The New Decade Program; #2 Apoyeque, Nicaragua

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Lurking in Plain Sight – Apoyeque, NDVP #2

The location of the Nicaraguan capital Managua is a volcanic one, active and highly explosive even if the past five centuries have been relatively quiet.

Few cities around the world can claim a more beautiful setting than Managua (pop 2.223 million, 2012), Nicaragua, on the shores of tranquil Lago Xolotlán, Lake Managua. When the Conquistadores arrived, Masaya volcano 20 km from today’s Managua was in eruption and provided Europeans with their fist view of a lava lake. In a letter addressed to the Spanish Emperor Carlos V dated 10th April 1525, the Governor Pedrarias Davila of Castilla de Oro (Panamá) mentions the presence of Masaya and Momotombo volcanoes:

“In this province of Masaya there is a large mouth of fire which never ceases to burn and during the night it is so big as if it reaches the sky, and with a height of 15 leagues (75km) there is light as if it was day.”

The central complex of Masaya. The caldera formed as recently as just over 2,000 years ago inside an older and larger caldera, the 30 kA Las Nubes caldera, which formed following the eruption of a major ignimbrite that can be seen in the Managua area.

Although an – ahem – slight exaggeration, it still gives us an idea how impressive the phenomenon must have been to the Conquistadores. Because of its resemblance to molten metal, it was speculated that it was in fact a pot of liquid gold or silver, and how to extract this vast reservoir of riches immediately occupied Spanish attentions when they were not concerned with theological attempts to classify the phenomenon. From the beginning of Nicaragua’s conquest, the Spaniards referred to the Masaya Volcano as “The Mouth of Hell” or simply “Masaya’s Hell” (“Infierno de Masaya”) with one brave soul; Mercedarian Fray Francisco de Bobadilla, climbing the volcano in 1529 to erect a cross with which to exorcise the “The Mouth of Hell”.

Masaya is classified as a complex volcano but is in reality a shield volcano caldera formed about 2.5 kA BP by a large, 8-km³ basaltic ignimbrite eruption that resulted in the formation of the 6 x 11 km caldera. The phreato-plinian Masaya Tuff has been identified as a 10-20 cm thick layer on the western side of Managua, opposite the Masaya caldera. It has been identified as the result of a wet pyroclastic surge that travelled 35 km from its source (Pérez and Freundt, 2006). After this eruption, a new basaltic complex has grown inside the caldera which includes Masaya and Nindiri cones with the latter the host of the Santiago, Nindiri, San Pedro and, confusingly, Masaya craters.

The 1,297m high Momotombo stratovolcano, here seen from across Lake Managua or Lago Xolotlán, is less than 5,000 years old. (Uncredited image)

Across Lago Xolotlán and some 45-50 km NW of Managua lies the Momotombo volcano and Volcanic Field with Volcán Cerro Negro, Central America’s youngest (April 1850) and one of the most active volcanoes in Nicaragua almost immediately to its WNW. Momotombo volcano is one of Nicaragua’s most familiar landmarks. It is a somma volcano, meaning it has grown on the edge of the caldera of a mainly collapsed, previous edifice; Monte Galán. It began growing about 4.5 kA BP at the SE end of the Marrabios Range and the symmetrical cone has now attained a height a.s.l. of 1,297 m with a prominence in excess of 1,100 m. If we only take into account the portion above the 400-m isoline, the edifice has a volume on the order of 3.5 km3. In addition to this, there are the products of the geothermal volcanic field on the ESE flank as well as lava flows from Momotombo that have flowed down the NW flank into the 4-km-wide Monte Galán caldera to consider. Also, it has formed a 391-m-high island; Isla Momotombito, offshore in Lake Managua. In all, the volcanic products of Momotombo over the past 4½ millennia is in excess of five cubic km. Consequently, Momotombo has a long record of mainly strombolian eruptions punctuated by occasional larger explosive activity with at least fifteen eruptions since the arrival of the Conquistadores. The most recent in 1905 produced a lava flow that travelled from the summit to the lower NE base of the volcano.

There are two facts to keep in mind concerning Momotombo. First, volcanism seems to be migrating in the direction of Managua; from Monte Galán via Momotombo and into Lago Xolotlán, which is a cause for grave concern in the case of a large-ish, submarine eruption. Second, the copious amounts of juvenile magma fed into and mixing with the crust to form the andesitic lavas. At the very minimum, the rate is in excess of a cubic km per thousand years.

Looking at the map, there are further areas 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.

Chiltepe Peninsula with Managua city to the right.

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

The Chiltepe complex and Nejapa Volcanic Field with the locations of some of the major eruptions marked (based on INETER map).

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

The 300 m high Apoyeke stratovolcano rises to the West beyond the Lago Xiloá maar crater in the foreground. (Uncredited image)

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 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)

Discussion

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.

Earthquake swarms such as this one in April 2014 at Momotombo stratovolcano and the Chiltepe complex clearly show the area to be very active. (INETER)

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 millennium, the trend shown by recent earthquakes; that volcanic activity may be migrating into Lago Xolotlán where there already has been a major eruption that formed Isla Momotombito, must be considered and closely monitored. And if this isn’t enough, Masaya is another potential MDE volcanic system and there are indications that the magma system of the older Apoyo Caldera is still more or less intact even if it at present is dormant.

Finally, I would like to give my thanks to Dr Hans-Ulrich Schmincke for making his and his fellow researchers’ work available online. It is as exemplary as laudable.

Henrik

Armin Freundt, Steffen Kutterolf, Heidi Wehrmann, Hans-Ulrich Schmincke, Wilfried Strauch ”Eruption of the dacite to andesite zoned Mateare Tephra, and associated tsunamis in Lake Managua, Nicaragua”, 2005
http://www.researchgate.net/publication/248256700_Eruption_of_the_dacite_to_andesite_zoned_Mateare_Tephra_and_associated_tsunamis_in_Lake_Managua_Nicaragua

Kutterolf, Freundt, Pérez, Wehrmann, Schmincke, “Late Pleistocene to Holocene temporal succession and magnitudes of highly-explosive volcanic eruptions in west-central Nicaragua” 2007. http://www.researchgate.net/profile/H_Schmincke/publication/228488958_Late_Pleistocene_to_Holocene_temporal_succession_and_magnitudes_of_highly-explosive_volcanic_eruptions_in_west-central_Nicaragua/links/0deec5164442f44371000000.pdf
Pardo N, Avellan D R, Macias J L, Scolamacchia T, Rodriguez D, 2008. The ~1245 yr BP Asososca maar: new advances on recent volcanic stratigraphy of Managua (Nicaragua) and hazard implications. J Volc Geotherm Res, 176: 493-512
http://www.bio-nica.info/Biblioteca/pardo2008volcanicstratigraphy.pdf
Rauch, Schmincke, “Nejapa Tephra: The youngest (c. 1 ka BP) highly explosive hydroclastic eruption in western Managua (Nicaragua)” J Volc Geotherm Res, 2010
http://www.sciencedirect.com/science/article/pii/S0377027310000545

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About Melly Rocks

20 something living in London while doing my Natural Science BSc with the Open University. Wannabe geologist and trainee volcanologist. Living life to the full and following the rocky road to my dreams...

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