The Sierra la Primavera volcanic complex is located just to the west of Guadalajara, Mexico. Activity started around 120 ka and proceeded to a caldera forming eruption 95 ka. A lake filled the caldera for 10 – 20 ka following the eruption. Dome formation within the caldera and resurgence started immediately following the caldera-forming eruption. Dome formation shifted to the southern margin of the caldera and continued 60 – 30 ka, with the most recent eruption 30 ka.
There is a vigorous hydrothermal system associated with the caldera, with geothermal exploration underway for nearly 40 years. A generation project in the 1980s was cancelled due to environmental concerns. There is a second attempt underway.
The Guadalajara metropolitan area has a population of around 5.3 million, the third largest metropolitan area in Mexico. It has the second most dense population in Mexico. Guadalajara has a strong and diversified economy, ranked the most business friendly city in Latin America in 2007. Commerce and tourism are about 60% of the economic activity. Industry is the rest, including tech, textiles, shoes. Most of the industrial output is sold domestically. The rest is sold N in the US. Tech is large enough that Guadalajara is referred to as the Silicon Valley of Mexico.
Climate is a humid subtropical climate with dry, warm winters and hot, wet summers. High altitude (1,566 m, similar to Denver, Colorado) also drives the weather. Average temperature yearly range 28° – 14° C, with record high of 41° C and record low of -1.5° C. Average rainfall is just over 100 cm.
The volcanic area associated with the volcanic complex has been overgrown and is now known as the Primavera Forest, El Bosque de la Primavera, or “Guadalajara’s lungs.” It is quite popular with residents and tourists and periodically fends off development attempts. The first of these in 1980 led to a popular reaction that eventually got the entire area declared as a protected area / wildlife refuge.
Hikers, visitors and campers enjoy a scenic region with a variety of birds and animals. The Rio Caliente is a hot spring fed river that serves as a natural jacuzzi. Some of the feeder streams are even hotter than the main stem. Volcanic features are visible in canyons with huge blocks of pumice and obsidian beds. Volcanic hydrothermal systems are notoriously variable. Those of La Primavera are no different with numerous extinct hot springs and fumaroles now producing cold water.
Guadalajara is surrounded by volcanic structures in addition to La Primavera. There is a local day trip that brings people to the caldera / dome field, a local cinder cone (Cerro del Cuatro) and a smallish stratovolcano (Tequila volcano). Extreme sports enthusiasts turned the 130 km day trip into a competitive 120 km, 35-hour run, the Trivolcano Challenge.
The Selva Negra Nature Reserve is located some 32 km SW of Guadalajara. It is notable for extensive deposits of high-grade obsidian. Pre-Hispanic natives used obsidian widely, with hundreds of mines and workshops. The reserve is littered with piles of broken, discarded knives, arrowheads, and other artifacts. There is so much of this stuff it is used in places to line hiking trails.
Volcanic monitoring and response in Mexico is the responsibility of Centro Nacional de Prevencion de Desastres (CENAPRED). There is no volcano observatory specifically designated to La Primavera, but there is a national and regional seismic network that would pick up future activity. There are multiple dedicated webcams in Guadalajara.
Region
We wrote about the Trans Mexican Volcanic Belt in multiple posts over the years. These have included San Juan, El Estribo, Michoacan – Guanajuato Volcanic Field, Paricutin, Nevado de Toluca, and Popocatepetl.
There are at least five volcanic features surrounding La Primavera, several of them located in and immediately bordering Guadalajara itself. Most of these volcanic centers are closer than 50 km from the center of La Primavera. We will work generally clockwise starting with Tequila at the 10:00 location
Tequila (El Volcano de Tequila)
Tequila is an extinct andesitic – dacitic stratovolcano located near Tequila, Jalisco. It tops out at 2,920 m. The crater is breached to the NE. It is located some 36 km from the center of the El Primavera caldera. It is surrounded by rhyolitic domes and obsidian flows dating 900 – 210 ka. There is a prominent summit spine that dates 210 ka, the last known eruption from the summit. The most recent activity took place on the flanks, erupting andesitic lava flows, creating a chain of domes on the SE flank. There are at least a dozen cinder cones younger than the main edifice along a local fault S and SW of the volcano. They are no younger than 40 ka. While there are active springs on the volcano and in its crater, they are all cold, indicating no active hydrothermal system.
