Ayarza is a double caldera located about 40 km NE from the active Guatemala volcanic front.  The two-lobed caldera lake was created by a pair of Krakatoa-style eruptions that collapsed a coalescing pair of stratovolcanoes.  Interestingly, there was no resurgent activity following the caldera-forming eruptions.  The lake floors are smooth, and sediment covered.  There are no obvious hot springs, fumaroles or other hydrothermal activity in or around the lake.

The two eruptions were 27 and 23 ka.  The calderas are filled with Laguna de Ayarza.  It has a surface area of 14 km2, a maximum depth of 230 m.  Walls from the surface to the lake bottom are steep with relatively little sediment.  Sediment layers on the bottom of the lakes are quite thick.  The double caldera measures 5 x 7 km.  The larger lobe is 4 km wide.  The smaller lobe is about 2 km. 

Ayarza is located some 49 km ESE from Guatemala City, both the capital and largest city in Guatemala with over 3 million in its urban area.  Guatemala City is built on the former Maya ceremonial center Kaminaljuyu, which rose to prominence in 300 BC.  The civilization collapsed some 600 years later in 300 AD. 

This is a preferred location for a major city despite multiple natural disasters including major volcanic eruptions, earthquakes and hurricanes.  Four stratovolcanoes are visible from the city.  Fuego and Pacaya are both active.  Agua and Actenango are not. 

Today, Guatemala City is the economic, governmental and cultural epicenter of the nation.  It is also the major transportation hub.  Surrounding lands support agriculture and ranching.  There are multiple coffee plantations located in close proximity to Ayarza.  The lake also supports a thriving tourist industry.   

Climate in this part of Guatemala is classified as a tropical savanna climate, bordering on a humid subtropical climate.  Its relatively high altitude of 1,500 m tends to moderate high temperatures a bit.  Average daily temperatures range between 26° – 15° C, with record highs and lows 6° – 8° C above and below those averages.  Average precipitation is around 127 cm/year.  It is normally quite humid. 

Volcano monitoring in Guatemala is done by Instituto Nacional de Seismologia, Vulcanologia, Meteorlogia e Higrologia (INSIVUMEH).  Smithsonian GVP lists 43 volcanoes in Guatemala.  INSIVUMEH currently lists and ranks 25 of them as active threats.  Ayarza is not considered active or listed as a threat.  As such, it is not actively monitored.  The closest active webcam appears to be in Guatemala City.  It is not dedicated to Ayarza.

Region

Volcanoes in this part of the Central American volcanic belt are closely spaced with many of them recently active.  The area is also heavily populated with 3 million living in the Guatemala City urban area 48 km W of Ayarza.  In order to keep this a manageable number of systems to mention, I will limit the mentions to volcanoes within 30 km from Ayarza.  Note that there are at least another 16 volcanoes closer than 100 km from Ayarza.  The currently active Fuego / Acatenago system is 89 km W from Ayarza.  Pacaya is 51 km W.  Tecuamburro is 44 km SW.  All distances will be measured from the center of the Ayarza caldera. 

Jumaytepeque 

Jumaytepeque is a small basaltic stratovolcano located some 18 km SE from Ayarza.  The 1,802 m volcano was constructed near the SE rim of the large Santa Rosa de Lima caldera.  It is newer than the most recent 23 ka tephra from Ayarza.  The volcano is relatively uneroded with activity over the last 10 ka possible.  There are two older cinder cones to the N that are covered by the Ayarza tephra. 

Cullapa – Barbarena 

The Cuilapa – Barbarena volcanic field is a monogenetic field with about 70 cinder cones.  It is located at the intersection of the local Jalpatagua fault with the Santa Rosa de Lima caldera, 31 km WSW from Ayarza.  The youngest cones may be newer than the last activity from Tecuamburro, less than 10 ka.  These cinder cones are so far undated.

The cones are generally less than 100 m high.  They erupted from fracture systems associated with the previously mentioned fracture system intersection with Santa Rosa de Lima caldera.  They are built on a basement of pyroclastic flows from Amatitlan caldera to the NW.  The most recent eruptions took place within the last few thousand years. 

