USING PETROGRAPHIC ANALYSIS TO STUDY THE CHANGES IN ERUPTIVE CHARACTERISTICS OF VOLCAN DE COLIMA

The Western Mexican Volcano Belt (WMVB) is influenced by the subduction of both the Rivera and Cocos beneath the North American plate and causes diverse and complex magma compositions. By comparing the 1913 eruption to other samples taken from eruptions from 1999-2015, petrographic trends through time were evaluated. The reaction rim thickness of hornblende was used as an indicator for ascent rate and the Michel-Levy method was employed to track changes in plagioclase composition over time.

From the data gathered, we concluded that the 1913 eruption was from a very wet magma based on the high abundance of hornblende. The 1913 hornblende had the thinnest reaction rims in the 1913 eruption show that it had the fastest ascent rateThe abundance of hornblende in 2015 was lower than that of 1913 hornblende concentration and also had thicker hornblende reaction rims, indicating slower ascent rate. The An% of the 2015 samples was also higher.

There is also evidence for magma mixing from comparison of ratios of OPX versus CPX data on pyroxene that showed an increase in OPX in a 2004 sample, due to OPX being an indicator of an addition of more primitive magma (Streck et al, 2006). The increase in vesicles in the 2004 eruptions compared to the other eruptions from 1999-2015 indicate a different eruption style.