Santiaguito has now been active for nearly a century, with vertical ash eruptions and a lava extrusion rate ranging from 0.1 to about 2 m3/s.  Its magma is in the range of 62 to 65 % SiO2 and has trended toward more mafic with time. Flow of magma is unsteady and in recent decades has produced blocky lava flows that advance very slowly, but collapse suddenly forming block and ash flows and surges. The block and ash flows are remobilized as lahars, hyperconcentrates and floods during the intense monsoonal rainy seasons that come from May to November. The area downslope to the south of Santiaguito is a productive zone for commercial agriculture and export crops, and the continual activity poses a challenge to assess a complex pattern of volcanic hazards.