Geology of Volcán Masaya
Masaya Volcano is a large basaltic shield volcano, located 20 km south of Managua, Nicaragua. It is composed of a nested set of calderas and craters, the largest of which is Las Sierras shield and caldera. Within this caldera lies Masaya Volcano sensu stricto, a shallow shield composed of basaltic lavas and tephras. This hosts Masaya caldera, formed 2500 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.
The floor of Masaya caldera is covered by poorly vegetated lavas, 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 pit, 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 possibly within Nindiri crater in 1852. (Excert from Rymer et al., 1998).