Late quaternary explosive silicic volcanism on St Lucia, West Indies

Abstract

Many explosive eruptions of dacitic magmas have occurred on St Lucia during the late Quaternary. These have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands, with co-eruptive air-fall and surge layers interbedded with palaeosols and epiclastic deposits. Vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits. Young magmatically related dacitic lava domes have been extruded in the Qualibou depression. The pumice succession can be divided into older quartz-poor deposits forming the Choiseul Pumice and younger crystal-rich deposits with abundant large quartz which are called the Belfond Pumice. The Choiseul Pumice groups together scattered remnants of the products of many eruptions of different low-silica dacitic magma types. The Belfond Pumice is the product of several eruptions of a high-silica magma type and 14C ages have dated these between 20900 to 34200 years B.P.The pumice flow deposits occur as small-volume valley fills. A granulometric study of Belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components. It is suggested that the narrow, winding and vegetated valleys on the island locally induced turbulence and the flows moved with large, highly fluidized and inflated heads, resulting in substantial loss of fine vitric ash. One ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses. This flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation. Ignition of this released the large quantities of gas needed to elutriate most of the fines.A model is suggested for the recent volcanic activity on St Lucia in which separate batches of silicic magma, each having a distinctive petrological and chemical character, rose into high level chambers over a large area. Eruptions of volatile-rich magma led to highly explosive pumice-forming activity from vents in the central highlands. Degassed and more crystal-rich magma was extruded later from the same vents or in the attenuated flank of the Qualibou depression to from lava dome complexes.

Volcano flank instability in the Lesser Antilles Arc: Diversity of scale, processes, and temporal recurrence

“…A very active fumarolic field is present in the central part of the structure. The last historical explosive event was phreatic and occurred in 1760 A.D. [ Tomblin , 1965; Wright et al , 1984, Lindsay , 2005]. …”

Ash-plume dynamics and eruption source parameters by infrasound and thermal imagery: The 2010 Eyjafjallajökull eruption

“…This is in contrast to strong plumes, characterized by an ascending velocity of the buoyant column larger than that of wind. Weak plumes are significantly more frequent and, when associated with long-lasting eruptions, can not only affect local communities but also cause prolonged disruptions in the global air traffic, with effects that go far beyond the direct impacts on the air transport industries (Ernst et al, 1994;Guffanti et al, 2010;Miller and Casadevall, 2000;Prata and Tupper, 2009;Rose et al, 1995;Scollo et al, 2009;O'Regan, 2011).Strong and weak plumes are characterized by significantly different dynamics (e.g., Morton et al, 1956;Briggs, 1969;Turner, 1973;Wright et al, 1984;Sparks, 1986;Carey and Sparks, 1986;Coelho and Hunt, 1989;Bursik, 2001) and result in very different tephra deposits (e.g., Ernst et al, 1994, Bonadonna et al, 2005. As a result, empirical and theoretical relations that link plume height and MER for strong plumes (e.g., Wilson and Walker, 1987;Sparks, 1986;Mastin et al, 2009) cannot be easily extrapolated to weak plumes, making the realtime characterization of the source term even more complex (Degruyter and Bonadonna, 2012).…”

Aberrant thermoluminescence dates obtained from primary volcanic quartz

“…Wrong attribution is unlikely, because the formations of Choiseul and Belfond are very characteristic and extend over a large geographical area. The 14 C ages are unlikely to be wrong because they come from several independent measurements on charcoal from different places and are coherent (Wright et al, 1984;Lindsay et al, 2005). Concerning the K-Ar ages, the unspiked method associated with specific protocols for sample preparation has been shown to be well-adapted to the measurement of young ages (Gillot and Cornette, 1986;Quidelleur et al, 2001).…”

“…Since the mid 1960s, various authors have studied the geology and stratigraphy of the volcanic unitsofthisisland (Tomblin, 1964(Tomblin, , 1979Martin-Kaye, 1969;Le Guen de Kerneizon et al, 1983;Vatin-Pérignon and Chevallier, 1983;Wright et al, 1984;Vatin-Pérignon et al, 1985). This volcanism is represented by widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands as well as young dacite domes extruded in volcanotectonic depressions.…”

“…Pure (a) Ages measured by unspiked K-Ar method (Quidelleur et al, 2004;Samper et al, 2007), (b) Geological evidence (young sub-active zone with fumaroles activity) and K-Ar determination, (c) BP uncalibrated 14 C based on multiple determinations on charcoals (Wright et al, 1984;Roobol et al, 1983;Lindsay et al, 2005).…”