Volcanologic map of Santiguito Volcanic Dome.

Of the fourteen extrusive units mapped on Santiaguito dome, 5 are volcanic domes. Santiaguito was originally formed in the center of the explosion crater created by Santa María's 1902 activity. This crater was volumetrically much smaller (0.5 km³) than the amount of material erupted during the 1902 event, and local slumping near the crater has occurred and is continuing along a series of east-trending faults. The general westward growth of the dome complex and many of the structural features on Santiaguito are controlled by these near-vertical faults. The domes are studded with Pelean spines.

Twenty-five new chemical analyses show Santiaguito's eruptive products to be soda-rich dacite of the calc-alkaline suite. The dome 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. The rocks of the dome complex are uniform, gray-brown phorphyritic dacite and andesite, usually with oxyhornblende phenocrysts as well as plagioclase. 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 extrusive vent. 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.

(Rose, 1972b)

Map showing units of Santiaguito, 1984 (Rose, 1984).

Pelean spines on the north slope of the La Mitad dome at Santiaguito. Photo by Bill Rose, December 1972.

View of Santiaguito from the summit of Santa María. The Caliente vent is fuming to the left. The endogenous units of Santiaguito are from left to right: Caliente, La Mitad, El Monje, and El Brujo.