The Apaneca Range is a group of basaltic to andesitic stratovolcanoes stretching west from the Santa Ana complex to the Guatemala border. The volcanic front in this part of Central America is very busy with multiple calderas, eroded volcanoes, and recently active systems jammed closely together.
This post will discuss what is formally defined as the Apaneca Range. It will not include the Santa Ana – Izalco system to the east (though the Wiki includes Santa Ana (Ilamatepec) as a member of the range) and will end with the Guatemalan border to the west. Granyia covered the Santa Ana – Izalco system in an excellent 2018 post.
The area is heavily populated with over 150,000 living within 10 km of the range, and over 6.2 million within 100 km. The largest town appears to be Santa Ana with nearly 375,000, located NE of the range and the Santa Ana volcanic complex. Route 13 runs parallel to the range, mostly on the north side and has the villages of Ahuachapan, population 130,000, Turin, 10,000, Atiquizaya, 34,000, El Refugio Ahuachapan, 10,000, , Chalchuapa, 73,000, and San Sebastian Salitrillo, 19,000, strung along it from W to E.
There are a pair of national parks along the range. Parque Nacional El Imposible is along the western third of the range. Parque Nacional Cerro Verde surrounds the Santa Ana volcanic complex on the eastern end of the range. The Ruta de Las Flores is a 32 km winding route through local villages and coffee plantations that has become a popular tourist destination. It cuts through the Apaneca Range generally from north to south. The region is increasingly popular with hikers. Summit Post is as good a place to start as any. It is described as one of the prettiest and most traveled areas in El Salvador.
Santa Ana is the second largest city in El Salvador. It is a major processing center for El Salvador’s coffee bean industry. Over they years, it has become a major tourist destination.
Being one of the largest regional industries, coffee plantations are numerous and can be easily found with an internet search. A quick search researching the region found Ally Coffee offering Santa Matilde Bourbon Natural coffee beans, Cervantes Coffee Roasters, George Howell Coffee, and El Carmen Estate (Finca El Carmen). Each of these links has some very nice local photography and descriptions of their businesses.
Climate of the region is warm year-round with an average temperature around 25 C and classified as a tropical savanna. Year-round humidity is high. The rainy season takes place during the summer with an average of 120 cm of rain. This falls off significant during the dry season. The Atlantic hurricane season takes place June – Nov, with flooding not uncommon when these hit.
There is a major hydroelectric power station that supplies electricity to most of the western part of the country.
Volcanoes
The range is part of the volcanic front created by magma generated by subduction of the Cocos Plate beneath the North American Plate. Volcanoes produce generally basalts, andesites, trachyandesites, dacites and rhyolites. Members of the range will be generally be discussed from west to east along the range.
The range has an active hydrothermal system with multiple hot springs available, mostly on the north side of the range.
There were four main stages of volcanic activity in the area of the Apaneca Range, also known at the Ahuachapan Region or the Ahuachapan – Cuyanasul area. Initial activity built several stratovolcanoes over the basement. These included Cuyanausul, Cerro de Apaneca, Cerro Empalizada. These were cut by a major caldera event that created the Concepcion de Ataco Caldera. Post-caldera activity built small mixed volcanoes (Las Ninfas, Laguna Verde, Hoyo de Cuajuste) and domes at Cerro San Lazaro. The most recent activity has been hydrothermal with the occasional phreatic blowout. There is an active hydrothermal electrical generation field associated with the range.
Cerro Empalizada marks the western edge of the caldera. As such, its original structure has been mostly destroyed by the caldera collapse, leaving a 1.5 km3 semicircular structure. Subsequent erosion by the Rio Asino carved a 400 m scarp between the river and top of the volcano. It consists of similar volumes of lavas and scorias that date sometime before 0.77 Ma. Lava units were erupted over a relatively short period of time, have massive layers, and the absence of soil formation between layers. It is located above Miocene volcanics of Sierra de Tacuba and is covered with pyroclastics of more recent events. The Apeneca and Las Nubes basalts are pre-caldera products.