Piedra Grande   

Piedra Grande occupies the northern third of the Tecuamburro graben.  The 1,600 m cone measures 7 km wide at the base and is located some 35 WSW from Ayarza.  It is eroded and faulted, flanked by younger andesites, basalts and ignimbrites.  Much of the upper part of the cone is hydrothermally altered and as such, undated.  The volcano is older than the 1.2 Ma neighboring and demonstrably younger El Sordo volcano.

Piedra Grande is one of a cluster of faulted, heavily eroded andesitic stratovolcanoes in the Tecuamburro graben.  Its summit is located on the N, with nested depressions open to the E.  There is a fumarole present at the summit.  El Sordo is a basaltic andesitic stratovolcano breached to the ESE.  One lava flow dates 1.18 Ma.  Los Sitios is a sequence of andesitic lava flows from two eroded and faulted scoria and cinder cones.  One of these dates 0.80 Ma. 

Ixhuatan 

The Ixhuatan volcanic complex is named after the town of Santa Maria Ixhuatan, located near the summit.  The 1,718 m high complex is located 31 km SW from Ayarza.  It is a 12.5 x 20 km shield cut by faults on the E side of the Tecuamburro graben.  A single lava is dated 1.16 Ma.  Most of the complex summit is heavily weathered.  Exposures along the summit are rubbly lava flow tops, scoria cones and vulcanian breccias.  Most erupted material are andesites to basaltic andesites. 

Silicic pyroclastic flow deposits are found in the Rio los Esclavos, W of the volcano.  The youngest center is a dacite dome complex at Cerro los Achiotes.  The domes and associated ashfall partly fill and extend from a horseshoe shaped amphitheater breached to the SE.  A N-S trending fault zone cuts the volcano.  It was the site of a seismic swarm 1979 – 1980. 

Cerro Santiago 

Cerro Santiago is the most prominent of a cluster of cinder cones and low shield surrounding Jutiapa in SE Guatemala.  The 1,192 m cone rises some 300 m above the surrounding plain, 29 km ESE from Ayarza.  Cerro Santiago is one of a pair of coalescing cinder cones capping a shield SE of the city.  Twin cones at Los Cerritos NE of Jutiapa cross the Interamerican Highway.  Volcan Culima is a steep-sided basaltic lava mound immediate E of the city.  Cerro Gordo (Amayo) is a craterless cinder cone surrounded by basaltic lava flows.  The lava flows all appear young but are undated.  They cover the area between Jutiapa and recent volcanic hills to the S.

Tahual   

The basaltic to basaltic andesite Volcan Tahual tops out at 1,716 m, about 700 m above the plains S of Monjas, 25 km E of Ayarza.  The summit is cut by a broad erosional crater that extends to the bast of the volcano.  It is narrowly breached to the NE.  There is a recent pyroclastic cone near the NE base that fed a short basalt lava flow.  The Laguna de Hoyo it to the N.  It is described as a steep-walled crater.  Both it and the NE flank cinder cone are located along faults that extend across the E base of the volcano to neighboring Retana caldera to the NE.  The volcano is popular among hikers. 

Cerro Alutate   

The 2,116 m Cerro Alutate volcano is a small recently active stratovolcano located some 25 km N of Ayarza.  The volcano is located well N of the Guatemalan volcanic front.  Other areas of recent volcanism are found NW and NE mostly S of the Motagual fault system. 

Calderas

No discussion of volcanism along this part of the Central American volcanic belt is complete without mentioning regional calderas.  Five large calderas are located in the N part of Central America:  Atitan, Amatitlan, Ayarza, Coatepeque, Ilopango.  Ayarza is the smallest and least active of these.  We will discuss it in more depth after briefly touching on its neighbors.

This is a busy part of the Central American volcanic belt, and there are many more suspected calderas in the region than these five.  That discussion will be reserved for another time. 