The region surrounding the volcano is famous for two industries based on volcanic products. First of these was ancient use of obsidian from the obsidian flows of the volcano. Obsidian was valuable in antiquity and was the center of the first export industry in Mesoamerica. It was bartered and widely worked. The word “tequila” is derived from “a place where you cut” as the proliferation of workshops left significant obsidian glass flakes as waste. The second and much more recent industry is farming of agave, which is harvested and turned into tequila, the alcoholic drink.
Northern Guadalajara Mesa
The Northern Guadalajara Mesa is a cluster of rhyolitic lava domes on the northern end of the Guadalajara Mesa, some 36 km NE of La Primavera caldera, 20 km N of Guadalajara. The highest dome is 1,820 m Cerro Jacal de Piedra, S of Cerro el Chicharron. These features are quite old at 1,400 ka. There are a pair of neighboring domes along a local fault to the E.
Cerro del Cuatro
Cerro del Cuatro is a well-defined cinder cone nearly in the center of Guadalajara, 17 km E of La Primavera. It is completely surrounded by the city, rising above the city, making it a favorite for mountain bikers. The cone is red scoria.
Southern Guadalajara
Southern Guadalajara is a chain of at least nine basaltic andesitic volcanic centers constructed along a NW – SE line through the southern portion of Guadalajara. These range from small cinder cones like Cerro San Bartolo, to larger composite volcanoes. The westernmost cones are stretched along southern Guadalajara, some of them have city streets and buildings. The largest volcanoes are at the SE end of the chain. These include Cerro el Molino and Cerro el Papanton de Juanacatlan. These volcanoes are difficult to find with Google Maps satellite view. The following three volcanic centers, Santa Cruz, Cerro de la Cruz / El Calvario and Cerro de Mazatepec are depicted in the 1980 Mahood paper as located on the NW end of this chain. I have not seen this connection made anywhere else.
Volcan Santa Cruz is not listed in any of the volcano databases. It is well visible in Google Maps satellite view. It is located 17 km SSE of El Primavera, on the SE outskirts of Guadalajara. It has a visible, well-defined crater, and is completely forested. One of the problems in Central and South America is the proliferation of mountains, towns, cities with identical names. Santa Cruz is one of these.
The Cerro de la Cruz / El Calvario is a cinder cone SSW of Guadalajara. It is breached to the E, with almost a quarter of the cone gone. It is located 23 km SE from La Primavera, 22 km SW from the center of Guadalajara. It is very close to Santa Cruz, only 7 km SE. The cone is forested and not built up.
El Cerro de Mazatepec is a scoria cone located 17 km SSW from El Primavera, 32 km SW from the center of Guadalajara. The Acatlan Volcanic Field is located to the SW of this cinder cone.
Acatlan Volcanic Field
The Acatlan Volcanic Field is located some 23 km due S from the center of La Primavera. It is an earlier caldera that La Primavera, having erupted some 650 ka. The eruption was massive, a VEI 7.2 ejecting at least 150 km3 of material (Acatlan ignimbrite) covering over 300 km2 with 80 m thick layer of ash and pyroclastics. Following the main eruption, resurgent activity built rhyolitic and dacitic lava domes, andesitic cinder cones, and lava flows. There does not appear to be any residual hydrothermal activity in the region.
The ignimbrite was erupted in a series of successive pulses from collapse of the eruption column that filled surrounding valleys, flattened the surrounding terrain, and spread wider once the surrounding terrain was flattened by deposits of the newly erupted material. The eruption appears to have proceeded from a compositionally zoned magma chamber that first produced white or pink rhyolitic pumice base topped by a black andesite. While the caldera proper is not well defined, its shape may be inferred by the location of domes emplaced by post-caldera activity following caldera formation.
