Effusive volcanism takes place where magmas are fluid and volatile-poor. The density difference between magma and crust and the pressures driving the ascent of magma are insufficient to create lava fountains. On Io, the most closely studied volcanoes where effusive volcanism is taking place, leading to the emplacement of extensive lava flow fields, are Prometheus and Amirani. At Prometheus in particular, study of the thermal emission and calculation of the effusion rate reveals the interior mechanisms and structure beneath the volcano by which magma is supplied to the surface. The closest terrestrial analogue to this style of activity is the current Pu'u 'O'o-Kupaianaha eruption of Kilauea, Hawai'i, where the typical mode of emplacement is that of inflating pahoehoe flows after magma transport through lava tubes over several kilometers.

Volcanic activity at Prometheus

Prometheus (Plates 6 and 7) is one of the most persistent of Io's volcanoes and the site of a volcanic plume that has been observed in every appropriate observation by Voyager and Galileo (e.g., Lopes-Gautier et al., 1999; Davies et al., 2006c). Study of the persistent eruption at Prometheus provides an insight into the mechanism of magma supply and heat transfer from the interior of Io to the surface and invites comparison with terrestrial analogues.