The massive compound San Salvador volcano dominates the landscape W of El Salvador's capital city of San Salvador. The dominantly andesitic Boquerón stratovolcano has grown within a 6-km-wide caldera whose rim is partially exposed at Picacho and Jabalí peaks, which themselves were formed by collapse of an older edifice about 40,000 years ago. The summit of Boquerón is truncated by a steep-walled crater 1.5 km wide and ~500 m deep that formed during a major eruption around 800 years ago. It contained a crater lake prior to an eruption during 1917 that formed a small cinder cone on the crater floor; a major N-flank lava flow also erupted in this year. Three fracture zones that extend beyond the base of the volcano have been the locus for numerous flank eruptions, including two that formed maars on the WNW and SE sides. Most of the four historical eruptions recorded since the 16th century have originated from flank vents, including two in the 17th century from the NW-flank cone of El Playón, during which explosions and a lava flow damaged inhabited areas.