Volcanic domes of Ahuisculco
The southern part of the area is now encompassed in the Selva Negra Nature Reserve, which serves as a wildlife corridor between Bosque la Primavera and Sierra de Quila. It has large obsidian deposits. It is located mostly to the W of La Primavera, some 24 km SW from the center of the caldera. Obsidian flows here do not appear to be connected to anything associated with activity from Sierra La Primavera.
La Primavera Caldera
La Primavera is a 95 ka old, 11 km diameter, rhyolitic caldera located some 50 km N of the triple junction between the N-S Colima, E-W Chapala and NW-SE Tepic Zacoalco grabens. It is located within the Tepic – Zacoalco graben along with neighboring volcanic centers like the Tequila and Ceboruco volcanoes (NW), lavas and cinder cones of the Southern Guadalajara volcanic chain, and the ancient ignimbrites of the Sierra Madre Occidental to the E. The caldera sits on basement rocks of Cretaceous granites (3 km deep), andesitic lavas (51 Ma), rhyolitic ignimbrites, basalts, andesitic basalts of the San Cristobal group (12 Ma), rhyolite and ashflow deposits of the Guadalajara group (7 – 3 Ma), and an andesitic lava / ash flow tuff dated 1.6 Ma.
Initial activity was effusive, erupting andesitic to rhyolitic lavas 145 – 100 ka. The caldera forming eruption took place some 95 ka. The VEI 6.9 eruption ejected at least 90 km3 of material, the Tala Tuff. The Tala Tuff was erupted in three pulses. The first and largest pulse sent an ignimbrite outside the caldera confines. The collapse took place during the eruption likely between phases I and II, creating a shallow caldera. Phases II and III of the eruption did not send ignimbrites outside the ring fractures. It appears this eruption proceeded from a single conduit within the caldera, as there is no evidence of eruptions from the caldera ring fractures.
Water quickly filled the caldera and persisted for at least 20 ka. Resurgent activity immediately followed the eruption, producing a few domes in the middle of the caldera and domes along the northern and southern ring fractures. Eventually, resurgent activity and intra-caldera eruptions ended the lake. Uplift disconnected with resurgent activity continued for some time after caldera formation.
The Giant Pumice eruption 86 ka, and Older ring domes were erupted soon after caldera formation. The next round of dome extrusion was the Younger ring domes around 75 ka, followed by the Southern arc lavas (exterior eruptive centers) 60 – 25 ka. Eruptive products have been primarily rhyolites from caldera formation to present.
There are at least 14 identified pyroclastic units younger than the Giant Pumice, the best identified base unit of the stack. It is a white to grey continuous bed 4 – 12 m thick dating 86 ka. The pyroclastic units were erupted during dome and lava formation. They are interlayered with lahar deposits. The youngest pyroclastic unit of this stack dates somewhere 45 – 26 ka.
Post-caldera explosive eruptions took place from four volcanic centers. Inside the caldera, the Nejahuete composite dome was the source for most, if not all explosive eruptions 94 – 71 ka. Outside the caldera to the S, explosive activity took place at San Miguel, Planillas and Tajo 92 – 27 ka, generally working its way W – E over that period.
Intra Caldera Activity
Several domes were emplaced inside the caldera and on the northern and southern parts of the caldera rim fault after the caldera collapse. The first of these were lava flows that eventually built the Nejahuete composite dome starting some 93.5 ka, the start of caldera resurgence. Around the same time, other eruptions on the S ring fault started erupting lavas that built the Pilas and Tule domes. At the time, the caldera was filled with water leaving layers of lake sediments.
The Nejahuete dome reactivated 86.4 ka. This eruption produced large pumice blocks that floated and sank to form the Giant Pumice, the best stratigraphic marker in the caldera. These (up to 6 m diameter) blocks were dispersed throughout the lake and on top of the Pilas dome. Sedimentation continued following the eruption.
