Temperature proxy data and their significance for the understanding of pyroclastic density currents

A major dome collapse of the Soufriere Hills volcano, Montserrat, on 26 December 1997 generated a devastating pyroclastic density current that destroyed vegetation and left a distinctive tar-like deposit on the surface. The deposit included a range of charred, mainly herbaceous angiosperm, axes and roots. Studies of reflectance of these charcoalified plants indicated mean reflectances of 0.72-1.16 for the three samples with a maximum reflectance of 1.77 recorded for all readings. These data provide minimum temperatures of the How of 300-425 degrees C, consistent with organic geochemical data obtained by C-13 solid-state nuclear magnetic resonance and gas chromatography-mass spectrometry. These temperatures have been used to calculate characteristics of the pyroclastic density current. We estimate flow front current densities near the ground of 1.8-3 kg/m(3), using constraints from a mean flow speed of 90 m/s estimated from seismic data. The mean temperature of the ash component is estimated as 400-610 degrees C.