Emplacement of continental flood basalt lava flows

We propose that continental flood basalt (CFB) lavas were predominantly emplaced as inflated compound pahoehoe flow fields via prolonged, episodic eruptions. Our most detailed observations come from the ~14.7 Ma Roza flow field of the Columbia River Basalt (CRB) Group. The Roza flow field seems to be typical of many flood basalt lavas. Individual flows show a wide range of pahoehoe surface features and a three-part internal structure in vesicularity and other textural parameters. This three-fold division into an upper crust, core, and basal crust appears to be diagnostic of the inflation process and is ubiquitous in basaltic lava flows over a remarkable range of sizes. The pahoehoe surface features and indications of inflation are inconsistent with rapid emplacement of these lava flows. Instead, we interpret the observations to imply that the Roza, and other CFB flows, were emplaced over an extended period of time. From the thickness of the upper crust, which we suggest formed while the flow was actively inflating, and an empirical expression for the rate of crust growth of Hawaiian inflated sheet flows, we estimate that individual Roza flows were emplaced over 5 to 50 months and that the Roza flow field was constructed over a period of 6 to 14 years. However, even,with this longer eruption duration, the average lava effusion rate of ~4000 m³/s is similar to that of the highest- effusion-rate eruption in recorded history (the 1783-4 Laki eruption in Iceland). Our observations of lava characteristics in other CRB flows and in the Deccan Traps suggest that this emplacement style is typical of many, if not most, CFB flows. Initial estimates of the volatile release from the Roza eruption indicate that prodigious amounts of S, Cl, and F were injected into the upper troposphere and lowermost stratosphere; thus this single flood basalt eruption could have had a significant effect on the global atmosphere If other flood basalt eruptions produced similar amounts of volatiles, volatile release might provide a link between flood basalt eruptions and mass extinctions.