New museum in Yucatán will explain Chicxulub Crater and demise of the dinosaurs

 Mexico's geography in the Press  Comments Off on New museum in Yucatán will explain Chicxulub Crater and demise of the dinosaurs
Feb 132018
 

A new museum 40 kilometers northwest of the Yucatán state capital – Mérida – is expected to open later this year to explain the nearby Chicxulub Crater, created by an asteroid impact 65 million years ago, and believed to be responsible for the demise of the dinosaurs.

The world’s largest crystals and strange microbes grow in caves in Chihuahua, Mexico

 Excerpts from Geo-Mexico, Updates to Geo-Mexico  Comments Off on The world’s largest crystals and strange microbes grow in caves in Chihuahua, Mexico
Feb 182017
 

The Naica caves, in the northern state of Chihuahua are home to the world’s largest natural crystals. The crystals are selenite, said to enhance sex drive.

The formation of the crystals caves is described in more detail below, but in February 2017, Penelope Boston, head of Nasa’s Astrobiology Institute announced that “bizarre and ancient microbes” had been found in these caves. The discovery came after nine years of work and was first announced at the American Association for the Advancement of Science conference in Boston. The life forms in the Naica caves survive by living on minerals such as iron and manganese and could be 50,000 years old. The find is still subject to peer-review and independent confirmation:

How were the crystals discovered?

Early in 2001, news emerged of a truly extraordinary discovery in caverns deep under the earth in the state of Chihuahua. Miners tunneling through the Naica Hills, south of Chihuahua City, in search of silver and zinc, found huge mineral crystals, far larger than any natural crystals previously seen anywhere else on the planet.

The monster crystals, over six meters long, are made of selenite, a crystalline form of the mineral gypsum (the number one ingredient in blackboard chalk!). For its pale translucence, this form of gypsum is known as selenite, named after Selene, the Greek goddess of the moon.

The largest crystals are over 11 meters long, and weigh more than 55 tons. Scientists believe they have found DNA from ancient bacterial life trapped in air bubbles inside the crystals.

The crystals formed when the caverns were completely filled with hot water (warmer than 50 degrees C) supersaturated with calcium sulphate (gypsum) for very long periods of time. The estimated growth rate, confirmed by uranium-thorium dating, is 1.5 mm per 1000 years, which means that the longest crystals took more than 500,000 years to form.

The engineer working for the mining company Peñoles that first reported these caves believes that many more caves probably exist, but stresses that they would never have been located at all if it had not been for the mine’s massive pumps (pumping more than 16,000 gallons of water out each minute) working round the clock for years. This pumping has lowered the water table from 100 meters below the surface to about 300 meters in the area of the mines. Since the caverns have been drained, the crystals are no longer growing, and the temperatures in the cavern have fallen by about 0.5 degrees C each year.

The Naica (“shady place”) hills have been actively mined for more than a century. Even though early prospectors discovered silver here in 1794, the first formal mining claim was not made until a century later in 1896, by one Santiago Stoppelli, and large scale mining only began in 1900.

Ten years later, super-large sword-shaped crystals of selenite were found in a cavern at a depth of 120 meters. Over the years, a steady stream of geologists and mineral collectors have visited this 70-meter-diameter cave, since renamed the Cave of the Swords, which is now equipped with paths, lights and a ventilation system. Even with this system, the temperature in the cave is a stifling 40 degrees C! Several typical examples of selenite crystals from this cave, ranging in length from 1.2 to 1.6 meters (4 to 4.25 feet), are displayed in the Smithsonian’s National Museum of Natural History.

But these specimens from the Cave of the Swords are small fry in size when compared with the latest discoveries, 300 meters below the surface, in the Cave of the Crystals. In two relatively small chambers, each the size of a small apartment, miners found incredibly large selenite crystals, some over six meters long. The crystals combine to form massive fifteen-meter-long columns, “the size of pine trees”, as well as hundreds of formations shaped like sharks’ teeth, jutting about a meter up from the cave floor. The overall effect is, in the words of Richard Fisher, an Arizona-based photographer and adventurer, like walking into an enormous geode.

The Future

Mining operations at Naica have been in the hands of the Peñoles group since 1961 and the mine, one of the most productive in Chihuahua, still produces ample quantities of lead, silver and zinc.

Even if the silver ore is eventually worked out, Peñoles realizes that the caverns might have a very bright future as a major geo-tourist attraction. The company has not only taken steps to safeguard the crystals, but was already developing a tourism plan, which involved the installation of an air-conditioning system in the caverns, since the temperature in the Cave of Crystals is a mind-boggling 60 degrees Celsius. This heat, combined with the 100% humidity in the caverns, is so suffocating and disorienting that researchers can only safely spend a few minutes at a time studying this fantastic sight.

The harsh conditions have not deterred crystal looters who are already reported to be breaking through padlocked doors and trying to chisel prize specimens off the cave walls. This is a dangerous business and the effort proved fatal for one would-be collector when the gigantic crystal he was attempted to sever broke away from the ceiling and crushed him as it fell. The combined physical forces of Newton and Darwin caused this particular plunderer to meet his maker!

It was announced in October 2015 that Peñoles had been forced to close its Naica mine indefinitely, following its failure to reduce the level of water in the mine following a flood in January 2015. The company was seeking to redeploy more than 400 workers. In 2014, the Naica mine produced 19,694 tons of lead, 15,399 tons of zinc and 1.9 million ounces of silver (25% 6% and 3% respectively of the company’s total output of each metal).

Whether or not attempts will be made in the future to allow access to view the crystals is unknown. It is perfectly possible that other miners in the region might be lucky enough to find even bigger crystals nearby!

These cavers may never be suitable for sustainable tourism. To preserve these magnificent crystals for future generations, the decision may have to be taken to leave the caverns under water, protecting the crystals that already exist and allowing new ones to grow.

These amazing crystals are one of the latest additions to the incredible list of natural wonders that can be enjoyed in Mexico!

For truly amazing images of the crystals, watch the Discovery Channel documentary “Naica: Secrets of The Crystal Cave” (2008).

Note:

  • This is an update of a post originally published in 2010

Mexico’s geology, relief and landforms are analyzed in chapters 2 and 3 of Geo-Mexico: the geography and dynamics of modern Mexico. Buy your copy of this invaluable reference guide today!

Basalt columns and prisms can be seen in various places in Mexico

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Oct 252016
 

Basalt is a dark, fine-grained, basic (low silica) igneous rock, often extruded as molten lava from volcanic fissures. Its low silica content means it can flow easily, often building up over the years to form large plateaus. As the basaltic lava cools, it contracts and solidifies. An extensive network of cracks often develop in basalt, which may extend many meters deep. These cracks tend to leave columns between them which are roughly hexagonal (6-sided) in shape. Among the more famous examples of basalt columns or pavements in the world are Giant’s Causeway (Northern Ireland), Fingal’s Cave (Scotland) and Devil’s Postpile (California, USA).

The best known location in Mexico to see basalt columns is about an hour’s drive north of Mexico City, at Santa María Regla, in the state of Hidalgo. These columns, attractively located on the side of a canyon, with a waterfall tumbling over some of them (see sketch), were visited by the famous Prussian scientist/geographer Alexander von Humboldt, during his exploration of Mexico in 1803-04. Some individual columns are 40 meters tall.

Basalt prisms at San Miguel Regla

Basalt prisms at San Miguel Regla, from Humboldt and Bonpland, Vues des Cordilleres et monumens des peuples indigenes de l’Amerique.

