Volcán Santo Tomás (also known as Volcán Pecul) is a large eroded stratovolcano located across a valley SE of Santa María volcano. The summit of the volcano is capped by late-Quaternary andesitic tephra. A winding ridge connects Santo Tomás to Volcán de Zunil, 4.5 km to the NE, a 3542-m-high stratovolcano that forms the topographic high point of the Santo Tomás - Zunil complex. Volcán de Zunil is located on the SW rim of the 4-km-wide, 600-m-deep Tzanjuyub caldera, which is breached to the south by the Río Masa. Several dacitic-rhyolitic lava domes are located on the caldera's northern flank and the NW flank of Volcán de Zunil. The youngest dome, Cerro Zunil, was last active about 84,000 years ago (K-Ar dating). No Holocene eruptions are known from Santo Tomás, although it was included in the Catalog of Active Volcanoes of the World (Mooser et al., 1958) based on its geothermal activity. Solfataras and thermal springs are located on the west side of the ridge between Santo Tomás and Zunil.