AGU Fall Meeting18 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. 2005

late Aug. 2005

Courtesy of La Prensa Gráfica

Courtesy of SNET

Courtesy of El Diaro del Hoy

Santa Ana crater

4 Feb 2007Orthorectified ASTER image

After28 June 2007

Before31 August 2005

3 Feb 2001

Orthorectified

ASTER image

Courtesy of El Diaro del Hoy

Crater lake evolution, 2000-2007

Low level activity early 2000

[Bernard et al., 2004]

Hydrothermal activity May 2000 - Feb 2002

[Bernard et al., 2004]

Low level activity Feb 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

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

LakeName

Lake Type

Representative Values

Temp. pHSize

(radius)

Power Output

Eruption History

Santa Ana pre-2005

high activity,cool acid brine

16-30ºC 0.7 to 2

100 m 0.5-20 MW

Phreatic eruption Oct 2005.

Yugama, KusatsuShirane

high activity, cool acid brine

8-33ºC 1 to 1.8

135 m 3-25MW

Phreatic eruptions; cool between eruption; hot before and aftereruptions.

Santa Ana post-2005

peak activity/variable mass

25-65ºC 0.4 to 1.2

<100m 13-830MW

Upwelling, several lake evaporations & minor phreatic eruption.

Laguna Caliente, Poas

peak activity/ variable mass

38-96ºC -0.87 to 0.26

140 m 150-550MW

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 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.

• 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

AcknowledgementsFunding Sources:Dept. of Geological & Mining Eng. & Sciences, Michigan Tech Univ.National Science Foundation

OISE & PIRE 0530109EAR 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 UniversityDr. Joop Varekamp

Servicio Nacional de Estudios TerritorialesDemetrio Escobar, Eduardo Gutierrez, Francisco Montalvo

Universidad de El SalvadorRodolfo Olmos & students

LaGeo S.A. de C.V.Carlos Pullinger, Marvin Garcia