A simple explanation for the space-based calculation of lava eruption rates

Knowing how lava effusion rates vary during basaltic eruptions can be of gr eat significance when trying to provide preliminary forecasts regarding how far lava will flow. However, problems exist in accurately determining effu sion rates using conventional field-based techniques. To ameliorate this pr oblem Harris et al. [J. Geophys. Res. 102 (1997), 7985-8003; Bull. Volcanol . 59 (1997), 49-64; J. Volcanol. Geotherm. Res. 102 (2000), 237-269] develo ped a method for determining effusion rates using infrared satellite data, and showed how the method could be used to provide realistic estimates of e ffusion rates, repeatedly during several eruptions at Kilauea (Hawai'i) Kra fla (Iceland), Etna and Stromboli (Italy). Harris et al. [J. Geophys. Res, 102 (1997), 7985-8003; Bull. Volcanol. 59 (1997), 49-64] indicate that thei r method allows instantaneous lava effusion rates to be determined thermody namically by equating the amount of heat lost by an active lava flow (deriv ed from the satellite data) to the amount of heat liberated by the cooling mass of lava. The purpose of this paper is to provide a simpler, alternativ e explanation. We find that rather than being used to calculate heat loss, Harris et al. [J. Geophys. Res. 102 (1997), 7985-8003; Bull. Volcanol. 59 ( 1997), 49-64; J. Volcanol. Geotherm. Res. 102 (2000), 237-269] actually use the satellite data to estimate the area of active lava present within the satellite's field of view at the movement of data acquisition. Thus, change s in the effusion rates they present can only be proportional to changes in this area. The active flow areas were then multiplied by a constant, the v alue of which is obtained from a crude approximation of the lava flows heat balance. Crucially, the absolute value of this term falls within the range of an empirically derived parameter that was found by Pied and Baloga [J. Volcanol. Geotherm. Res. 30 (1986),29-45] to explain strong linear correlat ions between eruption rate (i.e. the time-averaged effusion rate) and lava flow area for 34 historic Hawaiian flows. As a result, we find that the met hod of Harris et al. [J. Geophys. Res. 102 (1997), 7985-8003] does not yiel d instantaneous effusion rates, but instead provides a valid and useful way to estimate average effusion rates (i.e. the eruption rate) from measureme nts of flow area. (C) 2001 Elsevier Science BY. All rights reserved.