Acamarachi itself is a cone-shaped volcano with steep upper flanks and a lava dome on its northern side. While there is no indication of historical activity and the flank lava flows are old, some lava flows around the summit crater and a lava coulee north of Acamarachi on the saddle between Acamarachi and Colachi volcano may be postglacial in age. Acamarachi is formed by andesitic and dacitic rocks.