Timing magma ascent at Popocatepetl Volcano, Mexico, 2000–2001

Abstract

Magnetic anomalies may be used to constrain magma ascent and are useful as precursors to eruptions especially when correlated with other geophysical and geochemical data. In this paper we present multiparameter data on the magnetics, dome morphology, geochemistry and seismicity associated with the December 2000–January 2001 eruptions, the largest of the recent eruptions at Popocatepetl Volcano. A 6-month data period was studied in order to evaluate the precursors and post-eruption processes. Several cycles of dome construction and destruction occurred from September 2000 through February 2001. In December, large amplitude tremor associated with a higher effusion rate resulted in the formation of a large dome which filled the crater to within about 50 m of the lowest part of the crater rim. Seismic activity in December was marked by many volcanotectonic earthquakes and both high frequency and harmonic tremor. On December 12 and 13, an increase in the tremor amplitude was followed by ash eruptions with 1.7–5-km-high columns. Tremor amplitude increased again on December 15 and oscillated for the next four days. Activity remained high until the end of the month. On January 22, an 18-km-high plume produced ash and pumice fall to the east as well as pyroclastic flows and mudflows which reached 6 km from the crater. The eruption left three concentric explosion pits, partially destroying the December dome. Mixing of a mafic olivine-bearing melt with a more evolved magma triggered the larger eruption on January 22 as can be seen from the higher MgO concentrations in some of the ejecta and the presence of a dark andesitic scoria with lower silica content and a white andesitic pumice with higher silica content. Precursory negative magnetic anomalies up to 5 nT (−3.2 nT, −5 nT, −2.9 nT) were associated with the ascent of the larger batches of magma which preceded the increases in seismicity, before the December 2000–January 22 VEI 3–4 eruptions. No significant increases in seismicity were observed prior to the January 22 eruption, except for a magnitude-2.4 earthquake on the day of the eruption. There was, however, a −0.8-nT magnetic anomaly which lasted from January 10 to 12 and possibly to January 17. Most of the magma ascended in three large batches and several smaller ones from October 22 through December 10. Short-lived crater domes with small volumes were formed at Popocatepetl in September–October and November 2000 and in March 2001. Even the smaller negative anomalies from September to February were associated with the ascent of several batches of magma. They preceded the increases observed in the seismicity and growth of a dome by several days.