The 2004--05 eruption of Etna was characterised by outpouring of degassed lava from two vents within Valle del Bove. After three months of eruption lava volumes were estimated to be between 18.5 and 32 ¿ 106 m3, with eruption rate between 2.3 and 4.1 m3/s. Petrological analyses show that magma is resident in the shallow plumbing system, emplaced during the last South-East Crater activity. SO2 flux data show no increase at the onset of the eruption and SO2/HCl ratios in gas emitted from the eruptive fissure are consistent with a degassed magma. No seismic activity was recorded prior to eruption, unlike eruptions observed since the 1980's. The purely effusive nature of this eruption, fed by a degassed, resident magma and the fracture dynamics suggest that magmatic overpressure played a limited role in this eruption. Rather, lateral spreading of Etna's eastern flank combined with general inflation of the edifice triggered a geodynamically-controlled eruption.