A new geologic map of Santiaguito is presented, updating information from 1972 and reflecting continual growth of an active volcanic dome complex. The dome has been marked by unsteady extrusion of dacite lava, which has formed 34 time stratigraphic dome units as well as fragmental deposits that consist of dome-collapse debris, talus, and sediments. The dome is now mainly covered by blocky lava flows, and its overall composition is changing toward lower silica content.