Effects of Size and Temperature-Dependent Thermal Conductivity on the Cooling of Pyroclasts in Air

Plain Language Summary The knowledge of the cooling time scales of pyroclasts, in conditions of free to forced convection, is of paramount importance in microtextural analysis, development of welded deposits, and in eruption column and pyroclast flow modeling. We performed cooling experiments of heated rock inside a cylindrical wind tunnel under a range of air speeds. In order to estimate the heat transfer coefficients, we modeled the transient temperature distribution in the sample with temperature-dependent thermal diffusivity and heat capacity, which were obtained from laser flash measurements. For air speeds up to 15m/s and for sample temperatures up to 1383 K (1110 degrees C), a Nusselt-Reynolds (Nu-Re) relationship up to Re=3x 10(4) is established in this study. We find that the dependency of heat loss on size is particularly important for pyroclasts larger than approximate to 1mm. Our results suggest that the cooling time scales of pyroclasts could be large enough to cause postfragmentation modification of clast microtextures. We further show that clasts of few-centimeter and larger develop solid crusts with molten cores within time scales of a few minutes and therefore may inhibit postdepositional welding. Explosive volcanic eruptions commonly produce fragmented magma or pyroclasts of microns to decimeter in size. These extremely hot pyroclasts looses heat over time in the presence of colder ambient air. The cooling time scale determines the interior texture of pyroclasts, which provide valuable information about magma ascent and emplacement on the Earth's surface. Cooling also determines eruption column or pyroclast flow dynamics following volcanic eruptions. We study the cooling of pyroclast by combining laboratory experiments with natural lava and heat transfer calculations. We show the variation in cooling time scales for pyroclasts of different sizes and discuss its importance for understanding the pre- to post-eruptive volcanic processes.