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

Abstract

Knowing how lava effusion rates vary during basaltic eruptions can be of great significance when trying to provide preliminary forecasts regarding how far lava will flow. However, problems exist in accurately determining effusion rates using conventional field-based techniques. To ameliorate this problem Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003; Bull. Volcanol. 59 (1997), 49–64; J. Volcanol. Geotherm. Res. 102 (2000), 237–269] developed a method for determining effusion rates using infrared satellite data, and showed how the method could be used to provide realistic estimates of effusion rates, repeatedly during several eruptions at Kilauea (Hawai’i) Krafla (Iceland), Etna and Stromboli (Italy). Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003; Bull. Volcanol. 59 (1997), 49–64] indicate that their method allows instantaneous lava effusion rates to be determined thermodynamically by equating the amount of heat lost by an active lava flow (derived 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, alternative 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, changes 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 value 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 Pieri and Baloga [J. Volcanol. Geotherm. Res. 30 (1986),29–45] to explain strong linear correlations 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 method of Harris et al. [J. Geophys. Res. 102 (1997), 7985–8003] does not yield instantaneous effusion rates, but instead provides a valid and useful way to estimate average effusion rates (i.e. the eruption rate) from measurements of flow area.