Lava flows at Santiaguito are slow-moving, highly viscous lobate units which typically move only a few kilometers in time periods measuredby by months of years. The flows are tens of meters thick, and extremely blocky.

The dome lava of Santiaguito is a soda-rich dacite of the calc-alkalic suite. This lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa María and the older volcanic rocks in the area. Trace element analyses of the lavas, along with major element data suggest a differentiation by fractional crystallization under constant or increasing PO2. Sr isotope determinations could not detect contamination of the lavas by radiogenic crustal material; the Sr87/Sr86 ratio in Santiaguito rock averages 0.7043. The Santiaguito lavas are tridymite-bearing hypersthene dacites, characterized by strongly zoned plagioclase phenocrysts and oxyhornblende.

Thick (75 m) lobate active lava flow descending from the El Brujo vent at Santiaguito in February 1972. Such lava flows are part of the exogenous activity at Santiaguito. Photo by Sam Bonis.

Aerial view of dacite lava flow west of El Brujo Vent. Photo by Maurice Krafft, February 1983.