 of major volcanism in the vicinity of Managua. Apart from the already mentioned Masaya Caldera and Momotombo, there’s the 6 x 7 km wide Apoyo caldera (Laguna de Apoyo) about 10 km SE of the Masaya Caldera and 30 km from Managua, the collapse of which has been assigned a radiocarbon date of 23 kA BP. Volcanism at Apoyo began about 90 kA BP and the caldera was formed by two plinian eruptions that produced the Upper and Lower Apoyo tephras separated in time by about 100 to a few hundred years. Much closer to the Nicaraguan capital, to the NW and across a bay of Lago Xolotlán, lies the Reserva Natural Península de Chiltepe with the twin, 2½ km crater lakes or calderas Apoyeque and (Laguna de) Xiloá. In the GVP, it is listed as Apoyeke.
Located on the Chiltepe Peninsula, Apoyeque is part of the Chiltepe pyroclastic shield volcano, one of three large ignimbrite shields on the Nicaraguan Volcanic Front. As volcanoes go, it is rather unimpressive as in spite of a base diameter in excess of 10 km, the Apoyeque crater walls do not even attain 400 m a.s.l. with the highest point , “Volcán Chiltépe” – a lava dome on its eastern edge, almost reaching 500 m a.s.l. In fact, it is somewhat reminiscent of Taal minus much of the caldera lake. The summit crater or caldera is 2.8-km wide and 400 m deep, the floor of the lake-filled caldera lies near sea level. Immediately to the SE of Apoyeque lies the 2½ x 3 km lake-filled maar crater Xiloá. On the surface, it is a beautiful setting for recreation for the citizens of Managua, but once you take a look at the history of activity, a very disturbing picture emerges, especially given its location so close to the Nicaraguan capital.
The Nejapa Volcanic Field
From the Xiloá crater and through the western part of Managua City runs the Nejapa-Miraflores fault, along the eastern edge of the Managua Graben between the Mateare and Nejapa-Miraflores fault. It is a volcanic field in its own right – the Nejapa Volcanic Field (NVF). It consists of at least 30 volcanic structures of which at least 21 are vents, many of which are maars; phreatomagmatically formed explosion craters and lies almost entirely inside Managua City. Stratigraphy aided by radiocarbon dating suggests that 23 eruptions have occurred in the area during the past ~ 34,000 years with repose times of between 400 to 7,000 years.
The second largest crater of the NVF is the 1.9 km diameter Refinaderia crater, formed about 9.38 kA BP and host to the Managua refinery which has given it its name. Stratigraphy suggests that it was formed by two series of phreatomagmatic pulses accompanied by pyroclastic base surges with a short period of scoria eruption (Strombolian) in between. The second most recent eruption of the NVF occurred at 1,245 (+125/−120) years BP and is the source of the Asososca Tephra. The phreatomagmatic eruptions resulted in the 0.9 x 1.3 km Asososca maar, today one of the main sources of drinking water for the citizens of Managua.
The largest and youngest crater of the NVF is the 1.4 x 2.5 km Nejapa Maar. The associated Nejapa Tephra with a minimum volume of 0.09 km3 covers at least 10 square kilometres inside W/NW Managua City. Again, the erupting magma interacted with water and there are indications of base surges. Radiocabon dating of underlying and overlying paleosoils give an age bracket of 1,200 to 600 years BP for this eruption (Rauch & Schmincke, 2010). The size of the explosion craters in the NVF indicates that the typical maar-forming eruptions are VEI 3 in size with the greatest probably attaining VEI 4.
Chiltepe – Apoyeque
Apart from some recent earthquake activity, the Chiltépe Peninsula has been quiescent throughout the past five hundred years of recorded history. But over the past 17,000 years, there have been at least six major Plinian eruptions from four vents including the two from the Apoyeque and Xiloá vents respectively. If two previous eruptions where neither age nor source have yet been identified are included, the on-shore deposits over the past 60 kA total in excess of 37 km3. This is a summary of the 