Atitlan 

Lago Atitlan partly fills the 15 x 25 km Atitlan caldera, 119 km WNW from Ayarza.  There have been at least three caldera events over the last 12 Ma.  The most recent of these produced 250 – 300 km3, the massive Los Chocoyos Ash 85 ka.  This is largest known unit in Guatemala over that period.  It is found in three ocean basins from the Gulf of Mexico to the Equatorial Pacific near Ecuador.  The caldera is surrounded by several conical stratovolcanoes built post-caldera.  These include Tolman, Atitlan, and San Pedro.  We covered Atitlan in a 2020 post.  

Amatitlan  

The 14 x 16 km Amatitlan caldera erupted at least 9 pyroclastic units with a total volume of 60 – 80 km3 300 – 24 ka.   It is located 49 km WNW from Ayarza.  This caldera took a while to identify as the E and W rims coincide with faults that outline the Guatemala City graben.  The N rim is buried by pyroclastics.  There are ring faults, hot springs and layered pyroclastics.  The system has been quite active building resurgent stratovolcanoes Pacaya, Pacaya Viejo, Cerro Grande and the currently active Mackenney.  There is also a cluster of dacite domes on the S caldera floor. 

Pacaya Viejo collapsed 1,500 – 600 years ago producing a debris avalanche that made it all the way to the Pacific Coastal plain.  The modern Pacaya (Mackenney) cone grew inside the collapse scar.  The NW flank Cerro Chino crater was active in the 19^th Century.  Active over recent decades has been frequent Strombolian eruptions with intermittent lava flow extrusion.  This partly filled the caldera moat and covered the new cone flanks with lava.  A future explosive eruption has the capability to partly destroy the summit. 

Coatepeque   

The 7 x 10 km Coatepeque caldera is located some 87 km SE from Ayarza.  It was formed by collapse of a group of stratovolcanoes immediate E of the Santa Ana volcano.  The caldera rim height increases on its W side, partly truncating Santa Ana.  The caldera formed during a series of major rhyolitic explosive eruptions 72 – 51 ka.  Post caldera activity built basaltic cinder cones, lava flows, and half a dozen rhyodacite domes near the caldera lake margin.  The domes are mostly undated, though the youngest is estimated at less than 10 ka.  Hot springs are found near the lake margins.  There are no verified historic eruptions known.

Ilopango 

The 8 x 11 km Ilopango caldera is located immediately E of the El Salvador capital San Salvador, 143 km SE from Ayarza.  It produced four major dacite – rhyolite eruptions over the last 20 ka.  The most recent one perhaps 535 AD, that produced 85 km3 of the Terra Blanca Joven ignimbrite.  The most recent eruption sequence was dome building through the lake 1879 – 1880.  We published a pair of posts on Ilopango, the most recent in 2019. 

Ayarza Caldera

Given the lack of post-caldera activity over the last 23 ka, most discussion of Ayarza is based on the pair of caldera-forming eruptions 27 and 23 ka.  The lake was created by the eruptions, with the W lobe being from the most recent.  Both calderas have walls that rise nearly 600 m above the lake surface.  Ancestral stratovolcanoes are estimated at 2,200 m for the Mixta eruption (E lobe) and 2,400 m for the Pinos Altos eruption (W lobe).

The lake records post-collapse geologic history of the area.  The caldera floor appears to be more chaotic than a piston-style collapse controlled by ring faults.  The collapse appears to have taken place partly along inward-dipping faults that steepen at depth and partly in a zone of smaller blocks that tumbled into place.  There is a channel between the two lobes that could have been cut if the water level in the lake was 120 m lower than today.  At least 170 m of sediment was deposited in the main basin since the calderas were formed.

The steep walls of the W caldera suggest it is the younger.  Stream deltas do not extend any significant distance into the 240 m deep W caldera.  The 140 m deep E caldera is filling with coalescing deltas.  The smaller lobe appears to contain more sediment fill.  It is also older, with more time to be refilled with lake sediments and debris from the more recent eruption associated with formation of the larger caldera. 

