basis of this presentation.