11 - Pillan and Tvashtar Paterae: lava fountains and flows

Summary

The eruptions at Pillan and Tvashtar Paterae are very different, in terms of lava emplacement, to that seen at Pele. At Pillan, in particular, the eruption began in a spectacular explosive style and then transitioned to effusive activity. As at Pele, the explosive eruptions at these locations were driven by low-viscosity silicate magmas. At Pillan and Tvashtar Paterae, however, the explosive phases were short-lived, with thermal emission rapidly building up to a peak and then gradually subsiding as the eruption style changed. Finally, lava effusion came to an end and the emplaced flows cooled and solidified. The early stages of the eruptions were dominated by lava-fountain activity that fed large flows. Extensive pyroclastic deposits were laid down in both cases.

An interesting result from the study of the Pillan eruption was the indication of the presence of ultramafic magmas on Io. These magnesium-rich flows dominated volcanism on Earth aeons ago. If they were being erupted on Io, then the mechanisms of eruption and evolution of a process long extinct on Earth were being revealed. This possibility raised an intriguing question: could Io be an analogue for the early Earth (Matson et al., 1998; McEwen et al., 1998b)? The merits of the case for ultramafic magmas on Io were discussed in Chapter 9.