Volcan Mombacho

Mombacho is a 1,345 m stratocone located around 10 S of Granada.  It is bounded by Lake Nicaragua to the E and the Apoyo Caldera to the NW.  It is asymmetrical in shape.  The summit has been greatly modified by flank collapse and erosion, though it was originally symmetrical.  Drainage today are young gullies cut into the soft flanks of the volcano and Apoyo ignimbrite on its N flank.

Mombacho was built primarily of ash rather than lava flows.  The ash built a cone similar in size and shape to Momotombo to the N and Concepcion to the S.  Before the cycle of flank collapses began, the cone may have been as high as 2,000 m.  More recent eruptions extruded lava flows that covered much of the remaining cone.

The summit is carved in the N and S by two collapse amphitheaters.  Thin andesitic – basaltic lava flows and scoria layers are visible in the scars.  The collapse scar from the oldest avalanche deposit to the SE was covered by later eruptive products.  Lava units can be traced to multiple vents, from multiple eruptive centers.  Only three of them are observed today.  The cone is surrounded by multiple small cinder cones, domes and explosion craters.  Minor fumarole activity takes place near the three craters and within the two most recent amphitheaters.

Mombacho is a heavily eroded and vegetated volcano.  It may be the most massive of several neighboring Nicaraguan volcanoes, with a 7 km wide base.  There are a pair of amphitheaters.  The most recent one to the S is 1.5 km wide, with a floor 750 m lower than the highest peak on its rim.  The lowest point on the crater rim is 1,080 m.  There is a second peak on the NW rim at 1,222 m.  Two small, vegetated craters about 200 m in diameter are near the secondary peak. 

A number of lava flows are visible down the slopes.  Like most of the rest of the volcano, they are fully vegetated.  They are also sharply defined at the base of the volcano.  The flanks of the crater rim and sides of the valley to the NE are very steep.  While mostly vegetated, frequent landslides leave visible scars and a jagged crater rim. 

Mombacho is not older than 2 Ma and may be considerably younger.  Las Sierras and Apoyo caldera ignimbrites form the current basement.  The Apoyo ignimbrite is pumice rich and varies from a few meters to the N to a few centimeters to the S.  It is likely Mombacho existed during the Apoyo eruptions but was smaller than today.  Apoyo Ignimbrite is 20 m thick along the shore of the lake and continued into the lake.  The flow climbs 100 m up the N flank of Mombacho 5 km from the Apoyo caldera.  The Las Sierras ignimbrite may underlie all of the cone.  The extreme E side of Mombacho may be built on lake sediments. 

The most recent eruption was initially reported as 1570 AD.  This date of a hot eruption is not supported by INCER.  Around the same time, the S crater rim collapsed into a debris avalanche destroying the S flank and a village of 400.  Note that the date and the nature of this collapse is in dispute.  Recent analysis places it 1560 – 1570 AD. 

Reports of more recent eruptions are sketchy at best.  There was a report of an eruption in 1850, tied to a small parasitic cone on the N flank.  This may be confused with eruption of a volcano N of Lake Nicaragua of the same name.  To make things even more confusing, a small cone NE of Mombacho may have been active in historic times.  Once again, the only feature on the entire volcano that indicates any recent are landslide scars on the face of the two main flank collapse amphitheaters.  Everything else is covered with greenery. 

Mombacho has a complex fault system including normal, thrust and strike-slip faults.  Most of the strike-slip faults are radial, cutting from the summit to the base.  Other strike-slip faults due to regional tectonics are found S of the volcano.  Thrust faults are found near the base.  Small normal faults are found on the flanks near the summit.  Radial faults are associated with lateral edifice spreading.  The base appears to be sliding to the NE and SE.  Most cinder cones and explosion craters follow thrust zones.  The thrust faults may allow easier paths for magma to rise to the surface. 

Gas and water sampling of fumaroles at Mombacho suggest an active magmatic system below it, with a current balance between hot, magmatically heated fluids beneath a shallow, water-fed environment fed by meteoric waters.  A change in heat input from depth, regional seismic events should identify a shift in this balance and an increase in the possibility of eruption. 

