AGU Fall Meeting 18 December 2008 Photo courtesy of La Prensa Gráfica Crater lake evolution during volcanic unrest: case study of the 2005 eruption at Santa Ana volcano, El Salvador. Anna Colvin, Bill Rose, Demetrio Escobar, Eduardo Gutierrez, Francisco Montalvo, Rodolfo Olmos, Joop Varekamp, Matt Patrick, & Jose Luis Palma

Santa Ana volcanic complex, El Salvador Most active volcano in El Salvador High risk: 1 million people live within a 25 km radius (17% total population of El Salvador) [Pullinger, 1998], [DIGESTYC, 2008]. Courtesy of USGS

2005 eruptive crisis & phreatic eruption Incandescent fumaroles—not magma (!) Hot acidic lahar Eruption column to ~14km altitude 1 Oct late Aug Courtesy of La Prensa Gráfica Courtesy of SNET Courtesy of El Diaro del Hoy

Santa Ana crater 4 Feb 2007 Orthorectified ASTER image After 28 June 2007 Before 31 August Feb 2001 Orthorectified ASTER image Courtesy of El Diaro del Hoy

Crater lake evolution, Low level activityearly 2000 [Bernard et al., 2004] Hydrothermal activity May Feb 2002 [Bernard et al., 2004] Low level activityFeb Jun 2004 Hydrothermal activity Jun Aug 2005 long-range precursors (months) Fumarolic activity Aug & Sep 2005 short-range precursors (weeks) Hydrothermal activity Oct 2005 – Oct 2005 eruption

Geochemical trends

Seismic and gas flux trends

Relative contributions to degassing from lake and fumaroles Pre-2005: Majority of degassing escapes through subaerial fumaroles Post-2005: Sulfur scrubbing by precipitation of native sulfur

Schematic model: Post-eruption Sulfur spherules

Lake Name Lake Type Representative Values Temp.pH Size (radius) Power Output Eruption History Santa Ana pre-2005 high activity, cool acid brine 16-30ºC0.7 to m MW Phreatic eruption Oct Yugama, Kusatsu Shirane high activity, cool acid brine 8-33ºC1 to m3-25 MW Phreatic eruptions; cool between eruption; hot before and after eruptions. Santa Ana post-2005 peak activity/ variable mass 25-65ºC0.4 to 1.2 <100m MW Upwelling, several lake evaporations & minor phreatic eruption. Laguna Caliente, Poas peak activity/ variable mass 38-96ºC-0.87 to m MW Phreatic activity, lake disappears in April 1989 and liquid sulfur pools form. Comparison to other crater lakes Volcanic lake classification based on Varekamp et al. (2000).

Conclusions Analysis of crater lake evolution & integration with gas emission and seismic data has allowed for identification of eruption precursors and interpretation of a possible eruption triggering mechanism. Long-range precursors (Jun 2004-Aug 2005): — crater lake warming, LP seismicity, geochemical constancy. Short-range precursors (Aug & Sept 2005): — fumarole incandescence, high gas emissions, VT swarms, banded tremor.

Likely eruption triggering mechanism: magmatic intrusion does not reach the surface but induces overpressure in the hydrothermal system and triggers a phreatic eruption. On-going intrusion (?) more plausible with sustained high lake temperatures and may yet trigger a phreatomagmatic/magmatic eruption. Satellite & ground-based remote sensing will be vital for future monitoring. Conclusions

Acknowledgements Funding Sources: Dept. of Geological & Mining Eng. & Sciences, Michigan Tech Univ. National Science Foundation OISE & PIRE EAR DeVlieg Foundation Fellowship Collaborations: Michigan Technological University Dr. Bill Rose, Dr. Matt Patrick (now at USGS/HVO), Dr. Ann Maclean, Dr. John Gierke, Dr. Jose Luis Palma, Dr. George Robinson, RS4Haz graduate students Wesleyan University Dr. Joop Varekamp Servicio Nacional de Estudios Territoriales Demetrio Escobar, Eduardo Gutierrez, Francisco Montalvo Universidad de El Salvador Rodolfo Olmos & students LaGeo S.A. de C.V. Carlos Pullinger, Marvin Garcia