Volcanic plume and bomb field masses from thermal infrared camera imagery

Masses erupted during normal explosions at Stromboli volcano (Italy) are notoriously difficult to measure. We present a method that uses thermal infrared video for cooling bomb fields to obtain the total power emitted by all hot particles emitted during an explosion. A given mass of magma (M) will emit a finite amount of thermal power, defined by M c(p)(T-e-T-o), c(p) and T-e being magma specific heat capacity and temperature, and To being ambient temperature. We use this relation to convert the total power emitted by the bomb field to the mass required to generate that power. To do this we extract power flux curves for the field and integrate this through time to obtain total power (E). This is used to estimate mass (Q) in Q=E/c(p)(T-e-T-o). When applied to individual bombs we obtain masses of between 1 and 9 kg per bomb, or a volume of 970 and 6500 cm(3). These volumes equate to spheres with diameters 12 and 27 cm. For the entire bomb field we obtain volumes of 7-28 m(3). We calculate masses for 32 eruptions and obtain typical bomb masses of between 10(3) and 10(4) kg per eruption. In addition, we estimate that between 10(2) and 10(3) kg of gas and ash are emitted as part of a mixed plume of bombs, gas and ash. We identify two types of eruption on the basis of the erupted bomb masses and the ratio of the plume's gas-and-ash component to the bomb component. The first type is bomb-dominated, is characterized by bomb masses of 10(4) kg and has ash gas/bomb ratios of similar to 0.02. The second type is ash-and-gas dominated, is characterized by erupted bomb masses of 10(3) kg and has ash-gas/bomb ratios of around one, and as high as two. There is no correlation between the quantity of bombs and quantity of gas-ash erupted. In addition, while source pressure for each explosion correlates with the quantity of gas and ash erupted, the mass of bombs emitted varies independently of pressure.