Type of Edifice

Coatepeque caldera is a collapse caldera which has suffered two major collapses. In a NE-SW direction it has an elliptical shape measuring 11 x 7 km. Inside the caldera the lake with same name measures 5 x 6 km and has a depth of approximately 120 m. The bottom of the lake has a flat morphology, except for the southern side where the dome Cerro La Isla was emplaced.

The origin of the caldera can be traced back to about 70 ka (CEL, 1992), when a large plinian eruption, followed by edifice collapse occured. This eruption deposited a biotite bearing rhyodacitic tephra over large part of western El Salvador. A period of quiescence of approximately 20,000 years followed, time in which a small lake started to develop.

At about 50 ka a second major plinian eruption caused a collapse centered on the southwestern side. This eruption is considered to be smaller than the first (CEL, 1992), however it still managed to deposit large pyroclastic flows that covered most of the southern, eastern and northern areas surrounding the caldera, including the site were the city of Santa Ana is now built on.

Following the last major eruption the emplacement of basaltic scoria cones on the rims and inside the caldera as well as silicic dome extrusion marked the end of volcanic activity. The last volcanic activity is believed to be the extrusion of the Cerro Pacho rhyolite dome which is placed in the last 10,000 years.

Presently there is no evidence of volcanic activity, except for the weak fumaroles and hot springs found on the sides of Cerro Pacho and Cerro Afate domes. Large fluctuations (up to 10 m) of the lake level have occured in the past century. CEL (1992) believes it is caused by a combination of seismic fracturing and "self-sealing" of active faults and fractures.