 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 six most recent, major eruptions (Kutterolf et. al. 2007):
- ~17 kA BP. Apoyeque, the Lower Apoyeque Pumice, rhyodacite
- 12.4 kA BP. Upper Apoyeque Pumice, rhyodacite. Ash distribution calculations pinpoint the source to the western end of the bay separating the Chiltépe Peninsula from Managua, the submarine San Carlos crater. The researchers have retained the name UAP though
- 6.1 kA BP. Xiloá A,B and C, dacite, VEI 5
- 3-6 kA BP. Mateare, Dacite to Andesite (GVP: 2550 BCE ± 1500 years) VEI 5
- 2-4.1 kA BP. Xiloa(?), Los Cedros Tephra, dacite (1050 BCE ± 1000 years GVP) VEI 4
- <2.1 kA BP. Apoyeque, the Chiltépe Tephra, dacite, VEI 6 Initially phreatomagmatic followed by plinian then surge deposits and finally phreatic as the eruption died down. Eruption column height estimated at 35-40 km, slightly greater than the 1991 Pinatubo eruption.
Of these, the Mateare eruption was the subject of a study published in 2005 (Freundt et al). Through density maps of the fallout deposits, they locate the vent to the western side of the Chiltepe Peninsula in the city of Mateare. At the time of the eruption, the surface of Lago Xolotlán was between four and nine metres higher than today which implies that the vent probably was underwater.
Initially, the eruption was unsteady as outside water entered the vent which resulted in great phreatic or phreatomagmatic explosions. The Mateare A layer associated with this phase of the eruption is up to 30 cm thick but has been eroded in many places and mixed or overlain with “Mateare and Xiloá Sands”. These sands on top of the Mateare A layer at one outcrop lies 32 m above the present level of the lake and represents an exceptionally strong tsunami 23-28 m in height. From the location of the Mateare vent on the opposite side of the Chiltepe Peninsula, this wave would have reached the location of present-day Managua City within 15 minutes of the first, large explosion. This initial tsunami was followed by several more but not as great ones. After the initial stage, the eruption became Pl