Masaya is a caldera located in Masaya, Nicaragua, 20 km south of the capital Managua. It is Nicaragua's first and largest national park, and one of 78 protected areas of Nicaragua. The complex volcano is composed of a nested set of calderas and craters, the largest of which is Las Sierras shield volcano and caldera. Within this caldera lies a sub-vent, which is Masaya Volcano sensu stricto. The vent is a shield type composing of basaltic lavas and tephras and includes a summit crater. This hosts Masaya caldera, formed 2,500 years ago by an 8-km³ basaltic ignimbrite eruption. Inside this caldera a new basaltic complex has grown from eruptions mainly on a semi-circular set of vents that include the Masaya and Nindiri cones. The latter host the pit craters of Masaya, Santiago, Nindiri and San Pedro. Observations in the walls of the pit craters indicate that there have been several episodes of cone and pit crater formation.
Masaya continually emits large amounts of sulfur dioxide gas (from the active Santiago crater) and volcanologists study this (amongst other signs) to better understand the behavior of the volcano and also evaluate the impact of acid rain and the potential for health problems.
The floor of Masaya caldera is mainly covered by poorly vegetated ʻaʻā lava, indicating resurfacing within the past 1000 or so years, but only two lava flows have erupted since the sixteenth century. The first, in 1670, was an overflow from the Nindiri crater, which at that time hosted a 1-km-wide lava lake. The other, in 1772, issued from a fissure on the flank of the Masaya cone. Since 1772, lava has appeared at the surface only in the Santiago pit crater (presently active and persistently degassing) and possibly within Nindiri crater in 1852. A lake occupies the far eastern end of the caldera.