The basalt columns of Santa María Regla are one of the locations described in The first geography fieldtrip guide in Mexico. They are also one of Mexico’s top 13 natural wonders.

Basalt prisms at Santa María Regla

Other locations in Mexico where basalt columns or prisms can be seen include a quarry between the towns of Jamay and Ocotlán (Jalisco), the south-facing slope of the hills overlooking the town of Chapala (Jalisco) and the Salto (waterfall) de San Antón (Cuernavaca, Morelos).

Mexico’s geology and landforms are analyzed in chapters 2 and 3 of Geo-Mexico: the geography and dynamics of modern Mexico.  Buy your copy of this book today!

The deepest water-filled sinkhole in the world is in Tamaulipas, Mexico

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Oct 122016
 

As vertical shafts go, this is a seriously deep one! Long considered to be “bottomless” (because no-one had ever managed to find the floor), we now know it is precisely 335 meters (1099 feet) deep, making it the deepest water-filled sinkhole anywhere on the planet.

The El Zacatón sinkhole is on El Rancho Azufrosa, near the town of Aldama in Tamaulipas in northeast Mexico. The sinkhole or cenote is one of several located in the same area, though recent studies have failed to demonstrate any obvious underground connections between them. The term cenote is a Spanish rendering of the Mayan word d’zonot, “a hole in the ground”. The El Zacatón pit, which is about 110 meters (360 feet) across and roughly circular, contains a deep lake. The water is warm (averaging about 30 degrees C), highly mineralized and has a sulfurous odor. The name El Zacatón comes from the floating islands of grass (zacate) which blow across this lake from one side to the other with the wind.

The pit’s depth has attracted serious divers for many years. In 1993, Dr. Ann Kristovich dove to a new women’s world record depth of 169 meters (554 feet). The following year, two American explorers tried to reach the bottom of the sinkhole. Jim Bowden successfully reached a men’s world record depth of 282 meters (925 feet) but still did not touch the bottom. Tragically, his diving partner Sheck Exley died during the attempt.

The mystery of the sinkhole’s depth was finally solved in 2007. A multi-million dollar exploration, mainly funded by NASA, trialled the Deep Phreatic Thermal Explorer (DEPTHX) robot, designed to explore ice-covered Europa, Jupiter’s smallest moon. Partners on the DEPTHX project include Carnegie Mellon University, Southwest Research Institute, Colorado School of Mines, The University of Arizona, and the University of Texas at Austin’s Jackson School of Geosciences. In the words of a NASA press release, The Deep Phreatic Thermal Explorer (DEPTHX) is a 3,300-pound, computerized, underwater vehicle that makes its own decisions. With more than 100 sensors, 36 onboard computers, and 16 thrusters and actuators, it decides where to swim, which samples to collect, and how to get home.

Exploring El Zacatón pit was considered to be an ideal preliminary test of the DEPTHX autonomous robot, which is about the size of a go-kart. The robotic vehicle successfully generated a highly detailed sonar map of the sinkhole, and obtained samples of water and biotic material from the walls, discovering several new phyla of bacteria in the process. Its next challenge is to explore beneath the ice of West Lake Bonney in Antarctica.

At El Zacatón, the sonar study showed that the sinkhole has a total depth of 335 meters: the lake is 319 meters deep at its deepest point, and its surface is 16 meters below the height of the sinkhole’s rim.

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Sources / Further reading:

Mexico’s geology and landforms are analyzed in chapters 2 and 3 of Geo-Mexico: the geography and dynamics of modern Mexico.  Buy your copy of this book today!

How Lake Chapala, Mexico’s largest lake, was formed

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Sep 262016
 

Lake Chapala is Mexico’s largest natural lake. On the geological timescale of millions of years, all lakes are temporary features on the earth’s surface. Once formed, natural processes begin to fill them in and/or to drain them.

Map of Lake Chapala, Jalisco, Mexico. All rights reserved.

Lake Chapala resulted from drastic earth movements, accompanied by earthquakes and faulting which occurred some twelve million years ago. Lake Chapala collected on the floor of a rift valley. Movements along the parallel systems of faults that caused the rift valley still occur today. Evidence for this continued movement can be seen in the cracked or displaced walls of some local buildings.

Given its advanced age, it is not surprising to discover that Lake Chapala was once (thousands of years ago) much larger. In fact, though no-one has so far proven it beyond doubt, it may have been immensely large, covering an area seven times its present area, with a correspondingly long shoreline. At a later stage in its history, it became the deepest lake of an interconnected series of lakes which flooded the valley floors where the towns of Jocotepec, Zapotitan, Zacoalco and Sayula are today. The present Lake Chapala is thus probably only a small remnant of the original version.

The lake is under heavier pressure than at any time in its existence. Local towns and the nearby city of Guadalajara see it as an inexhaustible supply of domestic and industrial water. Tourists see it as a recreation resource, and the thousands of foreign retirees who have settled on its shores see it as a major reason for the area’s beneficial climate.

A case study of ‘residential tourism’ in the villages on the northern shore of Lake Chapala is discussed in chapter19 of Geo-Mexico: the geography and dynamics of modern Mexico.

The levels of flow of the River Lerma, the only river of any size entering the lake, are crucial to the health of the lake. In recent years, demands for Lerma water have multiplied many times over, principally for farms in neighboring states, but also for industries. All the other rivers entering the lake are much smaller and, with rare exception, flow into the lake only a few times a year during the rainy season. The hydrology of Lake Chapala is discussed in chapters 6 and 7 of Geo-Mexico: the geography and dynamics of modern Mexico.

The good news is that Lake Chapala was recently declared an Internationally Important Wetland by the Ramsar Convention, joining a large global network of similarly important wetland sites. The 13th International Living Lakes Conference was held in Chapala, March 22-25, 2010.

This is an edited excerpt from Tony Burton’s Western Mexico, A Traveller’s Treasury (4th edition) .

Cueva Cheve, Oaxaca, is one of the world’s deepest cave systems

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Apr 062015
 

Even though most people have never heard of it, Cueva Chevé is one of the deepest cave systems in the world. In 2003, a team led by American speleologist Bill Stone, explored Cueva Chevé, located in the mountainous, pine-clad Sierra de Juárez region of Oaxaca, to a depth of 1484 m (4869 ft). The Cueva Chevé system is thought to have some tunnels (as yet unexplored) that extend even further, to depths beyond 2000 m (6500 ft). By way of comparison, at present the world’s deepest known cave is the Krubera Cave, in the Republic of Georgia, which has a maximum explored depth of 2197 m (7208 ft).

Profile of Cueva Cheve

Profile of Cueva Cheve

How deep might the Cueva Chevé be?

In 1990, colored dye trace experiments showed that there was a hydrological connection between the Chevé Cave and a distant spring (resurgence). This shows that the Cueva Chevé system (including parts not yet explored) has a total vertical fall of 2525 m (8284 ft) over a distance of (north to south) of almost 19 km (11.8 mi).

Because the major risks in exploring any cave system include the possibility of sudden rises in water level, or unexpected water flows through the caves, expeditions to this region are limited to the middle of the dry season (ie February-April). When an expedition gets underway, staging camps are set up underground at intervals, but only in locations believed to be well above flood stage water levels.

Cueva Chevé (see cross section) is shaped like a giant L. The vertical shaft is about 910 m (3000 ft) deep and roughly 3.2 km (2.0 mi) of passages are required to get to the bottom. The remainder is a long, gradually sloping passage that goes on for another 3.2 km and drops roughly 605 m (2000 ft). The cave’s deepest known point is about 11 km (7 mi) from the entrance, where explorers have so far failed to get past a terminal sump.

