SULFUR OUTPUT AND MAGMA DEGASSING BUDGET OF STROMBOLI VOLCANO

STROMBOLI volcano in the Aeolian islands has been erupting continuously for more than 2,000 years(1), 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 km(3) yr(-1) of magma, 100-200 times more than the volume erupted. Over 5,000 years, this rate implies that 50-100 km(3) of intruded basalt would have been degassed, suggesting either that the volcanic pile has grown substantially by intrusion(6) 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.