Characterization of the recent ash emissions at Popocatepetl Volcano, Mexico

Nine representative ash emissions from 1994–1997 were studied to characterize the recent activity and the eruptive process at Popocatepetl. A series of tephra eruptions began on December 21, 1994 and intermittent activity continues to present. The first eruptions were phreatomagmatic but in mid-March 1996 they turned magmatic. Cumulative volumes (529–1810 × 103 m3), were determined for the first eruptions. However, when eruptions grew larger, more widely spaced (and magmatic), the volumes were then calculated individually (22–1107 × 103 m3), both using the Simpson Rule and based on 244 sampling sites. This numerical integration method is more precise than other methods especially since sub-mm isopachs are neglected in most cases. Dominant winds carried ash mainly to the east (January through April 1995 and April 1996) except for the summer months when ash fell on Mexico City to the northwest (October 28 1996 and June 30 1997). In March 1996, changing wind direction produced ash fall to the southwest as well. the first year, volume calculations indicated that emission rate was higher at the beginning of the eruptions and then declined and stopped. Activity resumed the following year with a similar pattern until larger amounts of magma ascended. Detailed studies of the ashfall provided constraints on the dynamics of the volcanic plumbing system. Tephra emission was related with clearing (December 1994 to March 1995), and clogging of the vent (May 1995 to February 1996), until a larger new ascending batch was able to clear its way to the surface (March 1996). After April 1996, dome formation and explosive destruction were related to individual small ascending magma batches. from December 1994 to early March 1996 was made up mostly of andesitic lithic clasts and plagioclase and pyroxene crystals with minor amounts of accidental and accessory minerals. In March 1996, prior to dome formation, glass was also detected. Afterwards, ash components were mostly dome lithics and scoria with minor amounts of plagioclase, pyroxene and olivine crystals from the incoming magma batches.