The air in the cave is relatively warm, with temperatures ranging from 47-52̊ F (8-11̊ C).

Chambers so far explored have been given prosaic names such as “Cuarto de las Canastas” (the Basket Room), “Cuarto del Elefante Negro” (the Black Elephant Room), and “Cañon Fresco” (Fresh Canyon), while named cave formations include the “Taller de Santa Claus” (Santa Claus Workshop). Several parts of the cave system have been found to contain human artifacts, the earliest dating back at least several hundred years.

How to get there

Cueva Chevé is about 140 km (86 mi) north of Oaxaca City via highways 190 and 131.

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Where are the mining concessions in Mexico?

 Books and resources  Comments Off on Where are the mining concessions in Mexico?
Feb 022015
 

A series of graphics prepared by Mexico City daily El Universal includes a map showing the details of all the mining concessions in Mexico. According to the newspaper’s analysis, one fifth of Mexico’s total land area is subject to mining concessions belonging to one company or another.

The six companies holding the largest areas of concessions are:

  • Altos Hornos de México (364 concessions totaling 3208 hectares)
  • Fresnillo PLC (1009; 1953)
  • Industrias Peñoles (922; 953)
  • Minera Fresco (779; 889)
  • Cascabel (116; 749)
  • and Grupo México (711; 607).

The map is probably the single most interesting graphic in the series. Zooming in (top left of map) allows the details of each concession to be viewed, including the concession holder, size of concession, minerals involved and whether or not the concession is “active”. Is there a mining concession near you? You might be surprised. Even in an area of Mexico that I have known intimately for many years, there are two concessions that I have never previously heard of!

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The mystery of the Alarcon Rise

 Updates to Geo-Mexico  Comments Off on The mystery of the Alarcon Rise
Dec 292014
 

Geologists have discovered that some strange things are happening off the southern coast of the Baja California Peninsula.

In essence, while most of Mexico rests on the North American plate, the Baja California Peninsula is on the gigantic Pacific plate. The Pacific plate is moving slowly northwest and the pressures in the zone where these two plates intersect, under the Sea of Cortés (Gulf of California), has caused a complex series of parallel faults which (further north) link to the California’s San Andreas Fault system. Thus, the Sea of Cortés is an area of heavy seismic activity.

The Alarcón Rise is a 31-mile-long (50 kilometer) spreading center at the mouth of the Gulf of California. Along ocean spreading ridges like the Alarcón Rise, the seafloor is splitting apart as lava wells up from underneath. Credit: (c) 2012 MBARI

Location of the Alarcón Rise. Credit: (c) 2012 MBARI

The Alarcón Rise (see map) is a 50km (30 mi) long “bump” under the Sea of Cortés. New seafloor is being continuously created along the Alarcón Rise as undersea magma rises to the surface and cools to become lava. As the plates continue to move, this lave is then carried away to either side of the Alarcón Rise, allowing fresh lava to take its place, and so on. The rate of sea-floor spreading here is a relatively slow 5 cm (2 in) a year.

The Alarcón Rise has been studied in considerable detail by geologists attached to the Monterey Bay Aquarium Research Institute (MBARI) in California. Using a sonar-mapping robot, they discovered new deep-sea hydrothermal vents.

But their most surprising find was a “Weird Underwater Volcano.”

In most zones of sea-floor spreading, the lavas are relatively low in silica and therefore free-flowing. Such lavas are known, on account of their chemistry, as “basic” lavas. (Lavas higher in silica, known collectively as acid lavas, tend to be more viscous and flow less easily. Acid lava volcanoes tend to erupt far more explosively than basic lava volcanoes.)

The curiosity of the Alarcón Rise is that while the vast majority of lava flows along the ridge are basic (basalt) lavas, those associated with at least one volcano are clearly acidic, not basic.

Researchers used a remote-control vehicle to collect samples and explore the volcano, which is 2375 m (7800 ft) below the surface. Samples of the lava show that it is primarily rhyolite with some dacite, with a silica content of up to 77%, the highest of any rock ever found along a mid-ocean ridge, according to Brian Dreyer, a geochemist at the University of California, Santa Cruz. The volcano with acid lava forms a small dome, about 50 meters (165 ft) in height and covers an area of about 1200 meters by 500 meters (4000 feet by 1640 feet). The dome is probably several thousand years old.

The lavas solidified quickly to form angular chunky blocks, some of which then rolled down the sides of the rise to form talus (scree). The individual blocks can be as large as cars or small houses.

The findings suggest that this particular volcano could give rise to hazardous eruptions. It is only 100 km (60 mi) from land and very close to the major tourist areas fringing the coast of Baja California Sur. Any major explosive eruption from this volcano could also cause a tsunami with the potential to devastate settlements on both coasts of the Sea of Cortés (Gulf of California).

Geologists are still working on trying to fully explain this apparent anomaly in lava composition. They have already discarded several ideas, and their current hypothesis is that the magma source was at some point contaminated by seawater, resulting in an unusually high concentration of volatiles such as water, sulfur and chlorine.

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World-class rock-climbing near Monterrey

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Nov 132014
 

Mexico’s varied geography has made it a premier destination for all kinds of adventure tourism, from caving and canyoneering to jungle treks, white-water rafting and rock climbing.

This 6-minute video shows mountaineer Alex Honnold climbing the 460-meter (1500-feet) high rock face known as El Sendero Luminoso near Monterrey in northern Mexico. What makes this climb special (and slightly scary to watch) is that Honnold climbs solo and without any safety measures such as ropes.

Interviewed for National Geographic Adventure before he had seen the video, Honnold said, “I’m not sure what the video shows, but my true solo was all alone with no photogs [photographers] or helis [helicopters]. We then went back and filmed on big portions of it. In my mind there’s a clear difference between personal climbing—the actual solo—and work days—the filming afterward.”

"The Spires" in El Potrero Chico climbing area (Wikipedia photo)

“The Spires” in El Potrero Chico climbing area (Wikipedia photo)

The El Sendero Luminoso rockface is in an area known as El Potrero Chico, a short distance from Monterrey, near the town of Hidalgo.

The Wikipedia entry for El Potrero Chico describes it as having “a large range of different climbs, most of them in the 5.8 to 5.13 grade. The type of climbing can range from steep overhanging face to easy slab. The rock is usually quite sharp. The climbs are mostly situated in a canyon at the entrance of the park, while the interior offers undeveloped mountain terrain with many mountain biking routes, ranging from very easy to expert options.”

According to Wikipedia, El Potrero is “considered one of the top 10 locations to sport climb in the world. In addition to well over 500 routes, the area boasts the second longest sport route in North America, Timewave Zero, with 23 pitches and over 2,000 feet (610 m).”

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Cananea in Sonora: one of the largest open-pit copper mines in the world

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Oct 022014
 

One of the largest open-pit copper mines in the world, the Buenavista del Cobre mine in Cananea produced over 200,000 metric tons of copper in 2012. The mine, opened in 1899, is located approximately 40 kilometers south of the border between Arizona and Sonora, Mexico. The open-pit mine was estimated to contain 26.874 million metric tons of copper reserve as of December 2012.

This mine was the location of a 2014 toxic spill: Toxic spill in Sonora copper mine causes environmental disaster

mina-cananea

Source unknown. Used at http://www.labartolina.com.mx

The ore is refined at an on-site concentrator, which has a milling capacity of 77,000 metric tons/day. The concentrate output is transported to the smelter at La Caridad by rail. Also present are an on-site leaching facility and two solvent extraction and electro winning (SX/EW) plants, with an annual production capacity of 55,000 metric tons of copper cathode.

