The Aldama volcanic field is located near the E coast of Mexico, some 100 km N of Tampico, and in turn, N of the Trans Mexican Volcano Belt.  It was active 8 Ma – 240 ka and covers some 1,000 km2.  What is interesting about an intraplate basalt volcanic field inactive for nearly a quarter of a million years?

The unique feature of this field is heat and volcanic gasses from magma powering it also powers an active hydrothermal system that created a network of caves and sinkholes (cenotes) in a thick limestone layer along the coast of the Gulf of Mexico, Sistema Zacaton.  El Zacaton is the deepest of these at over 350 m.  Cave formation in limestone is a common occurrence.  Cave formation powered by corrosive volcanically heated water (volcanogenic karstification) is not all that common, but has created the two deepest underwater cave systems in the world, Sistema Zacaton in Mexico and Pozzo del Merro in Italy. 

The closest town is Aldama, with 22,000 residents, some 30 km S of the center of the field.  The closest large city is Tampico, some 103 km S, with a population of over 930,000 in its municipality.

The Sierra de Tamaulipas is located 39 km W of the Aldama volcanic field.  It is somewhat higher than Aldama, 1,260 m compared to 600 m of Aldama.  It is also farther inland.  As such, weather here is a bit cooler due to altitude with temperatures ranging between 25° – 13° C.  Average rainfall is just under 96 cm/yr. 

The Sierra de Tamaulipas is described as a sky island, an isolated group of mountains measuring 110 x 64 km separated from the neighboring Sierra Madre Oriental, high enough to have cooler and wetter conditions than surrounding lands at lower elevations.  Lower elevations are covered with a tropical thorn forest.  Tropical deciduous forests are found at mid elevations.  Various scrub vegetation is found at higher elevations mixed with low thickets, savannah and pine oak forests at high elevations. 

Volcanic monitoring in Mexico is done by the Centro Nacional de Prevencion de Desastres (CE – NAPRED).  Responsibility for monitoring volcanoes within each Mexican state is farmed out to local universities.  The two states in this region are Tamaulipas and Chihuahua.  There are no active webcams for either Aldama town or the volcanic field.  The field is not actively monitored.

Region

The Eastern Mexican Alkaline Province (EMAP) is a belt stretching along the E coast of Mexico from the E end of the Trans Mexican Volcanic Belt in the S to the Rio Grande Rift in the N, a distance of nearly 2,000 km.  Volcanism in the belt is primarily intraplate, erupting alkaline magmas.  Volcanic fields are isolated, and not recently active.  The three main fields of interest in this post are the Sierra de San Carlos, Sierra de Tamaulipas and Villa Aldama volcanic field. 

The region also has a thick layer of limestone laid down during the Cretaceous.  Action of corrosive waters of the volcanic hydrothermal systems fed by meteoric waters in this limestone layer created an extensive system of caves and cenotes (sinkholes) in the region around the fields named earlier.  All distances will be measured from the Smithsonian GVP location listed for the Aldama volcanic field.  

Sierra de San Carlos

The Sierra de San Carlos – Cruillas magmatic complex is the farthest N of these fields, 160 km NW from Aldama.  It is roughly 60 km in diameter, covering 3,600 km2.  The field tops out at 1,800 m.  The central part of the complex are unerupted plutonic bodies, dikes and sills that intruded into the limestones many tens of millions of years ago.  Oldest volcanic rocks are 29 Ma, toward the center of the complex.  Youngest volcanism is located on the periphery, with lava flows and necks visible as eroded outcrops. 

Bernal de Horcasitas

El Bernal is an 820 m eroded volcanic neck located some 74 km SW from Aldama volcanic field.  It is quite old at 28 Ma.  The cone has been completely eroded away, leaving the vertical neck which is popular with climbers.  The neck is used as the symbol of the municipality of Villa Gonzalez and as a state icon for Tamaulipas.  It is part of the state coat of arms and used on commemorative coins.   It is a protected natural monument due to its prominence and remains of indigenous settlements found on its lower slopes. 

Sierra de Tamaulipas

The Sierra de Tamaulipas is a volcanic semi-tropical mountain range.  The field covers at least 3,300 km2, topping out at 1,260 m, some 39 km W of Aldama.  Once again, magmas intruded into and through the thick limestone layer 34 – 5 Ma.  The oldest intrusions toward the center of the complex cooled into granite plutons.  Mantle xenoliths are found in three locations, Mesa La Sandia, Mesa San Fernando and Cerro La Clementine. 

