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Popocatepetl is a composite andesite volcano with a partly glaciated summit, standing at 5452 masl, and forms part of the central volcanic front of the Mexican Transvolcanic Belt (Boudal 1992). It is approximately 55km SE of Mexico City, and 45 km NW of Pueblo city, making the surrounding region a fairly densely populated one.
Figure from http://opentravel.com/blogs/ten-most-dangerous-volcanoes-on-the-globe/
The volcano was built upon the partly eroded scar of an older volcano approximately 4-5 kyr ago (Cantagrel 1984). Beneath the volcano is a 3km thick cretaceous limestone, shale and evaporite sequence overlying a Palaeozoic metamorphic basement (Fries 1965). There have been around 15 eruptions since the 16th century. The two most recent eruptions occurred in the 1919-1927 and 1994-present and consisted of effusive and explosive phases. In the past few 1000 years, eruptions have been smaller producing pyroclastic flows up to 25kms away, however significantly larger eruptions are known to have occurred, some of Mexico City (55kms away) is built on pyroclastic flows from an eruption that occurred 14ka (Siebe 1996).
The 1994 eruption began in December after a repose time of 70 years. But signs of the eruption including increased seismic activity were recorded as early as 1990 (Delgado- Granados 2001). Onset began with a 7km high plume with ash being deposited 50 km E and NE. The emerald green crater lake also disappeared. In March 1996, a lava dome was observed, and since then switches between dome effusion and explosive episodes with continuous degasing have characterized the volcanic activity till present (Delgado- Granados 2001). One of the strongest explosions occurred June 30th 1997 producing a column 15, 000 masl (Delgado- Granados 2001). Due to the activity of the volcano it is an ideal study site, but also a hazard to the local population so it is already constantly monitored.
Figure from http://r4z0r-z.deviantart.com/art/Popocatepetl-s-crater-166142965?offset=10
Some work has been conducted on the gaseous emissions from the volcano. Delgado- Granados 2001 noted the amount of sulphur degassed was much larger than what would be expected form the erupted volume of magma, unless the magma was composed of 82% sulphur (very unlikely). Combining this observation with the high variance in SO2 emissions (Delgado- Granados 2001), the mingled mafic and silicic magmas within pumice and lava (Goff 2001), the broad temperature ranges found using geothermometers (930- 1080°C) (Goff 2001), and the fluctuations in MgO content of each eruption (as opposed to a gradually decreasing trend that would be expected of a non replenished source) (Martin Del Pozzo 2002), it is thought that there is a body of degassing silicic magma being regularly replenished by mafic injections.
Analysing CO2 emissions, it was found that the bulk of the CO2 was from a magmatic source with CO2/SO2 around 1-2 (Gerlach 1997). But Goff 2001 noted that CO2/SO2 levels do fluctuate. Since March 1997 till at least February 1998 CO2/SO2 have been 30 times greater than the average, but yet HCl/SO2 and HF/SO2 ratios have not changed. So there must be another source of carbon. Within the erupted pumice and lava Ca-rich silicates and Ca-rich xenoliths are found, and are thought to be from the underlying 3km thick Cretaceous carbonate sequence. Thus it was suggested by Goff 2001 that assimilation of wall rock had occurred, resulting in temporal elevation of CO2 emissions.
Gerlach 1997 mapped the CO2 concentrations in two different plumes from Popocatepetl in 1997. Although there are only two examples they tentatively made some observations, they noted that CO2 in plumes tended to form clusters and often bottomed clouds, and suggested that these clusters were gravitationally induced. They also point out the difficulties of making such maps due to the high background levels, anthropogenic inputs and variable wind speeds.
Figure from Gerlach 1997. Contour map of plume CO2 concentrations, after subtraction of best fit to ambient atmosphere, in a plane normal to wind direction and sectioning through the plume 4km downwind of the volcano on 07/06/1997. CO2 concentrations are shown at 0.2μmol-1
Boudal C, Robin C. Volcan Popocate ́petl: recent eruptive history, and potential hazards and risks in future eruptions. In: Latter, J.H. ŽEd.., Volcanic Hazards. Assessment and Monitor- ing. Springer-Verlag, Berlin, 1992;110–128.
Cantagrel J M, Gourgaud A, Robin C. Repetitive mixing events and Holocene pyroclastic activity at Pico de Orizaba and Popocatepetl (Mexico). Bull Volcanol 1984;47-7:735-748
Delgado-Granados H, Cárdenas González L, Piedad Sánchez N. Sulfur dioxide emissions from Popocatépetl volcano (Mexico). Journal of Volcanology and Geothermal Research, 2001;108:107–120.
Fries, K. Resumen de la geologia de la hoja Cuernavaca, Estados Morelos, Mexico, Guerrero y Puebla. Univ. Nacional Autonoma Mexico, 1965;50
Gerlach T M, Delgado H, McGee K A, Doukas M P, Venegas J J, Ca ́rdenas L. Application of the LI-COR CO2 analyzer to volcanic plumes: a case study, volca ́n Popocate ́petl, Mexico, June 7 and 10, 1995. J. Geophys. Res. 1997;102:8005– 8019.
Gerlach T M, Delgado H, McGee K A, Doukas M P, Venegas J J, Cardenas L. Application of the LI-COR CO2 analyser to volcanic plumes: a case study, volcan Popocatepetl, Mexico, June 7 and 10, 1995. J. Geophys. Res. 1997; 102;B4;8005-8019.
Goff F, Love S P, Warren R G, Counce D, Obenholzer J, Siebe C, Schmidt S C. Passive infrared remotesensing evidence for large intermittent CO2 emissions at Popocatepetl volcano, Mexico. Chem. Geol., 2001;177:133–156.
Martin-Del Pozzo A L, Cifuentes-Nava G, Cabral-Cano E, Sánchez-Rubio G, Reyes M, Martínez-Bringas A, Garcia E, Arango-Galvan C. Volcanomagnetic signals during the recent Popocatépetl (México) eruptions and their relation to eruptive activity. Journal of Volcanology and Geothermal Research, 2002;113: 415–428.
Siebe C, Abrams M, MacÂõas J L, Obenholzner J. Repeated volcanic disaster in prehispanic time at Popocate- petl, Central Mexico: pasta key to the future? Geology 1996;24:399-402.