The active, 2-kilometer-diameter Colorada Pit (top right of image below) is recognizable in this astronaut photograph by the concentric steps, or benches, cut around its perimeter (see larger image). These benches allow for access into the pit for extraction of ore and waste materials.

Cananea Mine, Sonora (NASA Earth Observatory, March 2008)

Cananea Copper Mine, Sonora (NASA Earth Observatory, March 2008) Click to enlarge

Water (black) fills the bottom of the pit and several other basins in the surrounding area. The city of Cananea, marked by its street grid, is northeast of the mine workings. A leachate reservoir for removal and evaporation of water pumped from the mine workings is located to the east of the mine (image lower left). The bluish-white color of deposits near the reservoir suggests the high mineral content of the leachate.

sonora-cananea-mina_de_cobre

Source unknown. Credit: https://yoreme.wordpress.com/

Text: NASA’s Earth Observatory and The 10 biggest copper mines in the world

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Popocatepetl Volcano continues its very active phase

 Mexico's geography in the Press, Other, Updates to Geo-Mexico  Comments Off on Popocatepetl Volcano continues its very active phase
Aug 092014
 

Popocatepetl Volcano, near Mexico City continues to be very active, with smoke and ashes belching up to 1000 meters above the crater rim. Mexico’s National Disaster Prevention Agency Cenapred, reports that the volcano had 82 “low intensity” exhalations on 7 August 2014, four of which contained “explosive material”.

Popocatapetl, 8 August 2014. Credit: Cenapred.

Popocatapetl, 8 August 2014. Credit: Cenapred.

The agency also reported that many mionr tremors hd been recorded, including one harmonic tremor lasting 56 minutes. Geologists believe that the volcano is currently destroying dome number 50 even as dome number 51 begins to form. Dome #51 is currently about 70 meters in diameter. Renewed explosions, together with some ash fall is predicted for the coming days.

The Volcanic Traffic Light remains at Yellow Phase 2.

For a series of images dated 8 August 2014, see Images of 8 of ago of 2014

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Mexico’s geomorphosites: Ceboruco Volcano

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Apr 072014
 

A short distance west of the crater lake of Santa María del Oro, in the west Mexico state of Nayarit, is Ceboruco volcano which has a cobblestone road to the top. The road starts from the old and picturesque village of Jala, eight kilometers off the main highway (Highway 15). The cornfields around Jala yield some of the largest ears of corn in the world, more than 30 centimeters (one foot) in length, a cause for celebration in the village’s annual August festival. Jala was declared a Magic Town in 2012.

The road up Ceboruco is a geologist’s or biologist’s dream come true, a slowly unfolding series of volcanic forms and different types of vegetation with abundant surprises even for the scientifically expert. Small wonder, then, that the great German botanist Karl Theodor Hartweg was so impressed with Ceboruco when he collected plants here in the nineteenth century. To read more about his discoveries, see The geography of garden flowers, many of which originated in Mexico.

ceborucoNear the top are several short but interesting walks, some in shady, thickly vegetated valleys hidden between towering walls of blocky lava, some along the many overlapping rims of the various old craters of which this complex peak is comprised. Wherever you choose to walk, a multicolored profusion of flowers and butterflies will greet your eyes.

On the south side of an attractive grassy valley at kilometer sixteen, fumaroles send hot gases and steam high into the air reminding us that this volcano is not yet irrevocably extinct. A massive Plinian eruption in about the year 1000 sent ash plumes into the air and devastated a wide area around the volcano. The huge blocks of lava near the summit date from a prolonged series of eruptions in the early 1870s.

Highway 15 cuts through Ceboruco’s lava field a few kilometers after the Jala junction. For those not wishing to brave the cobblestone road up to the volcano, this is a good place to stretch the legs and marvel at the inhospitable, black lava blocks which were spewed out more than a hundred years ago.

This is a lightly edited extract from my “Western Mexico: A Traveler’s Treasury” (link is to Amazon’s “Look Inside” feature), also available as either a Kindle edition or Kobo ebook.

Want to read more about Mexico’s geomorphosites? The link uses Geo-Mexico’s “Site Search” feature.

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World’s longest underground river flows deep beneath the Yucatán Peninsula

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Feb 102014
 

In January 2007, the world’s longest underground river was reported from Mexico’s Yucatán Peninsula. [Prior to that date, the honor was held by the Puerto Princesa Subterranean River in the Philippines]

The Sac Actun (“White Cave”) river system in the Yucatán Peninsula wanders for 153 km (95 miles) through a maze of underground limestone caves. It took British diver Stephen Bogaerts and his German colleague Robbie Schmittner four years to explore the caverns using underwater scooters and specially rigged gas cylinders, before they finally discovered a connection between the Yucatán region’s then second- and third-longest cave systems, known respectively as Sac Actun and Nohoch Nah Chich (“Giant Birdcage”). Following the discovery of a link, the entire system is now known as Sac Actun. The system has a total surveyed length (including dry caves) of 319 kilometers (198 mi), making it the longest cave system in Mexico, and the second longest worldwide. [The longest is the dry Mammoth Cave System, Kentucky, USA, which measures 643.7 km (400 mi) in length].

Sac-Actun cave system

Sac-Actun cave system

Vying with Sac Actun for the title of longest surveyed underwater cave system is the nearby Sistema Ox Bel Ha (“Three Paths of Water”), also in the Tulum municipality of Quintana Roo. As of August 2013, surveys had measured 256.7 kilometers (159.5 mi) of underwater passages.

The underground passages and caverns of the Yucatán Peninsula have been a favored site for cave explorers for decades. Formal mapping of the systems has taken more than 20 years of painstaking work. Access to the systems is via the hundreds of sinkholes (cenotes) that litter the surface of the Peninsula. The Sac Actun system alone includes more than 150 cenotes.

Water management was critical to the Maya as they developed their advanced civilization in this area, a region with very limited surface freshwater. Many of the cenotes in the Yucatán Peninsula have archaeological importance and were utilized by the Maya for ceremonies. Perhaps the best-known (and most visited) cenote is the Sacred Cenote (cenote sagrado) at the archaeological site of Chichen Itza.

The caverns of the Yucatán Peninsula were formed as a result of the slow solution of limestone over thousands of years by percolating, slightly acidic, rainwater. In some cases, cave formations, such as stalactites and stalagmites, have later grown in the caves, formed drip-by-drip from the slow deposition of calcium carbonate from calcium-saturated ground water.

Because the average elevation of the Yucatán Peninsula is only a few meters above sea level, the water in many of the caves is “layered”, with a lens of freshwater overlying a layer of salt water. Rainwater that soaks into the ground becomes ground water, which then moves slowly along the watertable to eventually reach the ocean.

Cave researchers are worried that tourist developments in the Yucatán Peninsula will have adverse impacts on underground water systems, both in terms of water quantity (because of the amounts of fresh water extracted for domestic and tourist use) and in terms of water quality, because even point sources of water pollution (such as excess fertilizers from a golf course) could contaminate underground water supplies over a wide area.

