1. 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

2. 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

3. 2005 eruptive crisis & phreatic eruption Incandescent fumaroles—not magma (!) Hot acidic lahar Eruption column to ~14km altitude 1 Oct. 2005 late Aug. 2005 Courtesy of La Prensa Gráfica Courtesy of SNET Courtesy of El Diaro del Hoy

4. Santa Ana crater 4 Feb 2007 Orthorectified ASTER image After 28 June 2007 Before 31 August 2005 3 Feb 2001 Orthorectified ASTER image Courtesy of El Diaro del Hoy

5. Crater lake evolution, 2000-2007 Low level activityearly 2000 [Bernard et al., 2004] Hydrothermal activity May 2000 - Feb 2002 [Bernard et al., 2004] Low level activityFeb 2002 - Jun 2004 Hydrothermal activity Jun 2004 - Aug 2005 long-range precursors (months) Fumarolic activity Aug & Sep 2005 short-range precursors (weeks) Hydrothermal activity Oct 2005 – 2007 1 Oct 2005 eruption

6. Geochemical trends

7. Seismic and gas flux trends

8. 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

9. Schematic model: Post-eruption Sulfur spherules

10. 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 2 100 m0.5-20 MW Phreatic eruption Oct 2005. Yugama, Kusatsu Shirane high activity, cool acid brine 8-33ºC1 to 1.8 135 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 <100m13-830 MW Upwelling, several lake evaporations & minor phreatic eruption. Laguna Caliente, Poas peak activity/ variable mass 38-96ºC-0.87 to 0.26 140 m150-550 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).

11. Conclusions Analysis of crater lake evolution 2000-2007 & 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.

12. 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

13. Acknowledgements Funding Sources: Dept. of Geological & Mining Eng. & Sciences, Michigan Tech Univ. National Science Foundation OISE & PIRE 0530109 EAR 0732632 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