Tacaná rises to a height of 4,030 m above sea level, standing 1,800 m above its basement. The total volume of Tacaná is between 20 and 30 km³, with uncertainty coming from the irregular basement topography. The topographic basement generally slopes from the NE (Guatemala) to the SW (Mexico). The topography and drainage patterns of the Guatemalan side are irregular and mountainous whereas on the Mexican side the land slopes consistently to the SW, resulting in a series of subparallel SSW trending river valleys. These valleys are notable since many of the most serious and extensive types of volcanic hazards are primarily influenced by gravity and topography.
The irregular basement topography restricts the extent and direction of various gravity-influenced volcanic hazards. Significant basement topographic highs exist at: 1- Las Majadas ridge, north of San Rafael (Guatemala), 2- the north trending ridge paralleling and east of Río La Laja (Guatemala), 3- the area southwest of Agua Caliente, between the Ríos Coatán and Zapote (Mexico), and 4- two areas south of the summit and northwest and west of Unión Júarez (Mexico). Another probably basement high exists to the east of the summit and west of Río La Laja. This arcuate block of basement may represent the remnants of a caldera which has been mostly covered by Tacaná's lavas. Skylab photography also indicates the presence of circular structures to the north and east of the summit of Tacaná, which could be ancient calderas.
(Mercado and Rose, 1992)