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The landforms of the El Pinacate and Gran Desierto de Altar Biosphere Reserve

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Nov 092013
 

The breathtaking scenery of the El Pinacate and Gran Desierto de Altar Biosphere Reserve in the northern state of Sonora affords visitors a dramatic combination of two very distinct landscape types: volcanic landscapes (El Pinacate) in the east, and sand dunes (Gran Desierto de Altar) towards the west and south.

pinacate-map-googleVolcanic scenery (El Pinacate)

The eastern section of the Biosphere Reserve, El Pinacate, is a dormant volcanic area of around 200,000 ha (2000 sq. km), centered on the El Pinacate Shield (or Sierra Pinacate) which has 3 main peaks: Pinacate, Carnegie and Medio. The El Pinacate Shield is a composite structure, comprised of extensive, successive black and red lava flows, some more than 20 km long, seperated by desert pavement. The El Pinacate Shield boasts a wide array of volcanic phenomena and geological formations. Most of the lava is basaltic (alkaline) in composition, making it relatively fluid when molten; it is mainly of the aa (blocky) type, though some pahoehoe (ropy) lava is also found. The total volume of lava is estimated at between 150 and 180 km3.

Elegante Crater, El Pinacate

Elegante Crater, El Pinacate (example of a maar) Credit: IUCN Tilman Jaeger

Besides the lave flows, the Pinacate area has more than 400 cinder cones (formed 1.2 million years ago) and several lava tubes. The lava flows and cinder cones are only a prelude to the most visually striking features in the reserve: 10 enormous, deep, and almost perfectly circular maars (steam explosion craters). Maars are believed to originate from a combination of explosion caused by groundwater coming into contact with hot lava or magma and subsequent collapse. The maars of El Pinacate are rivalled only by similar formations in Africa. The largest single maar is El Elegante, formed 32,000 years ago, which is 1,400 meters (4,600 feet) from rim to rim and 140 meters (460 feet) deep. It takes visitors a two to three-hour hike to reach its rim and be rewarded by a spectacular view.

The volcanic forms of El Pinacate are relatively recent in geological terms, most having been formed during the Quaternary Period, which began some 2.8 million years ago. The most recent volcanic activity in this area was only about 11,000 years ago. Some volcanologists believe that some of these craters could become active again in the future, with the potential to form volcanoes up to a few hundred meters in height.

Ron Mader, the founder of Planeta.com and a foremost authority on responsible tourism in Mexico, has marveled at the “bizarre and mind-boggling scenery” of El Pinacate., which so resemble the lunar landscape that between 1865 and 1970 it was used by NASA as a training ground for astronauts preparing for the moon landings. The lava field is so vast and sharply defined that it later turned out that the astronauts could easily recognize it from space!

Sand dunes (Gran Desierto de Altar)

The western and southern parts of the El Pinacate and Gran Desierto de Altar Biosphere Reserve have entirely different scenery. The Gran Desierto de Altar is North America’s largest field of active sand dunes (erg). Several types of dunes are represented here, the tallest reaching 200 meters in height.

The sand needed to form and maintain these dunes comes from the fluvial and deltaic sediments of the Colorado River delta (to the west), the beaches of the Sea of Cortés/Gulf of California (to the south), the River Sonoyta (to the east) and the smaller river and stream fans formed in those parts of the reserve where there are volcanic and granitic mountains.

Sand dunes of Gran Desierto de Altar

Sand dunes of Gran Desierto de Altar

Prior to the opening of the Sea of Cortés (Gulf of California), vast amounts of sediment accumulated in this region brought by rivers of which little trace remains today. The creation of the Sea of Cortés, 5.3 million years ago, shortened the rivers and increased their average gradient (rejuvenation), causing them to cut into the pre-existing landscape leaving behind river terraces, remnants of the former higher level floodplains.

The fields of sand dunes of the Gran Desierto de Altar cover more than 550,000 hectares (5700 sq.km.) Several different kinds of sand dunes are found here–linear, crescent-shaped (barchans) and star-shaped–and they can be simple, compound or complex, depending on seasonal changes in the direction and strength of the wind.

Although linear dunes dominate (70%), crescent-shaped complex dunes and star-shaped dunes are of more interest because they exist in only a few locations in the world. Spectacular and very large star-shaped dunes, up to 200 meters high, occur both singly and in long ridges up to 48km in length. Star-shaped dunes possibly evolved from crescent dunes which changed their direction of movement becoming “reversing dunes”. Side winds may account for the multiple arms of some star-shaped dunes.

Other features – Granite massifs

In addition, there are several granite massifs (inselbergs), such as the Sierra del Rosario, emerging like islands from the sandy desert flats and dunes. They range in elevation from 300 to 650 meters above sea level. They represent another remarkable landscape feature harboring distinct plant and wildlife communities.

Main source:

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Geotourism in Mexico: García Caves (Grutas de García) in Nuevo León

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Oct 032013
 

The Garcia Caves (Grutas de García) are located in the Cumbres de Monterrey National Park, 9 km from the small town of Villa de García, and about 30 km from the city of Monterrey (state capital of Nuevo León). The highest point in the park is Copete de las Águilas which rises to 2260 m (7,410 ft) above sea level, but its best known peak is Saddle Hill (Cerro de la Silla), the distinctive saddle-shaped hill that overlooks the city.

Much of the park, including the mountains, are composed of sedimentary rocks that were originally laid down as marine sediments and then subsequently folded, uplifted and exposed to erosion. The extensive areas of limestone in the park, which date from the Cretaceous period, have been subject to karstification over 50 to 60 million years, which has resulted in typical karst landforms such as sinkholes, caves, cave formations and underground streams.

The Garcia Caves, one of the largest cave systems in Mexico, are deep inside the imposing Cerro del Fraile, a mountain whose summit rises to an elevation of 1080 meters above sea level, more than 700 meters above the main access road. The entrance to the caves is usually accessed via a short ride on a 625-meter cable car that was built to replace a funicular railway.

The cave system was first reported in 1843 by the Marmolejo family who informed their local prist Juan Antonio Sobrevilla that they had stumbled across it while looking for firewood.

Grutas de García. Credit: María de Lourdes Alonso

Grutas de García. Credit: María de Lourdes Alonso

Guided tours of the cave system show visitors some of its 27 separate chambers along a 2.5-kilometer (1.6 mile) route. The full system extends more than a kilometer further into the mountain reaching depths of more than 100 meters (340 feet) beneath the surface. The limestone of the cave walls contains lots of marine fossils. The caves have extensive and impressive formations of dripstone, including stalactities, stalagmites and other forms.

Unlike the suffocating heat of the Naica Crystal Caves in Chihuahua, the cave temperature here remains about 18̊C (65̊F) all year.

The chambers and formations have been given whimsical and imaginative names such as

  • “El salón de la luz” (The Light Chamber) where the natural translucence of the ceiling rock allows light from the outside to filter through.
  • “La octava maravilla” (The Eighth Wonder), a natural column formed where a stalagmite growing from the floor joined a stalactite, growing from the ceiling
  • “El mirador de la mano”, a stalagmite shaped like a human hand.
  • “El Nacimiento” (The Nativity),
  • “La Fuente Congelada” (The Frozen Fountain),
  • “La Torre China” (The Chinese Tower),
  • “El teatro” (The Theatre), and
  • “El Árbol de Navidad” (The Christmas Tree).

Want to read more about caves in Mexico?

Visit John Pint’s website for a selection of his writing, with many original articles, illustrated with great photographs, about many individual caves in Mexico.

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The volcanic calderas of Mexico’s Volcanic Axis

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Jul 082013
 

There is still lots of work needed to fully unravel the geological secrets of Mexico’s Volcanic Axis which crosses the country between latitudes 19̊ and 21̊ North. Unlike most volcanic belts elsewhere in the world, this one does not appear at first sight to correspond to any plate boundary. Another of the mysteries of this volcanic region, where igneous upheavals have shaped the landscape for several million years, is the relative dearth of calderas, the “super craters” formed either by collapse or by giant explosions.

