Age and Magma Flux of Santa María Volcano, Guatemala: Correlation of Paleomagnetic Waveforms with the 28,000 to 25,000 yr B.P. Mono Lake Excursion

The United Nations has targeted Santa Maria volcano, Guatemala, as part of its multidisciplinary "Decade Volcano" program to mitigate volcanic hazards. As part of this program to spotlight one of the world's most dangerous volcanoes, and on the basis of paleomagnetic data, we represent a wholly new interpretation of the temporal evolution of the Santa María volcano. Santa María is a typical, moderate-sized (10 km³), basaltic-andesite stratovolcano. In 1902, a catastrophic Plinian eruption produced a crater in the south flank of the volcano exposing 250 m of interbedded lava flows, laharic material, and pyroclastic deposits comprising approximately 40% of the cone's infrastructure. A reanalysis of 25 block samples collected by Rose and co-workers and analysis of 48 cores from eight newly sampled lava flows reconfirms and refines the geomagnetic excursion. The resulting geomagnetic waveform is strongly correlatable with that of the 25 to 28 ka Mono Lake excursion. By inference, the latter 40% of the cone's volume (4 km³) was emplaced in a 1000-3000 yr period with cone construction ending about 25 ka. The corresponding volcanic flux rate ranges from 0.13 m³/s to 0.04 m³/s; a range similar to young, active stratovolcanoes along the Central American arc. Extrapolating a flux of 0.13 m³/s to 0.04 m³/s throughout the entire construction of Santa Maria suggests that cone growth began between 27.5-32.5 ka. However, up to 20 km³ of dacite tephra was ejected in the 1902 Plinian eruption, suggesting that the magma system feeding Santa Maria remained active during the past 25,000 yr. Assuming a geologically reasonable, constant flux rate of 0.13 m³/s, a cumulative volume of nearly 100 km³ of magma could have been stored in the magma chamber)s) beneath Santa Maria. Because only 20 km³ of magma was erupted in 1902, as much as 80 km³ of magma, or its solidified complement, remains in situ in the crust. Thus the growth of even a moderate-sized volcano in Central America is complemented by voluminous intrusion of magma into the crust.