The evolution of an active silicic lava flow field: an ETM+ perspective

Abstract

An active dacitic lava flow began advancing down the south flank of the Caliente dome unit (Santiaguito Volcano, Guatemala) during July 1999. By January 2002 continued activity had built a ∼1-km 2, 66×10 6 m 3 flow field. Using a time series of Landsat 7 ETM+ images and field observations the evolution and characteristics of this flow field are described. Throughout the period of emplacement, flow front advance rates have been extremely slow (2–13 m day −1), consistent with a viscosity of 10 9–10 10 Pa s. However, extremely efficient thermal insulation has allowed flow fronts to extend up to 3.75 km from the vent, in spite of extrusion rates of just 0.5–1.6 m 3 s −1. Surface temperatures along most of the flow are less than 75 °C, and typically ∼40 °C. Such surface temperatures result in extremely low rates of heat loss, and thus core cooling of just 0.03–0.09 °C h −1 or 0.01–0.12 °C m −1. The flow field itself clearly displays the same systematic, spatial variations in structure and morphology as described by Lipman and Banks [Prof. Pap. - Geol. Surv. U. S. 1350 (1987) 1527] for a basaltic ′a′a flow, where a stable, and then transitional, channel zone feeds a distal area of dispersed flow. The stable channel dominates the proximal and medial sections of the flow field, and comprises a 90–375-m-wide channel flanked by levees that are up to 117 m high. A marked increase in the flow area and thickness occurred between January 2001 and January 2002, and was coincident with an increase in the extrusion rate from 0.5–0.7 to 1.2–1.6 m 3 s −1. However, this increase in extrusion rate did not bring an increase in flow lengths, indicating that the flow may have reached its cooling-limited length around January 2001.