A study of lava flows at the summit concluded that Acamarachi's last eruption may have occurred during the Holocene. There are no young lava flows on the flanks and the summit crater is degraded. Possibly after the ice ages, north of Acamarachi a silicic coulee was erupted on the saddle between Acamarachi and Colachi. This coulee has well preserved flow structures and covers . No evidence exists for activity in historical times.