The Soufriere Hills volcano on Montserrat has been continuously active since 1995. In this time activity has been characterised by andesitic dome growth and collapse events, producing pyroclastic flows and vulcanian eruptions. However, little is known about earlier volcanism on Montserrat, which extends back 2.5 million years.
We have used chemical tracers to investigate the evolution of the four main volcanic centres on Montserrat: Silver Hills (SH)(2600-1200 ka), Centre Hills (CH)(950-550 ka) and the South-Soufrière Hills (SSH)-Soufrière Hills (SH) complex (174 ka to present) (Fig 1).
Silver Hills and Centre Hills centres are chemically indistinguishable, despite spanning 2 million years. High-precision Lead isotope measurements of the volcanic material (Fig 2) show that SH and SSH volcanics are distinct from the earlier centres and each other. SSH can be further separated into Suite A and Suite B. Soufriere Hills and SSH were active in at the same time, but reflect very different sources of their magma.
SSH has an unusual composition amongst Caribbean volcanics in containing a greater amount of fluid derived from the subducting ocean crust. In contrast, the contemporary SH and earlier CH & SH centres are sourced from greater depths where melted sediments were combined with the fluids. Both types of magma were present during the volcanism occurring at around 130 thousand years ago. Eruptions in the post 1995 Soufriere Hills activity are all comparable with the earlier sediment-rich volcanic
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