Cerro Laguna Verde (Cuyanausul)
The Cuyanausul volcano is 11 km2 volcanic structure. It was built with a sequence of alternative lavas and scorias. Lava samples date 1.7 – 1.3 Ma. The volcano is cut by a regional fault system. Its northern flank is partly destroyed by recent dome and hydrothermal activity, and partly covered by the more recent Laguna Verde volcano. The eastern flank is covered by ash and lapilli from the monogenetic Las Ranas volcano. It is a mostly basaltic volcano with a crater lake that occupies a 350 m crater at 1,606 m above sea level. The volcano itself tops out at 2,030 m.
The southern flank is covered by fragmentary debris, late phreatic explosion products from the crater. Lava flows are mostly basalts, with pahoehoe extending to cover 39 km2. Thickness 80 – 120 m gives a minimum volume of 3.1 km3. Age of these lavas is estimated at less than 100 ka. Basalts are covered by ashes from the Las Ranas volcano and the Coatepeque Caldera.
Cerro de Apaneca is very similar to Cuyanusul volcano, though smaller at 5 km2. It is less eroded with a peak of 1,816 m. The flanks have radial erosion ravines with steep slopes. The volcano is built mainly from lava layers.
Concepcion de Ataco Caldera
This is a 3.5 x 5 km caldera at the western end of the Apaneca Range. The VEI 6.8 caldera-forming eruption around 525 ka produced at least 63 km3 bulk volume. Offshore sampling has identified multiple ash layers chemically similar to Ataco Caldera dating 15.6 – 14.2 Ma, 8.1 Ma, 2.8 Ma and 1.0 Ma. These are interlayered with deposits from neighboring Coatapeque Caldera eruptions dating as far back as 21.1 Ma. The crater floor lies about 700 m below the rim. The resurgent Cerro Himalaya dome is 1.5 km in diameter.
Initial activity in the region erupted around 17 km3 of primitive lavas 1.7 – 0.77 Ma that covered the basaltic – andesitic lavas erupted from the Cuyanausul, Apaneca and Empalizada volcanoes. This was followed by a caldera eruption cycle that eventually ejected 63 km3 of silicic pyroclastics, breccias, surge deposits and ignimbrites creating the 5 x 3.5 km caldera. The youngest pre-caldera eruptions are dated at 0.77 Ma. The post caldera resurgent Himalaya dome was built 0.28 Ma.
Caldera forming eruptions produced 300 – 700 m thick pumice flows topped with pumice and ash. This is dacitic – rhyolitic material with a total volume around 63 km3. The near pyroclastic flows are estimated at 26.7 km3. The distant pyroclastic fall is estimated at 32.2 km3. And fine ash pumice estimated at 4.3 km3. Age estimates bracket the date of the eruption, with pre-caldera Empalizada basalts dated around 0.77 Ma and the resurgent Himalaya dome estimated at 0.28 Ma.
A subsequent, smaller post-caldera eruption in the NE part of the main caldera quickly followed the caldera eruption. It deposited younger pyroclastic materials. Fall deposits from the caldera-forming event are covered by surge deposits, scorias and ash from this eruption, the Cebra tuffs. These are banded light and dark pumices typically 25 m thick. There is no evidence of paleosoils or water interaction that could separate the caldera forming eruption and the Cebra tuff event.
There are multiple dacitic to andesitic domes around the caldera. They are located mostly along two collapse faults within the caldera itself. These date around 100 ka in contrast with the Himalaya dome at 280 ka. Estimated volume of these domes is around 7.2 km3. They are thought to be built during the final magma cycle.
The final stage of activity created an active hydrothermal system. That combined with the presence of four main fault systems created regions inside the caldera and nearby grabens favorable for development of geothermal systems.
