Volcán Tolimán is a large andesitic stratovolcano that rises above the S shore of Lake Atitlán. It was constructed within the Pleistocene Atitlán III caldera, near its inferred southern margin. A shallow elliptical crater truncates the summit, and a subsidiary peak to the SSW also has a shallow crater. In contrast to the tephra-covered surface of Volcán Atitlán immediately to the S, the surface is covered by thick lava flows. Many of the flows were erupted from vents on the flanks and form a highly irregular shoreline on the S side of Lake Atitlán. A lava flow that entered Lake Atitlán from the Cerro de Oro lava dome on the N flank was considered by Newhall et al. (1987) to be less than a few thousand years old based on the thickness of sediment accumulated on the sublacustral part of the flow.