While the toponym La Caldera is used fairly commonly in Mexico’s volcanic regions for a volcano or volcanic crater, geologists restrict the term to the much larger landform that results from the collapse or super-explosion of a volcano. Even so, there is still some debate among specialists as to the precise definition of the term caldera.

Geologists have proposed a threefold division of the Volcanic Axis, based on differences in the volcanic landforms, in terms of their type, structure, age, morphology and chemistry.

volcanic-axis

The western sector (see map below) extends from the western coast of Mexico to Lake Chapala (including the lake basin). The central sector covers the area between Lake Chapala and the twin volcanoes of Popocatepetl and Ixtaccihuatl, close to Mexico City. The eastern sector includes these twin volcanoes and extends as far as Mexico’s Gulf Coast.

Mexico's Volcanic Axis (Fig 2.2 of Geo-Mexico, the geography and dynamics of modern Mexico. All rights reserved.

Mexico’s Volcanic Axis (Fig 2.2 of Geo-Mexico, the geography and dynamics of modern Mexico). All rights reserved.

The only caldera recognized in the western section is that of La Primavera, the forested area west of Guadalajara, whose formation we considered in

In the central and eastern sections of the Volcanic Axis, several other calderas have been recognized. They include (from west to east):

  • Los Azufres
  • Amealco
  • Mazahua
  • Huichapan
  • Los Humeros
  • Las Cumbres

Los Azufres

The precise origin of the Los Azufres caldera, in Michoacán, is still debated. The caldera is the site of an important geothermal power station with an installed capacity of 188 MW. (Mexico is the world’s fourth largest producer of geothermal energy, after USA, the Philippines and Indonesia.) The geothermal heat in this area is also used to heat the cabins in a local campground, and to dry wood and process fruit.

Amealco

The Amealco caldera is in the central part of the Mexican Volcanic Axis, midway between the towns of San Juan del Río and Maravatio. It dates from Pliocene times and has been heavily eroded since. It is about 11 km wide and 400 m deep and was the origin of great sheets of pyroclastic flow deposits (ignimbrites) with a total volume of around 500 cubic km.

Mazahua

Mazahua is a collapse caldera, 8 km in width, near the village of San Felipe del Progreso in the western part of the State of Mexico.

Huichapan

The Donguinyó-Huichapan caldera complex is 10 km in diameter and in the central sector of the Volcanic Axis. It appears to be two overlapping calderas, dating from around 5 million and 4.2 million years ago respectively. The rocks from the older caldera are intermediate to basic in composition, while those from the more recent caldera are acidic (high silica) rhyolites.

Los Humeros

The Los Humeros caldera is in the state of Puebla, close to its border with Veracruz. It is 55 km west-north-west of the city of Xalapa (Veracruz), relatively close to Teziutlán (Puebla). The main caldera (summit elevation 3150 m) is about 400 m deep and roughly oval in shape, with a diameter which varies from 15 to 21 km. It was formed about 460,000 years ago by the collapse of the underground magma chamber. Prior to collapse, lava emitted from this vent had covered 3500 square km with ignimbrite. Later, two smaller calderas formed nearby, with ages of about 100,000 years (Los Potreros caldera) and 30,000 years (El Xalapazco) respectively. Volcanic activity in this area has been utilized to produce generate geothermal power (installed capacity: 40 megawatts).

Las Cumbres

The easternmost caldera in Mexico is Las Cumbres, 15 km north of Pico de Orizaba, Mexico’s highest volcano, and close to the state boundary between Puebla and Veracruz. The Las Cumbres caldera was originally believed to be an explosion super-crater, but geologists now think that it was created due to the partial collapse of the eastern flank of the original volcano, between 40,000 and 350,000 years ago. The collapse of the side of Las Cumbres produced a huge debris avalanche (total volume estimated at 80 cubic km, which extended up to 120 km in the direction of the Gulf of Mexico.

Lake Alchichica

According to Dra. Esperanza Yarza de la Torre in Volcanes de México (UNAM; 1984), Lake Alchichica in the Oriental Basin near Puebla occupies another caldera. The basin has several shallow lakes, known locally as axalpazcos (“sandy basin with water” in the indigenous Nahuatl language). These occupy shallow craters (or in one case a caldera) and are largely sustained by ground water. The largest of the lakes, in a caldera, is Lake Alchichica, which has a diameter of 1888 meters, an area of 1.81 square km, and lies at an elevation of 2320 meters above sea level. The rim of the caldera rises 100 m above the lake level. The lake is used for irrigation. This lake is claimed to be Mexico’s deepest natural lake with a maximum depth of 64 meters, and a mean depth of 38.6 meters.

Main sources:

  • Gerardo J. Aguirre-Díaz & Fred W. McDowell. 1999. Volcanic evolution of the Amealco caldera, central Mexico. United States Geological Society. Special Paper 334.
  • Esperanza Yarza de la Torre. 1984. Volcanes de México. UNAM.

Want to read more?

  • Use the site’s tag system (left hand side of the page) to find lots more posts about Mexico’s volcanoes, geology and landforms.

Watch La Primavera’s geological history unfold via a short video animation

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Jun 292013
 

Only days after we published our third post about the Primavera Forest, near Guadalajara, we were alerted to an excellent 9 minute video animation of how the area was formed. This short video about “The Exciting Geology of Bosque La Primavera” was produced by geologist Barbara Dye during her stint as a Peace Corps volunteer in Mexico.

The video can also be viewed in Spanish:

Dye has also written a beautifully-illustrated 72-page guide (in Spanish) to the geology of the Primavera Forest, entitled “La Apasionante Geología del Área de Protección de Flora y Fauna La Primavera.”

Previous posts about La Primavera:

How was the Primavera Forest caldera in Jalisco formed?

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Jun 102013
 

In a previous post, we described the considerable geotourism potential of the Primavera Forest near Guadalajara:

In this post, we take a closer look at how this unusual area was formed.

Stages 1 and 2 (see diagram):

140,000 BP. The magma chamber beneath the surface began to fill with magma (molten rock underground) and grow in size.

By about 120,000 BP, several lava flows and domes had formed, made primarily of rhyolite, a silica-rich (“acid”) igneous rock. After each eruption, the magma level underground would subside for a period of time before pressure built up again towards the next eruption.

Formation of a caldera

Fig. 4 of Bullard (1962) “Volcanoes in history, in theory, in eruption”. Based on van Bemmelen (1929) and Williams (1941)

Stages 3 and 4

So much pressure had built up by about 95,000 BP that there was a huge explosion, sending 20 cubic kilometers (4.8 cubic miles) of rock and ashes high into the sky. The explosion covered 700 square kilometers (270 square miles) with volcanic materials, known today as the Tala tuff (tuff is the geological term for consolidated ash). This massive explosion caused the upper part of the magma chamber to collapse, leaving a caldera that was 11 kilometers (6.8 miles) wide. The Tala tuff includes large quantities of pumice, a light and porous volcanic rock formed when a gas-rich froth of glassy lava solidifies rapidly.

This caldera filled with water, creating a lake.

Stage 5

This stage began shortly afterwards when a series of ring domes were erupted around the edge of the caldera as the magma deep below the surface started to push upwards again, eventually forming small islands in the lake. These eruptions formed more pumice, blocks of which would break off and start to float across the lake as they gradually sank to the lake floor.

