Santa María is built on a surface of Tertiary and Quaternary volcanic rocks. Extensive and varied non-volcanic rocks underlie the volcanics, although they do not outcrop in the immediate vicinity of Santa María.
The upper crustal section of the volcano is studied via the abundant xenolithic population available from the 1902 eruption materials, from the dome lavas and pyroclastic ejecta, and from other nearby volcanoes. The volcanic/nonvolcanic intersection is also used to explain the petrology of the upper crust.
Lithic fragments are found throughout the proximal portions of the 1902 fall deposits. Although andesitic volcanic rocks which represent material from the pre-1902 lavas and pyroclastic deposits of the Santa María cone are the most abundant type of lithic material, a variety of plutonic and metamorphic rocks are also represented. A suite of plutonic lithics consisting of quartz diorite, diorite, quartz monzonite, graodiorite and granite are present, most are medium- to fine-grained to holocrystalline rocks which appear to represent shallow plutons similar to those encountered in the Atitlan area (Williams, 1960) and at a variety of other locations along and north of the volcanic front of the Guatemalan Highlands (Bonis et al., 1970). Chloritized biotite hornblende quartz monzonite was intersected by drilling at the Zuñil geothermal field 6 km ENE of the summit. This indicates that the basement under Santa María probably consists of a shallow intrusive complex (Rose, 1987). The nonvolcanic rocks of the plutonic basement are generally calc-alkalic in character. It is likely that these are of tertiary age and are part of the igneous rock suite related to subduction processes not unlike the current regime. Observations of the drill core in the Zuñil hole showed that the upper contact of the plutonic rock was apparently a weathered surface. Thus, the plutonic rocks may have been at the surface before modern volcanism covered them(Rose, 1987).
Metamorphic rocks are also found as xenoliths within the 1902 deposits. Amphibolites and a variety of schistose metasedimentary rocks are present, although metamorphic lithologies are less common than plutonic ones(Rose, 1987).
An interesting occurrence of exotic lithic fragments is present in the dacitic ejecta erupted on November 3, 1929. These were 2-10 cm rounded fragments of nearly pure halite (with minor Ca sulfate) which fell in the Retalhuleu area about 26 km SSW of Santa María (Deger, 1931). The best explanation of such xenoliths is the presence of evaporite beds under Santa María.
The examination of the above map allows us to recognize the pre-Santa María topography of the area. Santa María is surrounded by topographic remnants of older volcanoes. To the south and southwest are large areas of eroded andesitic lava (QTa) which buttress the coastal slope and subject to rapid erosion and because they are covered partly by younger volcanics, it is difficult to see any constructional forms. A suggestion of WNW-trending ridges within these units occurs on the southern and eastern slopes of Santa María. A curving ridge of andesitic lavas west of Santa María is a conspicuous topographic feature. These ridges could represent the remnants of ancient craters. Northeast of Santa María a portion of a clearly expressed crater at Las Mahadas occurs. This andesitic volcano has been partly filled with Santa María lavas. North-northeast of Santa María is an eroded basaltic cinder cone called Volcán de Valle (Qvvb). This volcano has been surrounded on three sides by the lavas of Santa María. The andesitic lavas of the La Cumbre area, directly north of Santa María, do not suggest any clear volcanological form. Together, these occurrences of basaltic and andesitic volcanic rocks portray an irregular volcanic basement which resembles much of the volcanic highlands of Guatemala.
The older calc-alkalic andesitic lavas south and west of Santa María are chemically distinct from the sodic lavas of the modern Santa María and Santiaguito suite.
The sodic character of Santa María's lavas is evident even in the earliest cone lavas, and has been preserved in all the subsequent eruptives. Na enrichment in the calc-alkalic lavas of the volcanic front of Guatemala has been noted as part of a regional northerly trend in Central America (Carr et al., 1979; Carr, 1984) which correlates with increasing crustal thickness. This suggests that the thicker continental crustal section may influence the Na enrichment. Comparison of the Na2O/K2O variations in several volcanos of the Santa María vicinity shows that the Na enrichment is a very erratic, localized phenomena. While the lavas of all the examined volcanoes are calc-alkalic and andesitic, only Santa María and Chicabal show extreme Na enrichment.
The exposures of the 1902 explosion crater reveal that the symmetrical cone of Santa María formed as a result of repeated alternations of lava flows and pyroclastic block and ash deposits which resulted in a classic composite cone stratigraphy. Most of the lavas exposed in the crater were vented from the summit region (now 3772 m) and are now preserved as thin layers (1-10 m thick) on a slope of 30-35°. Some of the younger flows preserved on the outer surface of Santa María are thicker (>20 m), were erupted from lateral vents as low as 2200 m elevation and flowed as far as 4 km down slopes of 10-20°. The crater exposures display a regular quaquaversal stratigraphy without dramatic irregular unconformities, which suggests that the cone growth was regular and rapid enough to avoid deep erosion. Neither is there any evidence of significant collapses or failures of the cone. The summit is capped by a domal mass of andesite. This summit lava is the most silicic (57% SiO2) of all the cone lavas and can be viewed as the culmination of a consistent slow increase in silica and incompatible elements.
A section of 26 successive lava samples collected from the walls of the 1902 explosion crater of Santa María were sampled and described by Rose et al. (1977). This remarkable section represents about the latter 40% of the cone's volume. The lavas of Santa María are porphyritic two-pyroxene olivine-bearing basaltic andesites. Chemically, the rocks are calc-alkalic and consist mostly of medium-K basic andesites according to the classification of Gill (1982).
Beginning in 1922 dome extrusion occurred at Santa María, first focusing on the 1902 explosion crater, then expanding to the west along subsidence faults that developed as a result of the evacuation of magma in 1902. The dome complex that has resulted from the continuous pulsating extrusions from 1922 until the present is called Santiaguito, and has now reached a cumuilative volume of nearly 1 km³ (Rose, 1972, 1973b, 1987). The chemistry of the Santiaguito lavas are very similar, but slightly more mafic (63-68% SiO2) than the 1902 dacite and has not shown any significant chemical changes with time. The mineralogy is nearly identical to the 1902 dacite, except that the crystallinity is slightly higher, the amphibolite has been oxidized to oxyhornblende and usually has reaction rims of plagioclase, pyroxene and magnetite, and the glassy groundmass has partly devitrified to tridymite and cristobalite. The Santiaguito magma seems to be a degassed residual portion of the pre-1902 magma body. The chemical composition of the Santiaguito dacite minerals and the matrix glass have been tabulated. Remnants of andesitic magmas are encountered as inclusions with lamprophyric textures in the domal lavas of Santiaguito (S1120) or in one case as a large portion of one of the domal units (S1000). These are probably traces of the 1902 basaltic andesite which have been slightly hybridized with the dacite, and are not the result of local remelting of cone lavas as suggested by Rose (1972).