Santa María is located in the volcanic highlands of Guatemala, which parallel the Pacific coast of the country. The highlands were formed by the subduction of the Cocos plate under the Caribbean plate, which resulted in the formation of a line of stratovolcanoes that stretches along much of the Pacific coast of Central America. In Guatemala, these volcanoes overlie a basement of carbonate as well as igneous and metamorphic rocks; many xenoliths ("foreign" rock fragments) found in lavas erupted from the stratovolcanoes are composed of limestone, granite, and gneiss.