Masaya – Granada – Nandaime is a hydrothermal area in the S part of the Nicaraguan Depression near Diriomo, 45 km SE from Managua.  It is an active volcanic area including Masaya Calera, Apoyo Caldera and Mombacho.  Abundant hot springs and fumaroles are found N and S of the volcano.  La Calera is N of Mombacho.  It includes hot springs and dispersed fumaroles.  Rio Manares River – Melcatepe is SE of Mombacho.  There are several thermal springs in a zone with several swamps and small lakes. 

There were some preliminary isotope studies done by the International Atomic Energy Agency around Mombacho 2001 – 2004 as part of a larger program to improve geothermal resource management in Nicaragua.  To date, there has been no further geothermal exploration done at Mombacho. 

Flank Collapses

At least three large-volume debris avalanches took place at Mombacho.  Smaller rockfalls take place nearly continuously on the steep scarps of the collapse amphitheater.  The oldest of these is La Danta.  It is undated.  The second is the Las Isletas collapse to the N around 450 AD that created the Las Isletas de Granada in Lake Nicaragua 11 km NE from the summit.  The most recent El Crater collapse took place 1560 – 1570 (with 1570 being the most agreed upon date).  None of these took place during an eruption, meaning they were all cold collapses. 

The La Danta scar to the SE and its debris avalanche have been covered by more recent material.  The other two collapses took place via different mechanisms, flank spreading for Las isletas and hydrothermal altering of rock triggered by a combination of seismic activity and heavy rainfall for El Crater. 

Mombacho is most notable for how its flanks collapsed.  Volcanoes generally suffer flank collapses due to instability introduced in two ways.  The first is via magma injection, similar to the magma injection into the cone of Mount St Helens that made its cone unstable.  The second is via flank spreading.  We see ongoing spreading at Momotombo.  Mombacho appears to have a third type of instability, that of basement spreading. 

Mombacho is built on a thick layer of the 20 ka Apoyo pumice and older Las Sierras ignimbrite.  Las Sierras underlies the entire cone and is responsible for most of the cone deformation.  This layer is perhaps 300 m thick under the volcano.  Deformation of this layer under the weight of the new volcano triggers flank collapses and debris avalanches at Mombacho.  These collapses take place in sectors. 

The three debris avalanches from Mombacho extend about 10 km from the volcano to the S, SE and NE.  Each avalanche lobe covers around 20 – 30 km2.  The most recent of these was probably 1570 AD.  The Las Isletas avalanche extends at least 7 km into the lake, most likely causing tsunamis.  The Las Isletas and El Crater avalanches were caused by two different mechanisms but created similar debris flows, coarse material on top of fine material with hummocks.  The fine material provided a low friction slide layer.  Las Isletas carried an amount of the La Sierrias ignimbrite basement into the lake with it.  El Crater only carried hydrothermally altered parts of the cone with it. 

La Donta

The first, La Danta failure of the SE sector took place 20 – 1 ka.  It created an unnamed group of isletas as water inundated its hummocks.  The avalanche also dammed Laguna de Pichicha.  The failure scarp was filled by subsequent activity and is no longer recognizable as collapse scar. 

Las Isletas

Las Isletas ran 12 km from the amphitheater.  It dates in pre-Columbian times and may have been viewed by locals.  The deposit covers 57 km2 with an average thickness of 22 m.  Volume of both the deposit and amphitheater is 1.2 km3 and 1.1 km3 respectively.  Minor phreatic activity took place after the collapse.    This was a dry collapse that carried some basement ignimbrite along with it.  The failure plane was located partly in the ignimbrite. 

The long runout of the N collapse deposit reached Lake Nicaragua and formed a curved peninsula and cluster of Isletas de Granada, some 11 NE from the summit.  Las Sierras material are the central and lower parts of the deposit.  The failure deposited material well into the lake, forming the Las Isletas archipelago.  The failure plane cut into basement rock beyond the volcano’s foot, extending up to 5 km from the volcano.  The N and E base of Mombacho was spreading outward before the collapse. 

El Crater

El Crater may be historic, as there are accounts of a disastrous event in 1570.  The 1.25 km amphitheater is deeper than the Las Isletas.  It extends 2.3 km from the summit.  The deposit covers nearly 50 km2, reaching 12.4 km from the scar.  Collapse volume is estimated at 1.75 km3.  The deposit is clearly larger than the volume of the S amphitheater alone.  More than 2,900 hummocks have been mapped for this deposit.  Numerous lakes now exist in depressions between individual hummocks (blocks). 

