The volcanic rocks of Tacaná are calc-alkalic, typical for Central America. They are medium-K, according to Gill classification.
Mercado and Rose (1991) compared Tacaná's andesites with other Guatemalan and Mexican volcanoes, using data from Tajumulco, Tolimán, Santa María-Santiaguito, Nicolas Ruiz, and El Chichón. Rocks from all these volcanoes are broadly andesitic and differ mainly in the degree of enrichment of K and other incompatible elements. Tacaná andesites show an average K enrichment similar to Tolimán, higher than Santa Marí:a and Nicolas Ruiz, but lower than Tajumulco or El Chicón.
The K/Zr ratios of all these volcanoes range from 75 to 125. K/Rb ratios range from 250 to 500. Santa María is greatly depleted in Rb compared to Tacaná. Nicolas Ruiz and El Chichón are slightly enriched in Rb relative to Tacaná. Zr/Rb plots separate the Mexican volcanoes (Nicholas Ruiz and El Chichón) from Tacaná and other Guatemalan volcanoes.
Chemical composition of Tacaná ashes is shown in the following table.
Mercado and Rose (1991) conclude that Tacaná's recent eruptives (lava and ashes) are dominantly silicic calc-alkalic andesites with normative quartz. Chemistry of the rocks shows a moderate K enrichment, similar to other Guatemalan volcanoes found along the northern side of the volcanic front. The chemistry and mineralogy of Tacaná's lavas resemble those of the Guatemalan volcanic front more than the Chiapas Volcanic belt.