There are three phreatic explosions related to the Hoyo de Cuajuste, Laguna Verde and las Ninfas volcanoes. The rocks show intense hydrothermal alteration, corresponding to post-magmatic hydrothermal phase still active in the region. Deposits cover around 2 km2 with a volume less than 0.3 km3. Two of the events took place in the craters of Las Ninfas and Laguna Verde. A third event between the two volcanoes created the Hoyo de Cuajuste, a crater with steep vertical walls.
The surface hydrothermal zone includes fumaroles, boiling springs and mud volcanoes. This action has altered the volcanic rock creating clays. The zone extends from the main volcanic chain toward the Central Graben valley. The most prominent fumaroles are in the zones of rising fluids, primarily at the foot of Cuyanausul and Las Ninfas volcanoes. The hot springs produce alkaline carbonated hot water that precipitates silica.
Post-Caldera Volcanoes
Cerro Las Ninfas is located at the eastern end of the caldera and has been partly been destroyed by the Cerro Blanco Graben. Its southern flank is covered by phreatic explosion debris. The volcano has been mostly built by basalts. It is a stratovolcano with a 300 x 500 m crater lake at the western end of the range.
Cerro El Oro covers 6 km2. It is composed of scoria and some basaltic lavas. The volume of its lavas is around 2.5 km3.
Cerro Las Ranas is on the eastern part of the area and defines the SW edge of the Central Graben close to the Coatepeque Caldera. It is built by ash and lapilli layers and is one of the youngest volcanoes in the area. Coatepeque pumice partly covers the ash deposits which overlie lavas from Cuyanausul, Laguna Verde and Las Ninfas volcanoes.
Cerro El Aguila at 2,036 m is a post-caldera stratovolcano. It is the highest point on the range.
Cerro Los Naranjos is one of the youngest stratovolcanoes of the Apaneca Range volcanic complex. It tops out at 1,961 m and is located on the eastern end of the range, 6 km NW of Santa Ana.
Ahuachapan – Chipilapa Geothermal Field
This is a geothermal field on the northern slopes of Laguna Verde. It is located 80 km W of San Salvador and 15 km from the Guatemalan border, close to the towns of Ahuachapan, Atiquizaya and Turin, and area of 20 km2. There are three levels of liquid in the system, an underlying brine, a middle pressurized hot water meteoric system, and a layer of steam under the cap.
The original geothermal field has been producing electricity since 1975. Evaluation started in 1953. By 1970, four commercially productive wells had been drilled. Successful reinjection tests were done by 1971. Original field had three units, 2 x 30 MW and a third of 35 MW. Declining reservoir pressure and limited drilling of new wells has reduced operating output, around 45 MW om 1989 when Los Alamos National Labs did a Reservoir Analysis. This analysis was part of a regional geothermal study in Central America.
By 1979, there were 27 deep wells, 12 of them production wells and 4 reinjection wells. Temperature decline in several production wells was observed by 1980 and possibly tied to reinjection, which was stopped in 1982. Waste brine is now dumped into the Pacific Ocean via concrete channel.
Power generation rose 1975 – 1982 as new wells and units were brought online. Since then, it has declined due to reservoir drawdown and suspension of reinjection. The level of pressure drawdown is dependent on well depth into feed zones, with shallow wells showing little to no declines, and deeper ones showing much larger drawdowns.
Local faults and fractures control the mass and heat flow in the field. The thermal anomaly is controlled by major faults. Cold water recharge of the system takes place primarily through the fracture system.
Eruptions
Hydrothermal eruptions in the area of the Ahuachapan geothermal field took place before recorded history. Even though these were not well documented, geologic evidence suggests they took place many times in the past. Agua Shuca likely erupted in 1868, with hydrothermal eruptions at EL Playon and La Labor. There is a third example near Apastepequa lagoon near San Vincente. There are no recent studies that have defined the frequency of these events.
A small phreatic blast took place in 1990 in the village of El Barro, the SW portion of the Ahuachapan Geothermal Field, the Agua Shuca (dirty water) fumarole area, some 2-3 m in diameter with fumaroles, mud pots, and altered ground erupted violently producing a blast of wind, stones and boiling water. The blast destroyed several small dwellings within a 100 m radius. 14 were immediately killed with another 21 injured. The explosion was directional and lasted some 20 seconds according to survivors. Two days later, a quiet pond (heated but not boiling) formed in the explosion crater and the surrounding area was covered with mud.
