Located in southwestern Guatemala, near the triple junction of the North American, Cocos, and Caribbean plates (Figure 1), Santa María is the most notorious volcano in Central America. In October 1902, Santa María was the site of one of the Earth's ten largest historic eruptions. This eruption resulted in thousands of deaths and had global atmospheric impact. Since 1922, the explosion crater on Santa María's southwest side has been the site of volcanic dome extrusion at a vent called Santiaguito [Rose, 1972]. In 1929, pyroclastic flows, apparently resulting from the collapse of the active dome, flowed more than 10 km to the south of Santiaguito, resulting in hundreds of deaths and damage to villages and plantations [Sapper and Termer, 1930]. Santiaguito has remained active and is the site of vertical eruptions, avalanches, lava flows, pyroclastic flows, mudflows, and other hazards that are of concern to the rapidly growing populations that surround the volcano.