Global Volcanism Program | Image GVP-07264
The thick light-colored unit the volcanologist is pointing to is the Arce fall deposit, overlying a paleosol and mafic ash- and scoria-fall deposits. The biotite-rich rhyolitic Arce pumice-fall deposit was erupted from Coatepeque caldera about 72,000 years ago and was associated with formation of the SW part of the caldera. This outcrop is about 15 km NW of the caldera rim. Deposits of the 84,000-year-old Los Chocoyos Ash from Atitlán caldera in Guatemala lie about 3 m below the base of the Arce deposit, but are not visible in this photo.
Photo courtesy of Carlos Pullinger, 1996 (Servicio Nacional de Estudios Territoriales, El Salvador).