The January 2014 Eruption of Pacaya (Guatemala): Rheology and Morphology through Field Observations and Laboratory Experiments
Monday, 15 December 2014
The morphology of lava flows depends largely on rheological properties (viscosity, yield strength), which in turn vary with its physical properties (composition, crystal and bubble fraction, temperature). We are investigating how these properties vary in space and time (1) along and across a flow at a particular instant in time, and (2) for a single quantum of magma from vent to final emplacement. Our study area is Pacaya (Guatemala). Its latest effusive eruption begun on January 10th 2014 and produced two lava flows emitted from newly-formed vents, aligned NW-SE across the summit. Flow surface profiles were obtained with a Laser Range Finder, and extensive sampling was conducted on both flows while still active or shortly afterwards. The lavas are basalts (SiO2 = 50-51.5%), the SE flow being slightly more evolved than the NW one. Both flows are less alkali-rich than historical flows, even as recently as 2010. The mineralogy consists of plagioclase, olivine, pyroxenes, and minor oxides. The samples are highly crystalline (~60% phenocrysts + microlites) and considerably vesicular (≥25%, decreasing downflow), with mostly interconnected porosity. The flow velocity, estimated from videos taken in the field, decreased from ~2.8 m/s by the vent, where the flow surface is mostly incandescent pahoehoe, to ~0.5 m/s about 1 km downflow, where the surface is mostly dark `a`a. This velocity decrease is consistent with cooling and crust formation, as well as a decrease in slope from 45° to 4°. The effects of crystallization on the evolution of the 2014 lavas rheology are being investigated through concentric cylinder and parallel plate viscometry, and will be compared to the 2010 data. Lavas erupted in 2010 have a viscosity of ~75 Pa s at their liquidus temperature of ~1260˚C, increasing to ~250 Pa s at ~1240˚C (~25% crystals) and ~800 Pa s at ~1225˚C (~36% crystals). At subliquidus conditions, lava rheology is distinctly non-Netonian, and best fit with a power-law rheology, but yield strengths are low even at high crystal fractions (e.g. ~140 Pa at 42% crystals). Rheological gradients are therefore expected to be very strong even within the incandescent flow interior. Experimental study of the 2014 lavas, combined with observations of their emplacement, will allow us to better understand how evolving rheology affects final flow morphology.