Manual de Plantas de Costa Rica | The physical environment | 55 
the Osa Peninsula, and distant Cocos Island (maximum elevation 634 m), more than 
500 km to the southwest. 
Most of the human population of Costa Rica lives in the Central Valley (Meseta or 
Valle Central), a small area of relatively low relief straddling the Continental Divide in 
the heart of the country, and ranging in elevation from ca. 700 to 1500 m. The Carib- 
bean portion of the Central Valley drains to the sea via the Rio Reventazon, the Pacific 
portion via the Rio Barranca and the Rio Grande de Tarcoles. Here are located the four 
largest Costa Rican cities: San José (1160 m), Alajuela (951 m), and Heredia (1168 m), 
all on the Pacific slope; and Cartago (1426 m), on the Caribbean slope. 
The cordilleras 
The axial cordillera of Costa Rica consists of four separate, slightly offset ranges, of 
distinct character. The northernmost of these, the Cordillera de Guanacaste, is a loosely 
connected chain of volcanoes of moderate height, separated by comparatively low 
passes. The main peaks are (from northwest to southeast): Volcan (or Cerro) Orosi 
(1450 m); Volcan (or Cerro) Cacao (1659 m); Volcan Rincon de La Vieja (1895 m); Vol- 
can Santa Maria (1916 m); Volcan Miravalles (2028 m); Volcan Tenorio (1916 m); and 
Volcan Arenal (1633 m). The last-mentioned peak is somewhat isolated and lies off the 
Continental Divide on the Caribbean side, but is aligned with the others. Subsidiary 
volcanic edifices include Cerro Orosilito (ca. 1210 m), Cerro El Hacha (617 m), Cerro 
Espiritu Santo (979 m), Cerro Montezuma (ca. 1510 m), and Cerro Chato (1145 m). 
The Meseta Volcanica de Santa Rosa is a broad plateau of volcanic rock at the base of 
the Cordillera de Guanacaste, extending southward to the vicinity of Cafias. 
The volcanoes of the Cordillera de Guanacaste are all considered extinct except for 
Rincon de La Vieja, Arenal, and Miravalles (which is in the fumarole stage); however, 
it is well to recall that even Arenal was regarded as extinct until its eruption in 1968 
(Weyl, 1980). Volcan Arenal is the only Costa Rican volcano that has actually produced 
lava in historical times (Weyl, 1980); even the well-publicized eruptions from others 
have involved just steam and ash. 
The Cordillera de Tilaran extends from near the town of the same name southeast- 
ward to the Montes del Aguacate (maximum elevation 1541 m), between San Ramén 
and Atenas. It is offset slightly westward from the axis of the Cordillera de Guanacaste. 
Nestled between the two ranges is Lake Arenal, formerly Arenal Lagoon (and now 
much enlarged by impoundment), which drains circuitously to the Rio San Juan and the 
Caribbean coast. The Cordillera de Tilaran is both the shortest and lowest of the four 
major chains; the highest point is Cerro Los Amigos (1842 m), in the Monteverde Re- 
serve. Although there are no active volcanoes in the Cordillera de Tilaran, three peaks 
in the Montes del Aguacate represent presumably extinct volcanic structures (Weyl, 
1980): Cerro Mondongo (1020 m), Cerro Pelon (882 m), and Cerro Tinajita (925 m). 
Cerro Los Perdidos (1370 m), on the Caribbean slope south of Cerro Chato, is also 