Younger basaltic lavas are found in the W part surrounded by intrusive bodies.  These are concentrated in the Llera region, and named in many places as a separate Llera volcanic field.  Some of these basalts cover up to 100 km2 and date 5.0 – 2.4 Ma.  The Llera volcanic field is the closest field to the AVF.  Several volcanic neck structures, locally called Bernales, are found across the region. 

There are isolated volcanic outcrops S of Sierra de Tamaulipas on the coastal plain near Ebano, San Luis Potosi.  These are eroded, small volcanic necks.  Height ranging 30 – 140m.  Diameters range from 100 – 1,000 m.  The SW margin of the Sierra de Tamaulipas has several broad lava plateaus and volcanic necks of erupted basalts.  These flows extruded over the limestone basement and fluvial layers.  Magma is thought to erupt through regional faults.  There are some flows to the W. 

Villa Aldama volcanic field

The Aldama volcanic field (AVF) is located on the Gulf of Mexico coastal plan, on the SE Sierra de Tamaulipas margin.  It is irregular in shape, covering some 1,000 km2 surrounding Aldama town.  The field is bounded by the Carrizal and Berberena Rivers.  The field contains several volcanic structures created by medium magnitude explosive activity.  The field erupted alkaline basalts and some trachybasalts. 

The AVF erupted on the SE margin of the Tamaulipas Anticline.  It is the most extensive volcanic complex of the Eastern Mexican Alkaline Province.  Most of the activity took place 8.6 Ma – 240 ka, erupting juvenile basalts and trachytes.  The most juvenile lavas are in the W part of the field.  Oldest rocks in the field are alkaline basalts in S and N parts of the field.  These were followed by subalkaline basalts and hawaities.  The most evolved rocks are the youngest, in the center and E part of the field.  These formed cones and mesas.

Volcanic products were deposited over Cretaceous limestones and marls.  There are some isolated volcanic necks in the S and W of the field that are not chemically related to the field.  One of these is the previously mentioned El Bernal (28 Ma), Cerro Nopal (23.5 Ma) and Cerro Auza (20 Ma). 

NE Mexico is seismically active (though the AVF is not), which has allowed seismic study of the area.  Tectonic motion in this part of Mexico appears to have an extensional component parallel to the Rio Grande Rift to the N as it extends SE into Mexico.      

Magmas in the AVF and the greater EMAP do not show any subduction signature.  They are thought to be due to a sudden increase in mangle temperatures due to slab detachment, opening a slab window allowing ascent of hot mantle fluids.  This may explain the sharp onset of volcanism in the region 7.5 Ma.  Alternately, the mantle fluids may have been created by slab rollback.  The discussion on magma sources continues with no agreed upon explanation as yet.

The Aldama volcanic field erupted multiple notable volcanic structures including lava flows, cinder cones, tuff rings, maars and simple cones.  Given the humid subtropical conditions in the field, the lack of weathering and erosion indicate they are relatively young. 

Lava flows are widespread in the AVF.  There are some plateaus (mesas) covering less than 15 km2.  Most of the flows are alkali basalts.  The Ejido Santa Maria flow to the W flowed N-S.  It is 5 m thick.  The Cerro Laguna Seca flow to the E of this is thicker at 10 m, also flowing N-S.  Two other flows to the NE are 5 m thick.  There are three flows in the S part of the field.  Rancho San Jose is 7 km long, flowing NE.  Two other flows toward the S cover 8 km2.  The central part of the field near Aldama town along the Tigre River shows well developed jointing. 

Evolved flows are concentrated in the central part of the field.  They are typically 10 m thick, oriented N-S.  One of them is 20 m thick and has been mined for blocks for port facilities on the Gulf coast.  There is a flow in the E part of the field 5 m thick cut by a local road.

There are two small cinder cones some 7 km S of Aldama town.  They are constructed of layers of lapilli, bombs, ashes and lava flows.  Basement rock are earlier lava flows.  Local faults cut the pyroclastic material.  There are also a pair of tuff rings in the NW part of the field.  They show interaction of magma with shallow groundwater.  They are small, less than 50 m high.  Bombs are up to a meter in diameter. 

There are maars (locally known as jagueyes) in the central part of the field.  One is 700 m wide, 30 m deep.  The other is 950 m wide and 20 m deep.  Outcrops along the rims are pyroclastics, ash and lapilli.  The highest structures in the field are cones in its central part.  El Cautivo is the highest point of the field at 670 m.  The rest range 300 – 400 m.   

Volcanogenic karstification

Before we discuss this process, a few definitions are in order.  First, karst is a limestone landscape where dissolving bedrock created sinkholes, sinking / disappearing streams, caves, and springs.  Karst is associated with limestones, gypsum and marble.  Water enters the ground through cracks, fractures and holes dissolved in the bedrock.  It travels underground, sometime dissolving the rock, sometimes depositing rock, and is discharged from springs, often at cave entrances.

