Has Dynamic Equilibrium Been Re-established for the Fluvial Landscape on the 1991 Mt. Pinatubo Ignimbrite Sheet?

Ignimbrite forming eruptions are rare events, and here report on the first study of an actively eroding ignimbrite sheet at Mt. Pinatubo (15° N, 120.5° E) deposited on the 15^th June 1991. An important question in the evolution of ignimbrite landscapes is how long do they take to reach dynamic equilibrium. At Mt. Pinatubo, the implication is that once dynamic equilibrium is reached the magnitude of risk posed by sediment-flow-related hazards is greatly reduced. Large erosional events, which generate high volume lahars, are unlikely except in the event of a major disruption to the normal pattern of erosion. This study used three images produced by the SPOT (System Pour l'Observation de la Terre) satellite. SPOT data are in three spectral bands (0.50-0.59μ m; 0.61-0.68μ m; 0.79-0.89μ m) with a spatial resolution of one pixel equals 20m. These were used to generate five false color RGB images of the Mt. Pinatubo region), showing the pre-eruption (April 1988), early post-eruption (December 1991) and later post-eruption (December 1994, February 1996, December 1998) landscapes. Drainage networks that developed between 1991 and 1998 were delineated using these images. Geographically we concentrated on the western side of the volcano, where approximately two-thirds of the ignimbrite was deposited in just three drainage basins; Marella, Balin-Baquero and Bucao. The nature of the drainage networks, combined with field-based observations, suggest there are four stages in the landscape evolution of the Mt. Pinatubo ignimbrite sheet. Stage one; The emplacement of a pristine ignimbrite surface, which in-filled the pre-eruption drainages, but did not bury all the surrounding topography. Stage two; Large amounts of unstable, excess sediment were removed. Stage three; The landscape simplified towards the stable state of dynamic equilibrium. Stage four; Erosion continues but with the landscape in a state of dynamic equilibrium, thus large erosional events are unlikely. The Horton-Strahler classification method for drainage networks was used to find bifurcation ratios that were assumed to be representative of the complexity and development of the networks in each drainage basin. Comparison of these data suggest that the Marella and Bucao drainage basins had re-established dynamic equilibrium by 1995. However, Balin-Baquero did not appear to have re-established dynamic equilibrium by 1998 (the latest SPOT image studied). The analysis for this drainage basin was then supplemented by data from NASA airborne interferometric synthetic aperture radar instrument, TOPSAR. TOPSAR provides data with information in three spatial dimensions, allowing the generation of a DEM, which shows the volume distribution of the landscape around the volcano. The concept of a hypsometric integral was used along with the DEM data and emplaced ignimbrite volume estimates made in 1991, to estimate the volume of excess sediment that needed to be removed for the Balin-Baquero basin to enter the forth stage of evolution. Our most realistic estimate is that dynamic equilibrium was re-established in 2000 +/- 1 year, which is in agreement with an extrapolation of the trend shown by the bifurcation ratios and also with recent predictions made by PHIVOLCS scientists.