Volcan Almolonga is the remains of an andesitic stratovolcano topped with a 3.3 km caldera. It is located along the Zunil fault zone. The caldera is surrounded by multiple ring-dike dacitic and rhyolitic lava domes. The youngest, largest and only historically active dome complex is Cerro Quemado (Burned Peak).
The complex is located 8 km south of Guatemala’s second largest city, Quetzaltenango and a few kilometers from the neighboring village of Almolonga. There are over 1.1 million living within 30 km of the Almolonga complex and over 6.8 million within 100 km.
The volcano tops out at 3,173 m.
Quetzaltenango, alternately known by its Mayan name Xelaju or Xela, is the second largest city in Guatemala with nearly 800,000 people. It is located in a mountain valley around 2,330 m above sea level. The population is 95% Amerindian and Mestizo.
The city was already well established when the Spanish arrived, at least 300 years old and unofficially considered as the capital of the Maya by some number of the inhabitants.
Climate is subtropical highland, with daytime temperatures falling quickly after noon due to its altitude. During the dry season, it is not uncommon for the city to get any rain at all. The rainy season runs late May – October.
The economy is generally agricultural with a healthy livestock industry. There is a thriving hiking industry in the region and multiple guided and unguided hikes possible. The region also has numerous hot springs which are commercially available to visitors as spring-fed hot baths. Indeed, the word Almolonga is derived from a Spanish description “place from where the water springs forth.”
Due to the proximity of active hydrothermal fields, there are at least two geothermal energy fields in the area, Zunil I and II. A lot has been written about geothermal energy, exploration, and development in these fields, to the extent that it is difficult to sort through all the geothermal papers to find information on the volcanoes.
Smithsonian GVP’s description of Almolonga has extensive coverage of a landslide in 1990 caused by what was initially called a well blowout at the Zunil geothermal field. Initial reports had a geothermal well blowout triggering a landslide that killed 23. It may be that it was simply a landslide that damaged the wellhead.
The landslide took place at night on the E flank of the Santa Maria volcano. The landslide scarp was above the well pad, making it unclear what role if any the well played in causing the landslide. The site of a proposed 15 MW geothermal power plant and an older well were covered in debris. The damaged well blew steam after the slide, putting steam 10 – 20 m into the air.
The landslide appears to be related to hydrothermal alteration of the rocks making up the cone of the volcano.
The region surrounding Quetzltenango is a complex active volcanic field with at least six recently active volcanoes – Santa Maria – Santiaguito (10 km), Cerro Quemado, Zunil (9.5 km), Santo Tomas (Pecul) (12.2 km) and Siete Orejas (15 km). The region also includes multiple calderas starting with the huge and poorly defined Xela (Quetzaltenango) caldera. Smaller calderas include Almolonga, Zunil and Siete Orejas. There are a pair of active geothermal fields, Zunil I and II. All of these are packed in a region of around 300 km2.
With the exception of Almolonga, Cerro Quemado and the two Zunil hydrothermal fields, all the other stratovolcanoes and associated calderas are located south of the southern rim of the Xela caldera.
The largest volcanic feature in the immediate region is an older silicic caldera called the Xela (Quetzaltenango) caldera. It is about 20 km in diameter. Neither its creation date nor size of the associated eruption has yet been determined. The Vogpira database does not even list the caldera or the associated eruption. The actual caldera rim is poorly defined, but visible in part in satellite images. More recent volcanic activity from neighboring systems covered, removed and reworked much of the older deposits.
Almolonga caldera is located in the center left of the larger system. It is surrounded by several dome complexes, the most recently active one is Cerro Quemado just outside the Almolonga caldera rim at the 8:00 position on the clock.
There are a pair of highly active hydrothermal and geothermal fields south of Almolonga similar in size to the caldera. Zunil II is perhaps 5 km SSE and Zunil I is perhaps 8 km to the south. Zunil I straddles the southern Xela caldera ring wall while Zunil II is entirely located within the confines of the greater caldera.
