From early September to early November, during the 2004--2005 Mt. Etna effusive eruption, petrologic monitoring was enabled by near-vent sampling of erupting lava and onsite SEM-EDS capabilities at INGV-CT. Significant differences in composition and temperature of lava erupted from 2920--2620 m and 2820--2320 m vents, attest to variably evolved shallow magmatic conditions within a geometrically complex storage volume. The petrography and glass compositions of water-quenched samples indicate that the magma feeding the 2004--2005 activity was likely to have been stored in the shallow plumbing system during the 2000 and 2001 activity, where it experienced volatile loss and extensive crystallization. Both geophysical and petrologic data suggest that this eruption was not triggered by the intrusion of a new dike from depth, but it was more likely that existing shallow magma reservoirs were passively mobilized and erupted in response to edifice deformation.