Lascar
Lascar is the most active volcano in the Central Andes of northern Chile according to thermal satellite data presented in Oppenheimer 1993. It is in the very remote Antofagasta region, 34 km east-southeast from the village of Toconao (23°22’S, 64°44’W). Lascar is a calk-alkaline strato-volcano 5592 m above sea level. At such a high altitude the gasses emitted from the volcano have much longer atmospheric lifetimes, and are likely to have greater environmental effects (Mather 2004).
Lascar crater. From http://www.atacamaphoto.com/atacama/atacama95.
The volcano has demonstrated very cyclic behaviour. Matthews 1997 have identified four cycles since 1986. Cycles involve a period of dome building and lots of degassing. Mather 2004 shows us that Lascar is a significant emitter of SO2, HCl and HF. This dome then subsides back into the conduit where subsidence is accommodated along the visible concentric, cylindrical or inward dipping fractures, generating some of the observed seismicity. As the dome subsides, gas emissions decrease suggesting some form of inhibition. Small eruptions occur during the period of dome subsidence, releasing some of the pressure built up due to gas emission inhibition. The cycle finally culminates by a relatively large vulcanian- plinian eruption with columns over 10kms high. Major eruptions, signifying the termination of a cycle occurred on 14-16th September 1986, 20th February 1990, 19-20th April 1993, and 17th December 1993. Wooster 1997 identified significant thermal decreases using SWIR (short wave infrared) from satellites before the 1986 and 1993 eruptions. Between April 1992 and February 1993 there was a 30% decrease in 1.6μm radiation (Wooster 1997).
Picture of 1993 Lascar eruption. From http://www.cgd.ucar.edu/ccr/ammann/volc/Volc_Pic/Lascar1993.jpg
The third identifiable cycle Feb 1990- April 1993 culminated in one of Lascars largest eruptions, so large that tephra fell on Paraguay, Uruguay, Brazil, and Argentina including Buenos Aires 1500 km away. This large eruption appears to have changed the volcanoes behaviour (Matthews 1997). Prior to this eruption cycles were typically 2 years long, but after the 1993 the last identifiable cycle was just 8 months long, and after this lava domes have not been observed (Matthews 1997). However large, explosive eruptions still periodically occur with continuous degassing.
SO2-Ar*100-H2S*10 ternary diagram showing three components that explain the fumerole emission compositions. For Active Crater fumaroles collected in November 2002 (closed squares), May 2005 (open circles) and October 2006 (closed triangles for fumaroles with T > 150°C; open triangles for fumaroles with T<150°C) From Tassi 2009.
Tassi 2009 conducted some isotopic work. δC - CO2 ratios of emissions were around -1.47‰ to -3.34‰, consistent with CO2 originating directly from the mantle (Hoefs 1973). CO2/3He of 3.19 x 109 to 1.66 x 1010 are slightly higher than those found at MOR (mid ocean ridges), but are similar to other fumarole values in Central America, suggesting that some of the CO2 is from a subducted sediment component. Further analysis of fumarole discharges showed that the composition could be explained by the mixing of three different components, hydrothermal, magmatic, and meteoric (air) (see SO2-H2S-Ar ternary diagram). Interestingly and also expectedly, fumaroles displaying a higher magmatic component were the hotter more central fumaroles, and the cooler more peripheral fumaroles display a larger meteoric and hydrothermal component. Andres 1991 noted that the SO2 emitted from Lascar was significantly (50-100 times) greater than that calculated via petrological methods, indicating there is an excess of SO2 emission. This means that more SO2 is being emitted than can be accounted for by the erupted lava.
Lascars continuous degassing makes it an ideal study site. Plus the lack of cloud within the region allows satellite data to be used and improved (Flynn 2001).
CLICK HERE FOR COMPOSITION OF LASCAR GAS EMISSIONS
References
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Mather T A, Tsanev V I, Pyle D M, McGonigle A J S, Oppenheimer C, Allen A G.Characterization and evolution of tropospheric plumes from Lascar and Villarica volcanoes, Chile. J Geophys Res 2004;59:72–82.
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Matthews S J, Jones A P, Beard, A.D. Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas. Journal of the Geological Society, London, 1994b;151:815-823.
Matthews S J, Gardeweg M C, Sparks R S J. The 1984 to 1996 cyclic activity of Lascar Volcano, Northern Chile; cycles of dome growth, dome subsidence, degassing and explosive eruptions. Bulletin of Volcanology 1997;59:72–82.
Oppenheimer C, Francis P W, Rothery D A, Carlton R W T. Infrared image analysis of volcanic thermal features: Lascar Volcano, Chile 1984–1992. J Geophys Res 1993;98:4269–4286.
Tassi F, Aguilera F, Vaselli O, Medina E, Tedesco D, Delgado Huertas A, Poreda R J, Kojima S. The magmatic- and hydrothermal-dominated fumarolic system at the Active Crater of Lascar volcano, northern Chile, Bull Volcanol. 2009;71: 171-183.
Wooster M J, Rothery D A. Thermal monitoring of Lascar Volcano, Chile using infrared data from the along-track scan- ning radiometer: a 1992–1995 time series. Bull Volcanol 1997;58:566–579.