Nejahuete reactivated once again with a series of explosive eruptions that produced pyroclastic fall deposits on the lake, eventually on top of the sediments. The lake was nearly dry at the time of the eruption, due to either uplift or pyroclastic deposits filling the lake. The eruption continued with pyroclastic flows and ashfall deposits inside the caldera, finishing with an obsidian flow on top the Nejahuete dome some 83 ka.
The Alto dome also formed at this time. There are landslides, lahars and mudflow deposits dated about the same time (84 ka). A new extrusion of magma on the southern rim of the caldera built the Extahuatonte and Old Culebreado domes. The lake reestablished itself for the next 6 ka. A new eruption of Nejahuete took place 78 ka. This one was explosive with plenty of water involvement, producing hydromagmatic explosions, wet, dilute pyroclastic flows between lake sediment beds. The eruption increased in intensity with multiple wet, dilute pyroclastic flows and lithic-rich ashfalls due to conduit erosion.
Around 77 ka, an effusive eruption vented the Culebreado dome on the S rim of the caldera. Resurgence ended with extrusion of the Cerrito Colorado lava SW of Nejahuete domes 76 ka. There was intense remobilization of erupted materials and lake sediments suggesting the lake was disappearing.
The Pedernal lower lavas were erupted 73 ka, after a couple thousand years of repose. This was followed by extrusion of the Cuesta dome outside the SE caldera rim. Nejahuete dome reactivated around the same time with a sub-Plinian eruption and several grey, dense pyroclastic flows inside the caldera. The eruption progressed to a stable eruptive column. The last reactivation of this dome was 71.5 ka.
At some later date, San Miguel volcanic center S of the caldera erupted with a sub-Plinian column. The upper lava of the Pedernal dome was extruded along the W caldera rim. Existing deposits were almost completely remobilized by lahars and mudflows within the caldera, likely due to significant rains around 66 ka, a known global rain event. This event lasted around 2 ka.
Extra Caldera Activity
Activity outside the caldera took place 92 – 25 ka. It was mostly concentrated along a NW-SE fault S of the caldera ring fault. This created the San Miguel, Planillas and Tajo volcanic centers, the Puerta and Animas domes, and the Llano Grande lava. It was simultaneous with intra caldera activity centered at the Nejahuete dome.
The San Miguel volcanic center, Animas and Puerta domes were active 92 – 60 ka. Initial activity was eruption of lavas that created the San Miguel volcanic center. Effusive activity on the northern flank of San Miguel created the Animas and La Puerta domes 89 ka. The southern flank of San Miguel was destroyed by a flank collapse and debris avalanche to the S somewhere 92 – 84 ka. Subsequent lava flows were erupted inside the scar 84 ka. The center was quiet until 71 – 69 ka when a sub-Plinian eruption took place. This was a highly variable eruptive column interrupted several times by hydromagmatic explosions, pyroclastic flows, and pumice falls. Eruption of an obsidian dike and lava flow closely followed. The center was quiet until the next eruptive cycle started with a sub-Plinian eruption 60 ka that dispersed a thick pumice fall to the N. There were several explosive eruptions during this sequence ending with the Llano Grande lava to the SE.
The Planillas volcanic center was active at least 71 – 45 ka. The original lavas are undated. It is located 7 km to the E of San Miguel. Initial lavas ended with an obsidian flow. They were covered with pyroclastics from San Miguel erupted 71 – 69 ka. Activity resumed somewhere 69 – 60 ka with an explosive eruption column that started small and intensified to a stable large one. This eruption ended with column collapse, pyroclastic flows, ending with hydromagmatic explosions. A few hundred to thousands of years later, the volcano erupted again 60 ka. This explosive eruption dispersed pumice falls. The next explosive eruption took place 59 ka with a Plinian eruption. The eruption also produced lavas, pumices, and pyroclastic flows. It ended with a sub-Plinian column that was variable in strength, producing ash falls, wet pyroclastic flows and hydromagmatic explosions. The S flank of the volcano collapsed in a debris avalanche to the S. The volcano reactivated 58 ka with another sub-Plinian eruption that proceeded much like the previous sub-Plinian / Plinian eruptions. This one ended with column collapse and thick, dense pyroclastic flow. The last explosive eruption took place 45 ka with a Plinian eruption that produced a thick pumice fall, pyroclastic flows, ending with several lava flows that filled the amphitheater of the flank collapse.
