Cristobalite in volcanic ash of the Soufriere Hills Volcano, Montserrat, British West Indies

Crystalline silica (mostly cristobalite) was produced by vapor-phase crysta llization and devitrification in the andesite Lava dome of the Soufriere Hi lls volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by Lava dome collapse contains 10 to 24 weight pe rcent crystalline silica, an enrichment of 2 to 5 relative to the magma cau sed by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 pe rcent) of crystalline silica. High Levels of cristobalite in respirable ash raise concerns about adverse health effects of Long-term human exposure to ash from lava dome eruptions.