GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.3 273 The conversion of spectral radiance to lava mass or volume flowrate has been investigated by several authors in the last six decades (e.g., Yokoyama, 1957; Pieri and Baloga, 1986; Harris et al. , 1997; Dragoni and Tallarico, 2009; Harris and Baloga, 2009; Garel et al. , 2012; Vicari et al. , 2011) employed infrared satellite data (Moderate Resolution Imaging Spectroradiometer, Advanced Very High Resolution Radiometer, and Spinning Enhanced Visible and Infrared Imager) to estimate lava eruption rates at Mount Etna. They used duration and lava volume data of about 130 episodes (Behncke et al. , 2006; Andronico et al. , 2008) and defined six eruptive classes representing the lava volume in relation to the entire eruption duration. In each class the effusion rate starts from a low value, reaches its maximum value after at about quarter of the entire eruption duration, and slowly decreases until the end of the eruption is reached. The volume flow rate ranges between 10 and 140 m 3 /s, the emitted lava volume ranges between 0.03 and 0.20 km 3 , and the eruption duration is less than 90 days. In association with lava flows, erosion has been often mentioned and is well documented in tube-fed and channel-fed flows in basaltic lavas. Greeley et al. (1998) studied the role of the conduit erosion (thermal and mechanical) in active lava flows on Earth and other planets. Hulme (1982) for Kilauean lava flows proposed erosion by melting and estimated an erosion rate in the order of 2.5 m/month. Peterson and Swanson (1974) observed that to initiate melting at least a few days of continuous flow are required and that erosion rates are about 2 m/month. On Hawaii Kauahikaua et al. (1998) measured thermal and mechanical erosion between 1.5 and 3.0 m/month. On Etna Calvari and Pinkerton (1999) found evidence only of mechanical erosion inside lava tubes and channels. Dragoni and Santini (2007) assuming that the erosion rate of the wall of the tube is proportional to shear traction, calculated the erosion of the wall as a function of time and found that the shear stress produced by the flow makes elliptical tubes closer to the circular shape. We present a model to show the role of mechanical erosion in controlling the effusion rate of basaltic eruptions and, in particular, the increase in effusion rate that is commonly observed in these eruptions. We consider the emptying of a magma chamber through a vertical cylindrical conduit with elliptical cross section, and we investigate the possibility that the increase in flow rate is due to a widening of the volcanic conduit due to erosion of the conduit wall produced by the magma flow. Figures show the effects of mechanical erosion on the conduit width, expressed by the semiaxis B(t) and the volume flow rate Q ( t ). The curves of B ( t ) show that the erosion rate is in the order of field observations (Fig. 1). In Fig. 2 we recognize the typical dependence on time of Q ( t ) in basaltic eruptions, and we conclude that mechanical erosion can explain this dependence. In particular, the model can reproduce the shape of the curves found by Vicari et al. (2011) for Mount Etna, the maximum values of Q(t) eruption durations, and emitted lava volumes. The choice of values of parameters assures that the total lava volumes are comparable to field observations. We note different values of eruption duration, which depends not only on magma chamber radius but also on the parameters viscosity and the erosion rate per unit traction k . We note that in general, the value of k has a great influence on the magma flow, whereas the radius of magna chamber and viscosity have a smaller effect: in particular, Fig. 1 – Evolution of the semiminor axis of a volcanic conduit B(t) for a choice of values of parameters.