Finally, there are two additional calderas in the immediate vicinity. The Siete Orejas caldera intersects the larger caldera at the 8:00 position. The Tzanjuyub caldera associated with Volcan de Zunil is located at the 4:00 position. Both sit mostly outside the greater caldera rim.
Siete Orejas is an andesitic stratovolcano with a large caldera breached to the south. The most recent caldera forming eruption took place 125,000 – 85,000 years ago and produced a widespread ashfall pumice and pyroclastic flow deposit.
Volcan Santa Maria and Santiaguito are outside the larger caldera to the SW.
Volcan de Zunil is located to the ESE of the Xela caldera rim. It in turn is located on the SW rim of the 4 km diameter Tzanjuyub caldera. Zunil is connected by a ridge to Volcan Santo Tomas (Pecul) to the SW. There are multiple dacite rhyolite lava domes on the complex N and NW flanks. The most recent activity was from the Cerro Zunil dome around 84,000 years ago.
Basement to the region is a large granitic formation. It is overlain by multiple thick lava and ashflow tuff deposits. These are in turn overlain by thinner deposits of eroded materials, lahar deposits, landslide and pumice ashfall deposits.
The greater Almolonga volcano is an andesitic stratovolcano surrounded by several dacite and rhyolitic lava domes. Activity at this location began some 84,000 years ago. It collapsed prior to the caldera forming eruption of Atitlan. The collapse created a 3.3 km diameter caldera 350 m deep. As far as I can find, caldera formation at Almolonga has yet to be dated.
Since the caldera formation, subsequent eruptions built multiple domes around the caldera. The largest and most recently active of these is the Cerro Quemado.
There are multiple magma bodies beneath the region. The magmatic system powering the Zunil hydrothermal system may be independent from Santa Maria and Cerro Quemado. Gas emission analysis suggests that the Xela magmatic system may have a common origin with Cerro Quemado.
The caldera walls are constructed of interbedded andesitic lava flows, volcanic breccias and tephra units.
The Cerro Quemado dome complex was built by at least eight vents producing viscous lava flows and plugs. It built nine domes through four eruptive phases. It covers around 12 km2 with a total volume of nearly 2 km3. Its lavas are andesites and dacites. With one exception, Cerro Quiac thought to be much older, all the domes are of similar age.
There was a flank collapse with a lateral blast around 850 AD. The subsequent debris flow traveled at least 6 km to the SSW. Total coverage was 13 km2. One paper describes the collapse scarp as an avalanche caldera and measures it as a 1 x 1.5 km scar 90 m deep, essentially a horseshoe-shaped amphitheater on the western side of the dome.
The most recent eruption was in 1818 and produced a lava dome and a blocky lava flow stretching 2.5 km to the east. This eruption also produced an ashfall. All volcanic activity from the dome quieted down after this eruption, with hydrothermal fumaroles remaining active.
Lahars, flank collapse, debris flows, and future explosive activity are possible hazards from this volcano.
The University of Edinburgh combined GIS (Mapping and Geographic Information Systems) products produced a volcanic hazard map of the Guatemalan highlands. This map includes estimates of relative volcanic hazards including tephra falls, flank collapse, lahars and pyroclastic flows. The color code has five levels with brown being the worst hazard. Almolonga is listed along with neighboring Santa Maria as the highest threat region.
There was no observed deformation around Almolonga by INSAR 2007 – 2010.
Phase I and II eruptions from Cerro Quemado produced voluminous lava flows and domes in the central and southern section of the complex. They erupted from a circular group of at least eight vents. The typical eruption would produce a viscous lava flow and plug dome at the vent. One of these, the Paxmux flow extends 2.5 km from the vent and temporarily dammed a local river. This flow does not overlap any neighboring units.