The S scarp is nearly vertical and exposes hydrothermally altered rock.  The clay-rich material likely contributed to the weakening process leading to the S collapse.  Deposits from this collapse have been mapped to the S edge of Lagunetas de Macatepe Natural Reserve, 12 km from the summit. 

The El Crater collapse scar is at least 400 m above the cone basement, meaning failure took place entirely within the cone.  Avalanche deposits are thick lobes of hydrothermal clays, altered lavas and scoria.  Buttresses on the walls of the scar are altered andesite and basalt plugs.  A zone of intense alteration extends over much of the lower crater.  Its material is similar to that of the avalanche. 

The scar cuts the cone deeply to uncover an altered core, though does not cut all the way to the base.  It is larger than the N collapse.  Deposits contain a larger proportion of scoria that may have allowed easier fluid circulation and increased alternation.  Hydrothermal alteration is thought to be the main cause of weakness in this sector.  There is no juvenile material, meaning this was not a hot collapse.  The avalanche has more fine material than Las Isletas. 

Two items of interest of this event are what triggered it and a dispute over what it was.  Rock in this collapse suffered intense hydrothermal alteration before the collapse.  While alteration can weaken the rock, it alone can’t trigger a collapse.  However, an intense seismic swarm ending with a large earthquake can trigger a collapse in highly altered, wet rock.  Local accounts of the collapse have it taking place after a period of intense rainfall and a seismic swarm that ended with a rather large quake.  While heavy rainfall and seismic activity might have triggered the collapse, they did not play a role during transport. 

There is some dispute over the 1570 AD event.  The USGS Open File Report 01-455, Lahar Hazards at Mombach Volcano, Nicaragua makes the argument that the event was a massive lahar, similar to a hurricane caused lahar from Volcan Casita Oct, 1998 that killed over 2,500.  A 0.0016 km3 debris avalanche converted into a 0.004 km3 lahar that extended at least 12 km from the source. 

The report quotes local accounts of the 1570 AD account that destroyed the village of Mombacho relatively high on the flank of the volcano, killing at least 400.  The event took place during a stormy night and multiple earthquakes were involved.  Local accounts suggest that intense rain triggered a landslide that converted into a lahar that destroyed villages near the volcano. 

As of this writing, it appears that it was a flank collapse / debris avalanche rather than a lahar, as there are hummocks downhill and no evidence of water involved in mobilization of the flow. 

Finally, two craters at Plan de las Flores were formed after the collapse.  There was an existing crater lake prior to the 1570 event,

Eruptions

Smithsonian GVP carries a pair of eruptions from Mombacho over the last 10 ka.  The 1850 eruption is discredited and the 1570 eruption is uncertain.  The volcano does have an active hydrothermal system and fumaroles.  Gas emissions from fumaroles are carried in six Bulletin Reports 1980 – 2012.

The first of these reported a small intermittent plume rising from the SE section of the summit in late 1980.  The next report Mar 1982 discovered four previously unknown warm springs on the S side of the volcano.  These are not believed to be new features.  A fumarole loudly emitting gas on the S collapse crater was observed 1986 – 1987.  It continued to be active through Dec 1993.

The Nov 1994 Report discussed the fumarole active since 1986.  A strong sulfur odor with winds blowing toward the fumarole led to the discovery of two other previously unknown fumarole fields.  These are not new fields.  Data results from three fumarole fields became available from INETER in 1994. 

Seismic instruments were first placed on the volcano around 2000.  Since Dec 2000, INETER included status and seismic reports from Mombacho in their national monthly volcanic seismic report, Seismos y Volcanes de Nicaragua.  The most recent Bulletin Report Feb 2012 summarized field observations, seismic activity and thermal measurements 2001 – 2011.

Mombacho was most seismically active in 2000, with 14 shallow earthquakes.  There were only 6 other earthquakes detected for the rest of the decade. Fumarole temperature measurements did not happen regularly, partly because rockfalls covered some of the fields from time to time.  Temperatures were not particularly hot, 95° – 125° C, though nearly 400° C was reported in Aug 2009 and 2010.  It is unclear how many locations this was reported at.  Measurements were all taken with handheld devices.

Field visits constantly encountered landslides within the S crater.  Its steep walls on the E and W scarps constantly shed debris that collects in the crater.  Large rockfalls took place after major rain events (hurricane and tropical storm passage). 

