THE PRESERVATION OF WOODS IN VOLCANIC PYROCLASTIC FLOWS AND SURGES

Hot pyroclastic flows and surges from the Soufrière Hills Volcano, Montserrat, in 1997 entombed local vegetation. The hot ash charred the woods that comprised living, dead and rotted woods. Some of the woods remained buried in hot ashes for several months. Specimens were recovered from the 21th September flows from near Bramble Airport and were studied by scanning electron microscopy and by reflectance microscopy. All recovered specimens (up to 15 cm in diameter and 20 centimetres long) were charred throughout. Unlike charcoalified logs resulting from wildfires these tended to remain in tact and not break into the characteristic cubic shapes. Sem studies show excellent anatomical preservation of the wood. Wood from trees which may have been living when hit by the hot ash show evidence of sudden rupturing of the wood. All the studied specimens were angiospermous. Reflectance values from the woods indicate a range of temperatures to which the woods were subjected , mainly in the range 200-340^oC, possibly up to 450^oC. A suite of charcoalified logs and twigs have also been studied from the North Island of New Zealand. Specimens were studied from the Taupo Ignimbrites (1.8ka) and the Kaharoa pyroclastics (650a) (collected by Colin Wilson). The pyroclastic flows and surges incorporated vast quantities of vegetation (probably 1km³) and most of what has survived to the present day is charred: uncharred vegetation having rotted away. The specimens included both angiosperms and conifers and included logs and twigs ranging from 0.5-15 centimetres in diameter and up to 33 centimetres long. Sem shows excellent anatomical preservation of the woods. Reflectance data between sites indicates a wide variety of charring temperatures, from above 450^oC to 225^oC from 11km to 43km from the vent. These studies indicate that volcanic pyroclastic flows and surge deposits may preserve charcoalified plants that may yield data not only on the vegetation which was entombed but also yield data on the nature of the deposits themselves.