Monitoring so2 emission at the Soufriere Hills volcano: Implications for changes in eruptive conditions

Correlation spectrometer measurements of sulfur dioxide (SO₂) emission rates during the current eruption of the Soufriere Hills volcano, Montserrat, have contributed towards identifying different phases of volcanic activity. SO₂ emission rate has increased from <200 tonnes per day (td⁻¹; <2.3 kgs⁻¹) in the early stages of dome growth to >550 td⁻¹ (>6.4 kgs⁻¹) after July 1996, with the uncertainty associated with any individual measurement ca. 30%. Significantly enhanced SO₂ emission rates have been identified in association with early phreatic eruptions (800 td⁻¹ (9.3 kgs⁻¹)) and episodes of vigorous dome collapse and pyroclastic flow generation (900 to 1500 td⁻¹ (10.4 to 17.4 kgs⁻¹)). SO₂ emission rate has proved a useful proxy measurement for magma production rate. Observed SO₂ emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts, implying the existence of a S-rich magmatic vapour phase.