Travertine is a form of sedimentary rock, usually limestone deposited around mineral springs, particularly hot springs.  It is formed by the rapid precipitation of calcium carbonate.  In caves, it forms the various structures associated with wet caves, like stalactites, stalagmites.  In some places like Italy, it is used as a building material. 

The process of volcanogenic karstification takes place when volcanic heated waters, often with gasses dissolved, are active in formation of caves, sinkholes, and other structures in karst.  The dissolved gasses create acids in the water like carbonic and sulfuric acid.  The process is usually accelerated, sometimes significantly with the addition of heat and dissolved acids.   

This takes place when a groundwater system in soluble rocks interacts with volcanic activity below the surface.  It creates conditions for intense cave formation in focused geographic areas, often very deep.  It is possible there is no recognizable surface feature, which makes identification difficult.

Geologic processes involved in cave development require sufficient thickness of carbonate strata, preferential groundwater paths (fractures), some continuing flux of groundwater.  For volcanically powered formation, you need volcanic activity that provides heat and volcanic gasses (H, CO2, H2S).  Note that the first three items on this list are common to all cave formation.  They also require some form of acid in the underground water supply.

Sistema Zacaton (Zacaton karstic field)

Sistema Zacaton is located between Sierra de Tamaulipas and the Villa Aldama volcanic field.  It is a deep, phreatic karst system formed over the last 5 Ma.  The system is currently active, though the most significant cave formation took place during periods of active eruptions.  The primary features of Sistema Zacaton are large, water-filled sinkholes (cenotes).  They are oriented along regional fractures present for tens of Ma.  The name Zacaton is derived from free floating islands of zacate grass on the surface of the water in the cenotes.  Limestone in NE Mexico is quite thick at 2,000 m and quite old 130 – 80 Ma. 

Other active volcanically driven karst systems are found in the Turkish Obruks, Mammoth Hot Springs in Wyoming, and Cuevra de Villa Luz in Mexico.  Two other suspected active volcanically driven karst systems are the Rhodope Mountain (Mandan Chamber) in Bulgaria and the Western Edwards aquifer, in W Texas. 

There are at least 18 karst features within Sistema Zacaton, all crammed within 4 km of one another.  These are diverse including kilometers long cave passages, broad travertine flows, collapse features, travertine filled sinkholes, deep phreatic shafts, horizontal phreatic conduits. spring flow travertines and numerous minor karst features.  During the period of active volcanism, thick deposits of travertine precipitated from springs discharging hydrothermal, mineralized water. 

8 of the sinkholes have travertine floors.  The closed sinkholes were once open, deep, water filled sinkholes.  Travertine floors exist at current water levels.  They are likely at a lower, glacial water level.  Cenote Verde has a flat floor at 45 m depth that is thought to be a travertine floor rather than a collapse feature.  It is open and water filled to an unknown depth below this floor.  El Zacaton is the deepest of the sinkholes, explored in a free dive to 284 m below the water table in 1995, the world record SCUBA dive.  It has been robotically explored to 329 m. 

Waterflow from the Sierra de Tamaulipas moves down through permeable limestone until reaching volcanic intrusions along a fracture zone.  This is a groundwater mixing zone where meteoric water from Sierra de Tamaulipas comes into contact with hydrothermal, mineralized water.  Water from the cenotes have a similar calcium carbonate signature to most other karst waters found worldwide.  They do have high levels of CO2 dissolved in the waters, especially at depth.  These CO2 levels typically drive increased acidity. 

Over the last 200 k, the karstification process in the Sistema Zacaton area took place in four stages.  First was volcanic activity at Aldama volcanic field, which provided heat and volcanic gasses that accelerated the process along fracture zones and built travertine terraces.  Mammoth fossils were preserved in travertine during this period.  The process continued as water levels dropped.  Shafts opened through newly deposited travertines as CO2 outgassed from thermal waters.  Additional shafts opened in the next phase, as well as lateral phreatic caves.  Springs discharged hydrothermal waters along the margins of older travertines under high water table conditions.  The final stage saw water levels falling to current levels, final stage travertines deposited by water flowing from shallow lateral caves forming typical cave travertines.  Travertine caps formed over some open cenotes. 

