| Popis: |
An interdisciplinary approach correlating magnetic anomalies with composition of the ejecta in each eruption, as well as with seismicity, was used to study the effect of magmatic activity on the local magnetic record at Popocatepetl Volcano located 65 km southeast of Mexico City. Eruptions began on December, 1994, and have continued with dome growth and ash emissions since then. The Tlamacas (TLA) geomagnetic total field monitoring station, located 5 km away from Popocatepetl’s crater, was installed in December, 1997, in order to detect magnetic anomalies induced by this activity. Spatial correlation and weighted difference methods were applied to detect temporal geomagnetic anomalies using TLA’s record and the Teoloyucan Magnetic Observatory as a reference station. Weighted differences were applied to cancel the effects of non-vulcanogenic external field variations. Magnetic anomalies over a 2-year time span were classified into four types correlating them with geochemical, seismic and visual monitoring of the volcanic activity. Magnetic anomalies are believed to be caused by magma injection and gas pressure build-up, which is sensitive to vent morphology and clearing during eruption, although some anomalies appear to be thermally related, changes in the stress field are very important. Most magnetic anomalies are short time signals that reverse to baseline level. Decreasing anomalies (−0.5 to −6.8 nT) precede eruptions by 1–8 days. The presence of a mafic magmatic component was determined by mineral examination and silica and magnesium analyses on the ejecta from the 1997–1999 eruptions. Whole rock analyses ranged from dacitic (65% SiO 2 ) to andesitic (57% SiO 2 ) with 2–6.6% MgO. The higher MgO, lower silica samples contain forsteritic olivine (Fo90). SiO 2 does not increase and MgO does not increase with time, suggesting ascent of small magma pulses which are consistent with the magnetic data. |
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