ERUPTION AND EMPLACEMENT RATES OF CONTINENTAL FLOOD BASALT LAVA FLOW FIELDS

Application of the inflation principal to continental flood basalt (CFB) lavas has led to recent suggestions that rates of formation, and hence factors such as effusion rate, were low, and that emplacement was slow or gradual. We examine plausible rates of emplacement for CFB pâhoehoe-dominated flow fields (the products of individual eruptions). It is important to define parameters being compared. Effusion rate is either an instantaneous or time-averaged volume flux feeding a lava flow from a vent. Eruption rate, or mean output rate, is the total volume of lava extruded during a fixed period of time, or, for the latter, the whole eruption. Because CFB flow-fields only provide total volume of lava emitted, and occasional indirect estimates of emplacement durations for individual sheet lobes, mean output or eruption rates must be inferred by comparison with basaltic fissure eruptions with known output rates. Kilauea’s on-going eruption has produced a 2 km³ compound lava flow-field in order 20 years, probably far too slow to account for a typical CFB field (1000 km³, thus 10,000 years of eruption). Laki 1783-4 produced a 15 km³ flow-field in 8 months, of which 9 km³ formed in the first 1.5 months, a sustained mean eruption rate 0.2 cu km/day (including surges at > 1 km³/day), or > 2000 m³/s; much of the flow character is rubbly pâhoehoe. This was followed by a period of eruption rate 0.06 km³/day (640 m³/s), both phases venting from fissure segments a few km long. We predict that CFB eruption rates might be similar to that of Laki (at the higher rate, a 1000-km³ CFB flow field would form in 14 yrs), but will also fluctuate throughout an eruption. Such enormous mean rates from fissure vents (100s-1000s m³/s) would feed numerous inflating sheet lobes (each containing 0.05-0.5 km³ of lava), where most of the lava output is emplaced. However, each lobe need only have local injection rates of a few to 10s of m³/s in order to thicken within plausible time-frames (months to a few years). Significantly higher mean output rates (equating to much shorter eruption durations) rates do not provide sufficient time to explain the growth of inflated lobes with observed crustal thicknesses and show that, compared with almost all historic basaltic volcanism, rates at which typical CFB lavas formed were at times spectacularly high, especially for certain parameters.