The Tajo volcanic complex is the newest of the volcanic centers to the S of the main caldera. This one is located 5.5 km E of Planillas along a NW-SE fault line. Like the other centers, activity here began with undated lava flows. The earliest known explosive eruption from Tajo was 45 ka, which produced a sub-Plinian column, pyroclastic deposits, and pumice falls. The most recent activity here took place 26 ka with another effusive eruption of lavas N and S.
Geothermal
The presence of an active hydrothermal system made La Primavera a target of interest for geothermal development for over 40 years. There are multiple papers analyzing the water flow beneath and through the caldera published over that period. Hot springs occur at low elevations and at the margins of the complex. Steam vents are located at higher elevations. Hot water from the center appears to migrate laterally, cooling by boiling and conducting to escape from hot springs at the margins of the complex.
The initial project started in the late 1970s, with 13 steam wells drilled 1980 – 1989. Phase I would have generated 25 MW. It came under intense environmental opposition, partly because the developers were not particularly friendly to the forest, and partly because of the mess created by Planta Geotermica Los Azufres in neighboring Michoacan. A presidential visit went poorly, and the project was summarily canceled, abandoned around 1989. Environmentalists claimed to have worked for years afterwards to repair the damage. But the project came back, in the form of the Bosque de Primavera / Cerritos Colorado geothermal project, which underwent final review in 2016. The project may be real this time around, with articles warning it would be reactivated in 2020 which in turn triggered full-throated environmental opposition.
Tectonics
Tectonics of this portion of Mexico is complex, driven by subduction and rifting, splitting the Jalisco block in SW Mexico off the North America Plate. Participants in this include the Rivera and Cocos Plates, both slowly subducting beneath the North American Plate. Both are remnants of the long disappeared Farallon Plate.
The subduction is generally eastward subduction of the Rivera Plate beneath the North American Plate into the Middle America Trench. Convergence rate is relatively slow, some 0.6 cm/yr in the north to 2.0 cm/yr to the south. There are occasional large megathrust and intraplate earthquakes up to M 8.2 over the last century. The most seismicity related to Rivera Plate subduction is clustered SW of Colima. Initial plate slope is shallow to 10 km and then steepens to 50° below 40 km. The subducting Rivera Plate is very young, around 10 Ma.
Regional tectonics appear to control location of Sierra La Primavera, with two triple junctions in relatively close proximity. The first of these is located some 50 km S. the triple junction between the N-S Colima, E-W Chapala and NW-SE Tepic Zacoalco grabens. These rifts bound the Jalisco Block to the S and W. The second triple junction to the SE is located south of Colima. It is involved in the opening of the Colima Rift. This rift crosses the Middle America Trench and generally defines the boundary between the Rivera and Cocos Plates. The triple junction between the Rivera Plate, the Cocos Plate and the North American Plate lies just south of the Middle America Trench. Colima sits above what may be a window between the two subducting slabs which helps explain both its vigorous activity and chemistry of erupted magmas. We covered Colima in 2015. La Primavera may be similarly situated.
Conclusions
Neighboring Guadalajara is built on the Tala Tuff, so a large future eruption would be problematic, at best. Most likely future activity is expected to be eruption of a new dome, starting with effusive activity perhaps even progressing to a sub-Plinian / Plinian eruption depending on the magma source. We know that the hydrothermal system is large and vigorous, though not extremely hot. This indicates continuing presence of a magma body under the system. Given the number of people living close to this caldera and the number of people recreating in it on a daily basis, I would think it should be monitored.
Additional information
La apasionante geologica del Area de Proteccion de Flora y Fauna La Primavera, B Dye, 2012
Tectonic setting of La Primavera Caldera, e-education.psu.edu