A further series of eruptions in about 75,000 BP led to a second series of ring domes. A combination of tectonic uplift and sedimentation had filled the lake in by about this time.

More volcanic domes have been created at approximately 30,000 year intervals since, in about 60,000 BP and about 30,000 BP; these domes were almost all on the southern and eastern margins of the caldera, and include the lava domes of El Colli and El Tajo on the outskirts of Guadalajara.

Many geologists appear quietly confident that lava and ash eruptions in La Primavera are a thing of the past. They consider that the Primavera Forest’s fumaroles, hot river and hot waterfall represent the last vestiges of vulcanism and are no cause for alarm. On the other hand, others, including Gail Mahood who has studied this area far more than most, warn that hazard monitoring is justified in the case of La Primavera given its proximity to a major city and bearing in mind that any future eruption would be likely to occur on the southern and/or eastern side of the caldera.

The La Primavera Forest is only one of several calderas in Mexico’s Volcanic Axis.

If you prefer a short 9 minute video animation of how the area was formed, try this excellent YouTube video: “The Exciting Geology of Bosque La Primavera“, produced by geologist Barbara Dye during her stint as a Peace Corps volunteer in Mexico.

References:

  • Mahood G. A. 1980. Geological evolution of a Pleistocene rhyolitic center – Sierra La Primavera, Jalisco, Mexico. Journal of Volcanology and Geothermal Research, 8: 199-230.
  • Mahood, G.A. 1981. A summary of the geology and petrology of the Sierra La Primavera, Jalisco, Mexico. Journal of Geophysical Research, Volume 86.
  • Dye, Barbara. 2013. “La Apasionante Geología del Área de Protección de Flora y Fauna La Primavera”.

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Mexico’s geomorphosites: El Sótano de las Golondrinas (Cave of the Swallows)

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Apr 112013
 

El Sótano de las Golondrinas, in the municipality of Aquismón in the state of San Luis Potosí, is a massive limestone sinkhole (pit cave), one of the largest known in the world. In terms of depth, it is thought to be the second deepest sinkhole in Mexico and is probably in the world’s top 20.

The depth of sinkholes can be difficult to determine. For example, in the case of El Sótano de las Golondrinas, its surface opening is about 50 meters by 60 meters (160 by 200 ft) in size, but is on a slope. The depth on the high side is about 376 meters (1220 ft); the depth on the low side is about 330 meters (1090 ft).

sotano-de-las-golon

Below the surface (see profile) the sinkhole is roughly bottle-shaped. The floor of the sinkhole is about 300 x 135 meters (990 by 440 ft) in area. However, the sinkhole is believed to have formed from the collapse of the roof of an underground cave. As a result, the floor of the sinkhole is not solid rock but rubble that presumably came from the walls and former roof. A shaft on one side extends down at least another 100 m, suggesting that the true floor of the original cave lies at least that far beneath the current rubble-strewn floor.

US photographer Amy Hinkle shot some spectacular images earlier this year in this cave.  The accompanying article highlights the “secret garden” that “nestles 300 meters beneath the surface of the earth”.

The cave’s name (literally “basement of the swallows”) derives from the thousands of white-collared swifts that inhabit the overhanging walls of its interior. They spiral out of the cave every morning over a period of 25-30 minutes and return to their cave homes close to sunset. Large numbers of green parakeets also live in the cave.

The floor of the sinkhole is home to a rich plant life, as well as a diverse selection of  fungi, millipedes, insects, snakes, and scorpions.

The original cave is thought to have been formed by a lengthy period of water erosion along a major fault line in the lower Cretaceous limestone in the Sierra Huasteca (part of Mexico’s Eastern Sierra Madre). Over time, the cave became larger as a consequence of both the water erosion and due to mass movements (landslides, rockfalls) on its walls. Eventually, the size of the cave was so large that its walls could no longer support its roof which then collapsed into the cave, leaving the open air sinkhole seen today. Following heavy rain, short-lived waterfalls cascade down the sides of the sinkhole.

The first documented exploration of El Sótano de las Golondrinas was apparently in 1966. Since that time, the cave has become a popular destination for various adventure sports including rappelling, abseiling and base jumping (no longer allowed).

There are several other very deep sinkholes in the same general area, including Hoya de las Guasguas (with a 202 m deep entrance shaft) and Sótano del Barro (402 m in depth).

Some ornithological studies have found that the bird population of El Sótano de las Golondrinas is decreasing, perhaps due to the disturbance caused by the increasing number of human visitors. To limit disturbance, access and activities are more tightly controlled. For instance, descents into the cave are now strictly limited to daylight hours when the birds are absent, and a no-fly zone has been established around the cave, primarily to avoid helicopter disturbance.

El Sótano de las Golondrinas is yet another outstanding example of a geomorphosite in Mexico. Mexico has literally thousands of geomorphosites. Among those described in previous Geo-Mexico posts are:

References:

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The crater lake of Santa María del Oro yields evidence for climate change

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Feb 142013
 

A magnificent crater lake nestles in a centuries-old volcanic crater a short distance east of the town of Santa María del Oro in Nayarit.

The connecting road from Highway 15 first passes through the former mining town of Santa María del Oro and then rises slightly to offer a splendid view of the beautiful slate-blue lake (known locally as “La Laguna”), set in a ring of verdant hills. In recent years, the lake, a good example of a geomorphosite, has become important for tourism with accommodations ranging from RV spaces to a boutique hotel. It takes about an hour and a half to stroll round the track that encircles the crater lake. Other attractions include visiting an abandoned gold mine (which offers a glimpse into the area’s past), birding, mountain biking, swimming or hiring a rowboat or kayak to venture out onto the lake.

Crater Lake, Santa María del Oro. Credit: Tony Burton

Crater Lake, Santa María del Oro. Credit: Tony Burton

This usually quiet lake has proved to be a valuable source of information for geologists and climatologists investigating the history of climate change in this region of Mexico.

The researchers who published their findings in 2010 in the Bulletin of the Mexican Geological Society extracted a sediment core from the deepest part of the lake. The relatively small area of the drainage basin surrounding the lake and the relatively steep slopes of surrounding hills mean that the sediments entering the lake are rarely disturbed after they are deposited. Wind and wave action are limited. The depth of the lake (maximum 65.5 meters) also helps to ensure that sediments remain undisturbed for centuries. This gives perfect conditions for a reliable sediment core.

Santa María del Oro. Credit: Google Earth

Santa María del Oro. Credit: Google Earth

The team analyzed the titanium, calcium and magnetism levels of successive thin slices of the core. By comparing the core with historic records and previous tree ring analyses from the same general area, they were able to accurately date each slice. The titanium levels in each slice allowed the researchers to quantify how much runoff occurred in that year, a proxy indicator of precipitation.

The team identified 21 significant drought events over a period of 700 years. The six most marked droughts occurred in 1365–1384, 1526, 1655-1670, 1818, 1900 and 1930-2000. They found periodicities of 25, 39, 50, 70 and 117 years for drought events, meaning that droughts occurred at fairly regular intervals of about 20-25 years.

The researchers then looked at the possible correlation between periods of drought and two distinct climatological factors: a shift to the south in the position of the Inter Tropical Convergence Zone (ITCZ) in summer and the occurrence of El Niño Southern Oscillation (ENSO) events. When the ITCZ does not extend as far north as usual during Mexico’s summer rainy season, states such as Nayarit and Jalisco receive less than their normal amount of rainfall. During ENSO events, rainfall is also diminished in central and western Mexico.

