Abstract
The 2015 reactivation of the Cotopaxi volcano urges us to understand the complex eruptive dynamics of Cotopaxi for better management of a potential major crisis in the near future. Cotopaxi has commonly transitioned from andesitic eruptions of strombolian style (lava flows and scoria ballistics) or nuées ardentes (pyroclastic flows and ash falls) to highly explosive rhyolitic ignimbrites (pumiceous pyroclastic flows), which entail drastically different risks. To better interpret geophysical and geochemical signals, Cotopaxi magma storage conditions were determined via existing phase-equilibrium experiments that used starting materials chemically close to the Cotopaxi andesites and rhyolites. The results suggest that Cotopaxi's most mafic andesites (last erupted products) can be stored over a large range of depth from ~7km to ≥16km below the summit (pressure from ~200 to ≥400MPa), 1000°C, NNO +2, and contain 4.5–6.0±0.7wt% H2O dissolved in the melt in equilibrium with ~30–40% phenocrysts of plagioclase, two pyroxenes, and Fe-Ti oxides. These mafic andesites sometimes evolve towards more silicic andesites by cooling to 950°C. Rhyolitic magmas are stored at 200–300MPa (i.e. ~7–11km below the summit), 750°C, NNO +2, and contain ~6–8wt% H2O dissolved in a nearly aphyric melt (<5% phenocrysts of plagioclase, biotite, and Fe-Ti oxides). Although the andesites produce the rhyolitic magmas by fractional crystallization, the Cotopaxi eruptive history suggests reactivation of either reservoirs at distinct times, likely reflecting flux or time fluctuations during deep magma recharge.
Highlights
After 13 significant eruptions since 1534, a major eruption in 1877, and a VEI 1 eruption in 1949, Cotopaxi volcano started to reactivate in 2001-2002 (Kumagai et al, 2010; Molina et al, 2008)
The ignimbrites of Colorado Canyon Series and of Chalupas Caldera are more potassic by nearly 2 wt% K2O with respect to the Cotopaxi rhyolites and, are comparable to the Wangrah_AB421 granite (Klimm et al, 2008; Fig. 2)
The AB421 200 MPa and NNO phase diagram suggests that the Canyon Colorado Series and Chalupas Caldera ignimbrites could have crystallized at 200 MPa, ~700 °C with a H2O content close to saturation (~6.5 wt%), i.e., under conditions only slightly different than those inferred for the Cotopaxi rhyolites
Summary
After 13 significant eruptions since 1534, a major eruption in 1877, and a VEI 1 eruption in 1949, Cotopaxi volcano started to reactivate in 2001-2002 (Kumagai et al, 2010; Molina et al, 2008). In August 2015, phreatomagmatic eruptions followed four months of precursory activity including a major seismic crisis (Gaunt et al, 2016). This crisis was small to moderate in intensity and ash volume erupted (8.6x105 m3; Bernard et al, 2016), this does not preclude a major magmatic crisis in the near future. The densely populated surroundings of the volcano make the monitoring and forecasting of the volcano’s activity of highest priority and urge us to understand the complex eruptive dynamics of Cotopaxi.
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