Puebla, Shield volcano in Nicaragua.

Las Lajas is a broad basaltic shield volcano in central Nicaragua, 22 km north of Lake Nicaragua in Boaco department. Its summit is cut by a caldera 7 km across with steep walls and 650 m deep, crossed on the southeast side by a narrow canyon where the río Mapacha flows toward the lake. Five coalescing andesitic-dacitic lava domes stand in the caldera centre, including Cerro el Castillo, Cerro Ochenta Pesos and Cerro San Ildefonso, with further domes on the outer flanks.

Las Lajas is the largest Quaternary-age volcano east of the Nicaraguan graben. McBirney and Williams (1965) took its youthful shape as Holocene, but Plank (2002) obtained three Miocene radiometric dates, so the main edifice may be older; Van Wyk de Vries (1999) had already judged it probably Pleistocene, with young flank cinder cones possibly Holocene. No Holocene eruption is known.

Comarca las Lajas is one of several villages built inside the volcano's caldera, and the same name is used for the community and the massif. The caldera floor is drained by the río Mapacha, which leaves through a narrow canyon on the south-east side and flows into Lake Nicaragua.

The caldera rim stands about 650 m above its floor, and the Mapacha river cuts a narrow canyon through the south-east side on its way to Lake Nicaragua. Access to the interior villages, Comarca las Lajas among them, follows tracks down the steep caldera walls, so allow time for the descent and climb back out. The pyroclastic domes at the centre — Cerro el Castillo, Cerro Ochenta Pesos, Cerro San Ildefonso — are the landmarks to orient by.

The caldera's southeastern side is cut by a narrow canyon where the río Mapacha flows toward Lake Nicaragua. Inside the 7 km-wide, 650 m-deep depression stand five coalesced andesitic-dacitic lava domes, among them Cerro el Castillo, Cerro Ochenta Pesos and Cerro San Ildefonso.