Eruption cyclicity of at La Soufrière, St Vincent over the past 5000 years - defining cycles and constraining activity

La Soufrière Volcano on the island of St Vincent, West Indies, has experienced four historical explosive eruptions of varying magnitude occurring in 1718, 1812, 1902 and 1979, which define a crude 80-100 year periodicity to the explosive activity. Recent fieldwork and extensive radiocarbon dating have established the stratigraphy of these historic as well as of two recent prehistoric events (that occurred in ~1440 and 1580 AD), showing that basaltic andesite scoria-rich pyroclastic density currents (PDCs) were typical in the last 600 years. Our radiocarbon dating shows that a similar broad periodicity of 80-140 years for explosive eruptions over the last 600 years, although there may have been a general decrease in magnitude and vigor of the more recent eruptions. Products on the eastern flank represent two or three notable eruptions of scoria–rich PDC and radiocarbon dating indicate that these relate to two clusters of dates, one around ~2500 and another at ~4500 yr BP. These dates suggest that the broad cyclicity of the period between 1000 and 5000 yr BP was characterized by larger, less frequent, eruptions with relatively minor or no activity between these larger events. The spatial distribution of the products indicates that PDCs in the last 600 years were significantly controlled by the pre- existing crater topography, whereas the older events (2000-5000 yr BP) were not. This suggests that the eruptions between 2000 and 5000 were significantly larger and more vigorous than those in the last 1000 years. It is possible that the size and time scale of these different events results in a bias, with smaller events being missed or the time period studied is too short to completely capture longer-term cycles. Further geochronology will refine the nature of eruptive cyclicity.