Tectonics

The Central American volcanic arc in Nicaragua and Costa Rica is formed by the subduction of the Cocos Plate beneath the Caribbean Plate.  Subduction takes place 8 – 9 cm/yr.  Unusually thick oceanic plate in the S part of the region formed due to the Galapagos hot spot 90 – 15 Ma. 

The volcanic arc erupts through accreted terranes onto the Chortis and Chorotega blocks.  The major boundary between the two is obscured by sediments and young volcanic material but is located along the political border between Nicaragua and Costa Rica.  The Pacific region of Nicaragua accreted to the Chortis Block and is likely an old volcanic arc.  The Chorotega Block forms much of Costa Rica.  It is formed out of thickened oceanic crust, likely part of the Caribbean Large Igneous Province.  Basement rocks here are only exposed near the Pacific coast.

There is a discontinuity, described as a step in distance from the trench about 50 km N of the Nicaragua – Costa Rica border.  Slab depth beneath the arc is typically 150 – 180 km in Nicaragua, 80 – 95 km in Costa Rica with smooth changes across the step area.  Plate geometry does not cause the step.  The arc I located in mountainous terrain in Costa Rica and within the low Nicaragua Depression in Nicaragua.  It migrated trenchward over the past 25 Ma, reaching its current location 7 – 1.5 Ma.  The oldest dated lavas within the depression are 330 – 65 ka. 

Crust in the Nicaragua Depression is 11 – 18 km thinner than in the center of the Highlands 100 km NE.  It is unclear if the crust thinning is a recent event, as there is relatively little identifiable extension.  There is also very little basin subsidence.  It is possible that older and more recent volcanic material in the depression have different origins.  These discrepancies are not yet reconciled.  It is possible the depression represents forearc geometry before the new terrane was attached.

Conclusions

Mombacho is a currently dormant volcano with no historic eruptions, though it does have an active hydrothermal system.  The action of that system altering rocks in the cone and movement of the cone on a weak basement layer triggered at least three flank collapses over the last 10 ka.  The largest threat from this volcano are additional flank collapses, lahars and other landslide / debris avalanche events. 

As a reminder, hydrothermal alteration is progressive, so deformation of these volcanoes likely begins long before failure.  This means that deformation should be seen before failure, identifying basement and sector spreading, allowing prediction of type, location and effects of possible collapses.  Once locations and structure of potential collapses is identified, monitoring can identify rates of movement. 

Boating toward Mombacho from Las Isletas Grenada.  Note the blocks at the toe of the debris avalanche.  Image courtesy Next Adventure Nicaragua, Jun 2019

Additional information

Fumarolic gases at Mombacho volcano (Nicaragua): presence of magmatic gas species and implications for volcanic surveillance, Tassi, et al, Apr 2007

Lahar hazards at Mombacho volcano, Nicaragua, USGS Open-File Report, 01-455, Vance, et al, 2001

Emplacement mechanisms of cointrasting debris avalanches at Volcan Mombacho (Nicaragua) provided by structural and facies analysis, Shea, et al, Nov 2007

Catastrophic collapse at stratovolcanoes induced by gradual volcano spreading, van Wyk de Vries, & Francis, 1997

Mombacho (Nicaragua): seismicity and fumarole characteristics from 2000 to 2011, JA Herrick, Feb 2012

Volcanogenic tsunamis in lakes:  examples from Nicaragua and general implications, Freundt, et al, 2007

Collapsing volcanoes:  the sleeping giants’ threat, Shea & vany Wyk de Vries, Geohazards, mar – Apr 2010

Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front:, Geochimica et Cosmochimica Acta v 70, p 5215 – 5235, Elkins, et al, 2006

Modeling the propagation of volcanic debris avalanches by a depth averaged finite element solution, Sosio, et al, May 2010

Spreading volcanoes 1, Bogia, et al, May 2000

Systematic morphometric chatacterization of volcanic edifices using digital elevation models, Grosse, et al, 2011

Nicaragua country update, A Zuniga Mayorga, Proceedings World Geothermal Congress 2005, Apr 2005

Seismic evidence for fluids in fault zones on top of the subducting Cocos Plate beneath Costa Rica, Van Avendonk, et al, May 2010   

Crustal structure along the southern Central American volcanic front, MacKenzie, et al, Aug 2008