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 Plinian and deposited the up to two metres thick Mateare B layer. Then as the source of dacitic magma was exhausted the eruption changed and more andesitic magma was erupted to produce the 20 cm thick Mateare C pumice and Mateare D ash and lapilli layers. (Freundt et al 2005)
With the Nejapa-Miraflores fault which marks the eastern boundary of the Managua Graben running all the way to the twin craters Apoyeque and Xiloá, there is a case to be made for the latter marking the Northern end of the Nejapa Volcanic Field. If we accept this, the NVF is shown to be capable of highly explosive dacitic to rhyodacitic eruptions ranging from VEI 4 and up to VEI 6 with less than a thousand years between over the past 6-7 thousand years. If compared with the preceding period up to about 17 kA BP, it would seem that the interval between larger eruptions has decreased significantly with the most powerful eruption <2.1 kA BP.
In most of these eruptions, including the final one at Masaya 2.1 kA BP, water plays a major part which results not only in pyroclastic base surges but also a very great risk of devastating tsunamis. Should a repeat of the Mateare eruption occur on the Eastern side of the Chiltepe Peninsula today, the danger of a major tsunami striking Managua City and its 2.223 million inhabitants within minutes of the first, great explosions is overwhelming. Even a repeat of the Nejapa eruption could be catastrophic on a scale never seen before. The greatest danger of course comes from the pyroclastic base surges travelling through densely populated areas to the city centre a scant 5 km away. However, many of the (residential) buildings in Managua are of such a standard that even 2 cm of ash poses a serious risk of roof collapse. The Nejapa eruption deposited such a blanket up to at least five km from the vent.
In addition to the dangers posed by the Apoyeque-Xiloá complex and Nejapa Volcanic Field, the Momotombo volcano must be considered too. Highly active over the past 5,000 years and with a magma supply of >1 km3 per mil