Biomats coat the walls of three main cenotes, Zacaton, Caracol and la Pilita.  They are thin, purple-red blankets.  They cover dissolved calcium carbonate rock etched into fingers or a soup as much as a meter thick.  Bacteria oxidizes H2S (hydrogen sulfide) into H2SO4 (sulfuric acid).  These types of bacteria are often found at groundwater sites where they use chemical gradients to capture energy.  The bacteria oxidizing sulfur also accelerates the formation of sinkholes and caves as it converts it into sulfuric acid.

Tectonics

Tectonics of this part of Mexico are tied to the Eastern Mexican Alkaline Province (EMAP) or Eastern Alkaline Province (EAP), that stretches nearly 2,000 from the Rio Grande S along the Gulf Coast of Mexico to the Trans Mexican Volcanic Belt (TMVB) SE of Veracruz. 

There are at least six recognized volcanic fields in the EMAP.  Most of these are located in the vicinity of Veracruz on the E end of the TMVB.  A few are scattered to the N along the EMAP.  Volcanic activity is therefore treated in two parts, activity associated with propagation of activity W-E through the TMVB and propagation of activity from N-S along the EMAP.

Volcanic activity in Veracruz state took place in three episodes, 15 – 11 Ma, 7.5 – 3.0 Ma, and 2.5 Ma – present.  The first and third pulses are related to propagation of flat slab subduction E causing activity in the TMVB.  The second pulse is more complex and related to possible (and as yet unproven) extension or due to a transient thermal anomaly and mantle melts triggered by tearing and detachment of the subducted slab,

Volcanic activity between the Veracruz fields and the US – Mexico border were originally tied to SE propagation of an extensional regime related to S propagation of the Rio Grande Rift in Texas and NM.  There is considerable argument whether this extension actually exists.  There are faults running parallel to the W end of the Gulf of Mexico and the Sierra Madre Oriental that do provide zones of weakness for intraplate magmas to reach the surface.  Activity near the TMVB is far more vigorous and consistent than activity along the remainder of the EMAP to the N.

Conclusions

The Aldama volcanic field is yet another example of a recently active intraplate basaltic volcanic field.  Eruptive forms are consistent with other basaltic fields found worldwide.  The most interesting part of this field is residual volcanic heat and gasses powering intense karst forming activity in the thick limestone layer the system erupted through.  The subtropical location provides sufficient water to keep the hydrothermal system active, though sea level changes due to glacial cycles changed water levels in the various cave systems significantly over time. 

Additional information

Geology, geochronology and tectonic setting of late Cenozoic volcanism along the southwestern Gulf of Mexico:  The Eastern Alkaline Province revisited, Ferrari, et al, Sept 2005

Aldama volcanic field, Smithsonian GVP

Constraints on the origin of the Cenozoic intraplate Aldama volcanic field, Tamaulipas, NE Mexico, Rivera-Garcia, et al, Sept 2023

Geology, Local geologic history of Sistema Zacaton, University of Texas

The Sam Marcos fault:  A Jurassic multireactivated basement structure in northeastern Mexico, Chavez-Cabello, et al, Jan 2007

Arsenic mineral in volcanic tuff, a source of arsenic anomaly in groundwater:  City of Chihuahua, Mexico, Ren, et al, Feb 2022

Neogene – quaternary continental margin volcanism:  a perspective from Mexico, edited by Siebe, et al, 2006

Structural transect of the southern Chihuahua fold belt between Ojinga and Aldama, Chihuahua, Mexico, PH Hennings, Dec 1994

Volcanogenic karstification: implications of this hypogene process

Understanding Zacaton:  exploration and initial interpretation of the world’s deepest known preatic sinkhole and related karst features southern Tamaulpas, Mexico  M Gary

Volcanogenic karstification of Sistema Zacaton, Mexico, Gary & Sharp, 2006

Advances in hypogene karst studies National Cave & Carst Management Symposia NCKRI Symposia 1, National Cave and Karst Research Institute, University of South Florida, 2022

Geochronology and geochemistry of the El Salvador plutonic complex (Sierra de Tamaulipa, NE Mexico):  cenozic tectonic implications of the eastern Mexican alkaline province, Pena-Alonso, et al, Jan 2024

Volcanogenic karstification of Sistema Zacaton, Mexico, Gary & Sharp, 2006

Sistema Zacaton:  volcanically controlled hypogenic karst, Tamaulipas, Mexico, M Gary, Aug 2017

Late Cenozoic intraplate-type volcanism in central and northern Mexico:  a review, Aranda-Gomex, et al, Jan 2007

AMCS Bulletin 19 / SMES Boletin7, 2006 Abstracts, pp 140 

Mantle xenoliths and their host magmas in the Eastern Alkaline Province, Northeast Mexico, Trevino-Cazares, et al, Dec 2005