Pyroclastic eruptions associated with Santiaguito are generally minor in scale; associated with vertical ash eruptions and dome or lava flow collapses.

Most of the pyroclasts were formed by vesiculation and disintegration of highly viscous magma near vent areas. The same process occurs occasionally near the end of blocky dacite flows, when it generates nuées ardentes (Rose et al., 1976). Vesiculation and fragmentation by pressure release may be the main cause of tephra formation. However, as the region is characterized by high rainfall, it is possible that there may be some magma/water interaction, which results in phreatomagmatic eruptions.