Stage III volcanism was partly explosive. Events in this period included dome growth, catastrophic collapse, debris flow, lateral blast, pyroclastic flows and small debris flows. The presence of juvenile dome rock in late stage III suggests this started with dome growth. Dome collapse and debris avalanche were immediately followed by a lateral blast and associated pyroclastic flow that covered the neighboring valley below the dome. The eruption ended with emplacement of a small Pelean dome.
Timing of this eruption is thought to be 850 AD due to carbon dating of charcoal below the flows. The debris avalanche covers 13 km2 of the neighboring valley, extends about 6 km north of its source, and is 5 – 7 m thick at its farthest end. Average thickness is over 10 m. The volume of the avalanche is 0.13 km3, identical to debris removed forming the scarp on the dome.
The pyroclastic blast deposit covers 40 km2 and climbs more than 500 m up the opposing slope of neighboring Siete Orejas. The deposit is relatively thin or absent on the lee side of objects and hummocks in its path. There is a secondary pyroclastic flow deposit overlying the lateral blast deposit. The secondary flow did not issue from the collapse scarp.
The most recent eruption in January 1818 produced a blocky lava flow from the east central flank of Cerro Quemado. Minor pyroclastic and tephra falls put up to a meter of tephra on some nearby hills. Locals reported earthquakes before and during the eruption.
The 1818 lava flow is 2.5 km long and around 120 m thick. It was erupted over the course of six months, similar to that of the lava dome at Mount St Helens. The flow surface has blocks tens of meters across, spines, and rugged flow ridges.
Hot rock avalanches and pyroclastic flows and surges are commonly generated by the collapse of lava flow fronts and domes. Danger from these are typically confined to valley bottoms. Future dome growth here will likely be limited to the central dome complex.
Debris flows from Cerro Quemado are relatively uncommon.
Tectonic activity in Guatemala is controlled by subduction of the Cocos Plate under the trailing edge of the Caribbean Plate at 7 -8 cm/year. General movement of the Cocos Plate is NNE. General movement of the Caribbean Plate is to the ENE. The North American Plate to the immediate north is moving generally to the WSW.
This subduction created the Central American Volcanic Arc, stretching 1,200 km from western Guatemala to eastern Costa Rica. The dip of the Cocos Plate is shallower at the ends of the arc, 40 degrees in Guatemala and 30 – 60 degrees in Costa Rica, and steeper in the center, 65 – 75 degrees in Nicaragua. Crust thickness varies 40 – 45 km in Guatemala, thinning to 30 – 35 km in Nicaragua. Crust thickness may be related to edifice height along the arc, with higher mountains over the thicker crust sections.
There are three regions of volcanic activity – the volcanic front, the secondary front, behind the volcanic front. Magma generation along the volcanic front is primarily due to flux melting. As you get to the secondary front and behind the front, decompression melting may be equally important.
Subduction related volcanism started at least 30 Ma and continues to the present. Silicic volcanism is prominent in Guatemala, with explosive eruptions producing multiple calderas and widespread pyroclastic and ashfall deposits.
The Almolonga – Cerro Quemado complex is thought to be part of a larger more complex system including Santa Maria and Santiaguito. Their location appears to be controlled by the Zunil fault, which runs at right angles to the volcanic front.
Bulk rock chemistries for the Almolonga volcanic complex (also known as the Almolonga Volcanic Field – AVF) and the neighboring Santa Maria / Santiaguito are quite similar with a few exceptions. This may mean that they are both part of the same magmatic system.
The Almolonga Volcanic Field is part of an active and quite explosive part of Guatemala. The system has a very active neighbor in Santa Maria / Santiaguito which appears to share the same magma source. At least in recent millennia, Almolonga has a repose time measured in hundreds of years between eruptions. Volcanic threat analysis of the combined system does not exclude Almolonga – Cerro Quemado from the highest risk part of the region. It still has an active and visible hydrothermal system and should be considered an active, dangerous volcanic system.