A comparison of the recent eruptions of Mt. Pelée, Martinique and Soufrière, St. Vincent

Abstract

The simultaneous eruption of Mt. Pelee, Martinique and Soufriere, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuees ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelee produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufriere material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufriere material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelee reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelee tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufriere magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.