Interior of the 1902 explosion crater of Santa María, as viewed from near the Horqueta camp at the west entrance of the crater. Photo by Bill Rose, 1974 (see Rose et al., 1977).

Santa María volcano is a typical, moderate-sized (10 km³), basaltic-andesite stratovolcano. In 1902, a catastrophic Plinian eruption produced a crater on the south flank of the volcano, exposing 250 m of interbedded lava flows, laharic material, and pyroclastic deposits comprising approximately 40% of the cone's infrastructure (see above picture).(Conway et al., 1994)

Since 1922 a lava dome (Santiaguito) has protruded from the 1902 crater. Santiaguito continues to grow on the SW slope via the extrusion of dacite lavas. The rocks of the dome complex are uniform, gray-brown porphyritic dacite and andesite, usually with oxyhornblende phenocrysts as well as plagioclase. Two types of units are shown on Santiaguito: domes and flows.

Dome units are generally more grayish in color than the flows, are less vesicular, have more inclusions, and show little or no evidence of flow in response to gravity after extrusion. They comprise a much larger volume than the flow units. Spines and slabs stud the summits. The largest spine now preserved is on the La Mitad Dome; it is 200 m long and 70 m high. The shape of the dome units is sometimes circular, as in the case of the La Mitad and the El Brujo units, suggesting a simple central extrusive vent. The Caliente and El Monje Domes are not as simple. The Caliente unit was extruded from two or more vents and the El Monje dome can be subdivided into two elongate units on the basis of aerial photography control at various dates. Extrusive vents for the El Monje domes were apparently along fissures striking eastward.

Flow units at Santiaguito are generally darker and more brownish than the domes, are much more vesicular, and show clear evidence of downslope movement. Still they are substantially more viscous than most basaltic flows and many flow units solidified on the sides of domes. The movement of these flows is characteristically by rockfalling of oversteepened flow fronts. Most flows are rather small in volume, but the three most recent ones are significantly larger, the largest extending more than 2 km. The thickness of these alrger flows is commonly greater than 50 m but is variable. Where exposed, the flows have a vesicular top as much as 10 m thick. Vesicles as large as 20 cm are common. Some flows seem to exhibit little evidence of downslope movement. This may be due to the lack of slope at the point of extrusion, or to the tendency for flows become more sluggish in their waning stages, "stacking up" above their vents.