Sulphur output and magma degassing budget of Stromboli volcano

STROMBOLI volcano in the Aeolian islands has been erupting continuously for more than 2,000 years¹, and probably as many as 5,000, following a major flank collapse^2,3. Here we describe air-borne measurements of the plume flux of SO₂ during 1980-93, which show that the volcano emits very large amounts of gas, mostly by open-conduit degassing between explosive outbursts, while exuding little basalt. Microprobe analysis of sulphur in the K-rich (shoshonitic) basalt, along with data for primitive basalts in the region^4,5, suggests that the time-averaged SO₂ flux is produced by intrusive degassing of 0.01-0.02 km³ yr^-1 of magma, 100-200 times more than the volume erupted. Over 5,000 years, this rate implies that 50-100 km³ of intruded basalt would have been degassed, suggesting either that the volcanic pile has grown substantially by intrusion⁶ or, more probably, that a large magma storage system is emplaced at a shallow level within the crustal basement. Our results indicate that Etna and Stromboli alone provide 10% of the global budget of volcanic SO₂.