There is a discrepancy in caldera forming eruption dates for Ayarza.  VOGRIPA lists the Mixta pumice and ashfall as 32 ka, Smithsonian GVP lists it at 27 ka.  Similarly, the more recent Pinos Altos pumice and Tapalapa ash flow are dated 28 ka by VOGRIPA and 23 ka by Smithsonian GVP. 

Both eruptions were major rhyolite eruptions that produced large volumes of loose, easily erodible ash and pumice lapilli inside and along the rims of the calderas.  Some of this slumped into the lake on the E side.  More has been washed in by local streams and runoff.  The remaining walls are layered andesites and flow-banded rhyolite / rhyodacite.  The ancestral cone was built by andesites.  The rhyolites appear to be deposited during caldera forming eruptions.  There are some limestones seen as basement rocks. 

 The Tapalapa ash flow from the most recent caldera-forming eruption at Ayarza dates around 23.1 ka.  A volcanic mudflow is found on the N side of Ixhuatan volcano, believed to come from this eruption.  The mudflow is found along the Rio Los Esclavos, whose headwaters are at the W base of Ayarza volcano, along with cobbles and boulders of a basalt from the Cuilapa – Barbarena cinder cone field.  Finally, it is relatively young, in a high stratigraphic position just below the youngest Tecuamburro ash flow. 

Eruptions

VOGRIPA carries the two eruptions in its database as follows:  32 ka Mixta pumice / Tephra (MFT)  VEI 6.0  Basalt rhyolite  9.0 km3 bulk  28 km plume; 28 ka Pinos Altos air fall, Tapalapa Ash, Sabana Redonda Tephra, VEI 5.9 rhyolite 7.3 km3 bulk  29 km plume

As previously discussed, there are discrepancies between VOGRIPA eruption dates and currently accepted eruption dates (VOGRIPA is older for both eruptions), there is also a discrepancy in estimated bulk volume erupted in each eruption, with VOGRPA being larger.  A third discrepancy has the most recent eruption being smaller, another possible disconnect as it created a larger caldera. 

Ayarza pyroclastics are a pair of rhyolitic sequences separated by a meter-thick paleosol.  Both eruptions produced air fall tephra overlain by an ash flow.  Each sequence was due to a single, Krakatoa-style caldera forming event. 

The lower pyroclastic sequence base is an air fall unit mostly N of the calderas.  It is covered by an ash flow deposit.  The ash flow deposit is mainly rhyolite ash and pumice.  The upper ash flow has basalt pumice fragments and banded hybrid pumices.  This sequence dates 27 ka and is much thicker to the N where local surge deposits are found.  The unit is informally named the Mixta (mixed) unit with air-fall and ash-flow deposits. 

Mixta is described as a spectacular basalt – rhyolite mixed product out of the smaller, E lobe of the caldera.  The Mixta eruption was more likely triggered by local tectonism than by injection of basalt into the existing rhyolite as it appears a stratified magma body was tectonically disrupted, triggering an eruption. 

The upper pyroclastic sequence is separated by a meter thick paleosol from the older event.  The younger sequence once again starts with a major air fall unit immediately covered by a fine-grained ash flow sequence.  The first is named the Pinos Altos air fall; the ash flow sequence is called the Tapalapa ash.  There is a thin air fall tephra, Sabana Redonda air fall within the ash.  This eruption dates 23 ka, the last recognized activity at Ayarza.

The Pinos Altos airfall is two sets of well sorted rhyolitic tephras on the S flank and a single set on the N.  The lower set contains larger and more abundant lithic fragments.  The upper set is described as either a more intense phase of the eruption or a second pulse. 

The Tapalapa ash immediately covers the air fall.  It is an unconsolidated, unstratified fine grained pyroclastic deposit of rhyolitic ash.  Small pumice blocks are found in the base or mid-section of the unit.  The Tapalapa unit appears to have phreatomagmatic origin which magma encountered water immediate before or during the eruption.  It is unknown if the water was ground water or an ancestral Laguna de Ayarza impounded in an earlier caldera, as almost nothing is known about the pair of ancestral stratovolcanoes that went caldera.  Both Pinos Altos and Tapalapa deposits are chemically identical.  The Tapalapa ash is thickest to the W at 4 – 5 m thick.  It is confined to topographic lows. 