The Ahuchapan geothermal field was in operation for 15 years before the explosion. It is located in a highly fractured zone at the N base of Laguna Verde volcano. Agua Shuca is located on a known fault some 2 km S of the main power plant. It is also outside the geothermal field drawdown area. A test well drilled into Agua Shuca was not developed due to low permeability of the rocks. A large landslide in 1981 ½ km N of Agua Shuca took place shortly after a high-pressure reservoir test at the maximum power extraction rate. That test was stopped due to a dramatic pressure decline.
A more recent report of the Oct 1990 event changed the total eruption duration to 10 – 20 minutes. The eruption was described as a directional geyser, produced a crater 30 m in diameter, 15 m deep. Blast effects ended abruptly around 130 m from the crater. Local damage was downed trees, limbs, neighboring roofs and walls. Deposits were thickest to the N, 1 m thick near the crater and 0.3 m some 20 m from the rim. Total deaths ended up at 26 with another 13 injured not making it out of the hospitals.
Tectonics
El Salvador is located on the Caribbean Plate close to the western and northern margins. To the N, the Caribbean Plate slides along the North American Plate along the Cayman-Motagua fault. In continental Central America, that line is the complex Motagua fault system, about 100 km N of the N border of El Salvador. Movement along this fault system is around 2 cm/yr.
To the W, the Cocos Plate is subducting under the Caribbean Plate along the Middle America Trench. Rate of subduction is around 7.8 cm/yr. The subduction along this portion is oblique with a compression component and a strike-slip component parallel to the volcanic front associated with NW transport of a fore-arc sliver of the Caribbean Plate. The strike-slip component is around 1.4 cm/yr generally parallel with the southern coastline. This gives two earthquake types in the region – megathrust due to the subduction component and strike-slip earthquakes related to the NW component.
This E-W movement translates into a fault system, the El Salvador Fault Zone (ESFZ) which meets the Motagua fault system W of El Salvador’s western border. Over the course of this interaction, a series of N-S trending grabens have formed bounding regions of nearly E-W extension. Historical and recorded seismicity suggest extensional rates along these grabens is 0.8 cm/yr. This fault zone is laterally discontinuous, divided into large segments.
The Cuyanausul area has complex tectonics. It is located near the western end of the Motagua and El Salvador fault systems, near N-S grabens associated with intraplate deformation of the Caribbean Plate. The area has recent faults and graben in the NE part of the region likely caused by E-W extension mostly hidden below the volcanic structures. Active volcanism is mostly absent in the pull-apart basins.
The following video takes a tour though Parque Nacional El Impble, Ahuachapan, El Salvador. Everything you see is associated with volcanic activity.
Conclusions
The Apaneca Range is an example of a complex post-caldera line of volcanoes along the El Salvador volcanic front. Most recent activity in this region has taken place at Santa Ana (Ilamatepec) – Izalco volcanic complex to the E of the range. There is still magma available, demonstrated by active hydrothermal systems, but most of the activity along the range following caldera formation has been smaller complex volcanoes with the occasional monogenetic eruption. Residual magma drives a vigorous hydrothermal system in the region. As of this writing, it would seem that future activity in the Apeneca Range would be overshadowed (at least for now) by Santa Ana – Izalco.
Additional information
Smithsonian GVP – Apaneca Range
Volcano Discovery – Apaneca Range
Geology of the Ahuachapan – Chipilapa, El Salvador, CA geothermal zone, Partida, et al
The Ahuachapan geothermal field, El Salvador: Reservoir analysis, USDOE, OSTI.GOV, 1989
Geochemical evolution of the Ahuachapan geothermal field, El Salvador, CA, FEM Lopez, 1994
Hydrothermal eruptions in El Salvador: A review, Handal & Barrios, Jan 2004
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