Boqueron (Big Mouth)
This volcano is a few kilometers from downtown San Salvador and partly surrounded by its suburbs. The small cone in the center of the main crater formed in the 1917 eruption that also produced large lava flows on the north flank of the volcano. In 1977, the crater walls had a network of paths providing access for farmers, who grew flowers on the steep slopes.
The large steep crater allowed us to get more than 20 samples in stratigraphic order (from oldest at base to youngest at top). We determined that the early history of Boqueron was typical of an arc volcano with aluminum-rich lavas (calc-alkaline in geo-jargon). These magmas evolve at moderate depths in the crust. The recent magmas are different, they are iron rich (tholeiitic in rock- speak), indicating evolution at shallow depths. These geochemical groups are separated by a thick tephra layer exposed in the crater wall. We speculate that a major explosive interval changed the plumbing system of the volcano, creating a shallow magma chamber, where crystallization at low pressure now creates Fe-rich lavas.