Tectonics

The major tectonic feature of central Guatemala is formed by the subduction of the Cocos Plate under the Caribbean Plate.  This has created the 1,500 km long Central America Volcanic Arc stretching from southern Mexico to northern Panama.  Volcanoes are closely spaced, but clustered, averaging 27 km between clusters.  Due to active subduction, massive earthquakes are common.  A M 7.5 earthquake in 1976 killed 26,000 and opened fissures under Lake Atitlan dropping its water level by two meters in a month.

The volcanic front along the arc is 165 – 190 km from the offshore trench.  The seismic zone underneath the Arc varies from less than 100 km in Guatemala to 200 km in eastern clusters of small composite cones, shields, domes and cinder cones.  The volcanic front is located with a shallow seismic zone with strike-slip normal faults and grabens.  Much of the terrain dropped by the action of grabens has been filled with volcanic debris.  Basaltic lavas are usually present along the arc, but far more common in areas with thinner crust like Nicaragua.  Lavas range from basalts to rhyolites.  Individual vents within volcanic centers usually have more than one magma batch, indicating zoning of the underlying magma reservoirs.

There is a line of calderas dispersed along and slightly behind the main Arc.  These typically erupt silicic tephras and pyroclastic flows. 

There is a poorly defined secondary line of volcanoes 20 – 75 km behind (NE) of the volcanic front.  These are typically single composite cones.  They have significantly different chemistry than volcanoes in the main Arc.

Back Arc volcanism occurs in Central America.  It overlaps the second line and extends to the main front.  These are typically clusters of cinder cones, small shields and lava fields in areas of extension.  There is no historic activity in the back arc. 

Conclusions

Ayarza is an interesting example of a previously active volcanic system that went caldera in back-to-back eruptions 27 ka and 23 ka.  The interesting part to me is the complete lack of activity following the two caldera forming eruptions.  Note that the pair of eruptions obliterated all previous activity from the system.  It is today quiet with no obvious manifestations of current volcanic activity, though there is most certainly activity relatively close (within 50 km). 

Additional information

Explosive eruptions of the Ayarza calderas, southeastern Guatemala, Peterson & Rose, Jul 1985

A geophysical and geological study of Laguna de Ayarza, a Guatemalan caldera lake, Poppe, et al, Jun 1985

An evaluation of the geothermal potential of the Tecuamburro volcano area of Guatemala, Heiken & Duffield, Sept 1990

Petrologic and structural contrast of the quaternary volcanoes of Guatemala, Bull Volcano 27, 61(1964), Williams & McBirney, Dec 1964

Determinacion del estado Trofico de la Laguna de Ayarza, Ubicada en el Departamento de Santa Rosa, Republica de Guatemala, Salvador & Mayorga, Vol 7 No 1, 2012

Evaluacion del estado Trofico de la Laguna de Ayarza, utilizando el modelo de simulacion Wasp 7.41, Rodas & Moran, Vol 9 No 1, 2014

X-ray mineralogy of cores from two Guatemalan cauldera lakes, Poppe, et al, USGS Open-File Report 81-1333, 1981

Pacific offshore record of Plinian arc volcanism in Central America:  1.  Along-arc correlations, Kutterolf, et al, Feb 2008

Miocene to Holocene marine tephrostratigraphy offshore northern Central American and southern Mexica:  Pulsed activity of known volcanic complexes, Schindlbeck, et al, Sept 2018

Pleistocene tephra and ash-flow deposits in the volcanic highlands of Guatemala, Koch & McLean, Apr 1975 

An improved age framework for late Quaternary silicic eruptions in northern Central America, Rose, et al, Jan 1999

An evaluation of the geothermal of the Tecuamburro volcano area of Guatemala, LA-11906-MS, LANL, Sept 2001

Volcano observatories and monitoring activities in Guatemala, Palma, et al, 2021