Of the 21 droughts identified and studied, 7 proved to be statistically linked to ENSO events, 10 to ITCZ movements, and the remaining 4 events were closely linked to a combination of both.

As the study concludes, titanium analysis of sediments may allow for a more refined record of climate change in the period prior to reliable historic or instrumental records which might improve the understanding of how and why climate change occurred in past

Santa María del Oro is also worth visiting because it is only a short distance away from the edge of the canyon of the River Santiago and the El Cajón hydro-electric power project, one of three major HEP projects located along that river.

Source article:

Susana Sosa-Nájera, Socorro Lozano-Garcí, Priyadarsi D. Roy and Margarita Caballero. Registro de sequías históricas en el occidente de México con base en el análisis elemntal de sedimentos lacustres: El caso del lago de Santa María del Oro. Boletín de la Sociedad Geológica Mexicana, Vol 62, #3, 2010, p 437-451.

Santa María del Oro and surrounding areas are described in chapter 24 of the recently published 4th (Kindle/Kobo) edition of my Western Mexico: A Traveler’s Treasury (Sombrero Books, 2013).

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The distinctive street pattern of Venta de Bravo, Michoacán

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Jan 262013
 

The small settlement of Venta de Bravo, in the municipality of Contepec in the state of Michoacán, has a very distinctive street pattern. As the image shows, it has a circular “center”, surrounded by a series of concentric circular streets (see image), connected via regularly-spaced radial streets. The regularity of the pattern is not quite perfect. Based on the photo, the imperfections probably result from variations of topography.

Venta de Bravo, Michoacán. Credit: imagenesaereasdemexico.com

Venta de Bravo, Michoacán. Credit: imagenesaereasdemexico.com

The village has about 1300 inhabitants and is at an elevation of 2290 meters above sea level. This is clearly a “planned settlement”, and one almost certainly quite modern in origin. I haven’t ever visited Venta de Bravo and don’t know its history, but would certainly be interested in finding out more if you have any pertinent information or can suggest likely sources.

An online search for Venta de Bravo will turn up numerous articles about the seismically active 45-km-long Venta de Bravo fault, as well as references to the small “Rayón National Park” which is only a few kilometers away in the Sierra of Tlalpujahua and extends as high as 2770 meters above sea level (Cerro del Gallo).

Colima Volcano erupts, destroying lava dome first created in 2007

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Jan 142013
 

Colima Volcano (aka the Volcán de Fuego) is one of the westernmost volcanoes in Mexico’s Volcanic Axis, which straddles the country from west to east. The Volcano’s summit is only 8 km (5 miles) from the inactive Nevado of Colima volcano, Mexico’s sixth-highest peak, which rises 4260 m (13,976 ft) above sea level. (Curiously, despite their names, the summits of both volcanoes are actually located in the state of Jalisco and not the state of Colima.)

The elevation of Colima Volcano is officially given as 3820 m (12,533 ft) above sea level. In the past 400 years, it has been the most active volcano in Mexico, having erupted at least 30 times since 1576.

It is also considered to be one of the country’s most dangerous volcanoes. Numerous villages in its shadow keep a wary eye on its level of activity, and emergency evacuations have become a regular event in the past fifty years.

Colima Volcano, 11 Jan 2013. Photo: Protección Civil.

Colima Volcano forms new crater, 11 Jan 2013. Photo: Edo de Jalisco Protección Civil.

On a geological time-scale, the volcano first erupted about five million years ago in the Pliocene period, long after activity ceased at the nearby, and higher, Nevado de Colima. It quickly developed into a large volcano which partially blew apart or collapsed during Pleistocene times to form a caldera, five kilometers across. A new cone developed inside the caldera. This is the Volcán de Fuego we see today.

The cone is built mainly of pyroclastic materials (ashes and volcanic bombs) of andesitic composition together with some basaltic lava, making it a classic example of a composite volcanic cone.

Historically, the eruptions of the volcano have fallen into a definite cyclical pattern with periods of activity, each lasting about 50 years, interspersed with periods of dormancy. The first cycle of activity (after the Spanish arrived in Mexico) was between 1576 and 1611. Major eruptions occurred in 1680 and 1690, and further complete cycles occurred between 1749 and 1818, and from 1869 to 1913. Most geologists agree that current activity is part of the fifth cycle, which began in 1961.

A three year sequence of prior activity (2003 to 2005) is shown on this series of NASA satellite images.

Hazard Map of Colima Volcano (2003) Credit: Universidad de Colima, Observatorio Vulcanológico

Hazard Map of Colima Volcano (2003) Credit: Universidad de Colima, Observatorio Vulcanológico. Click for full-size image (large file size)

In each major cycle, the first results of renewed activity force new lava into the existing crater, forming a dome. Once the crater has filled up, any additional lava is ejected from the crater and flows down the volcano’s flanks. If the lava is unable to escape (relieving the underground pressure), the dome is liable to explode, which is exactly what happened a few days ago:

As on several previous occasions, once the subterranean pressure that caused the activity has been relieved, activity should cease, and the volcano will enter another less dangerous dormant phase. Even during this phase, a plume of hot gas often billows out from the volcano.

The dome that was destroyed in January 2013 began to build in 2007. The explosive activity on 6 January and 10 January 2013 left behind a new crater 220 meters (720 ft) across and about 50 m (165 ft) deep. According to the Jalisco-Colima Scientific Committee (which oversees the hazard analysis posed by the volcano), the events of 6 and 10 January emitted an estimated  1.5 million cubic meters of material, which formerly formed the dome. The 10 January explosion, which occurred at 21:40 hrs local time, sent incandescent material down the west flank of the volcano. An ash column rose about 3000 meters into the air before traveling north-eastwards on the wind towards the city of Ciudad Guzmán.

Thermal imaging shortly after the 10 January explosion showed that the temperatures in the crater are below 200 degrees Centigrade, which indicates relatively little gaseous build up and limited risk of further major explosions. Even so, a prudent 7.5 km exclusion zone is being maintained around the volcano.

Update (29 Jan 2013):

Another explosion at 3:58 am on 29 January 2013 created a plume of ash and cinders that rose more than 3000 meters above the volcano. The ash fell of nearby villages, including Los Mazos, Ejido Atenquique, Tuxpan and Huescalapa.

The area around the volcanoes is described in more detail in chapter 15 of “Western Mexico, a Traveler’s Treasury” (4th edition; Sombrero Books, 2013).

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How were the canyons in the Copper Canyon region formed?

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Aug 062012
 

According to a local Tarahumara Indian legend, the canyons were formed when “a giant walked around and the ground cracked.” However, geologists believe that a sequence of volcanic rocks varying in age from 30 to 135 million years was slowly uplifted to an average elevation of 2275 m (7500 ft) while being dissected by rivers.

Mexico's Copper Canyon

Where did the rocks come from?

The Sierra Tarahumara is part of the Western Sierra Madre, an extensive volcanic tableland, affected by grabens (rift‑valley faulting) and faults which deprive it, especially on its flanks, of any homogeneous appearance. Its eastern side merges gradually into the Chihuahua basin and range landscape; its western side is much steeper, marked by major normal faults of considerable vertical extent, and by deep canyons.

Stage 1. The lower rocks.

McDowell and Clabaugh (1979) describe two different igneous sequences, which were separated in time by a prolonged period of non‑activity. The older series is mainly comprised of intermediate igneous rocks between 100 and 45 million years old; it shows evidence of lava flows and violent eruptive activity which produced andesitic pyroclasts. There are also layers of siliceous ignimbrites. This lower series includes many rich mineral deposits, though it outcrops in only restricted parts of the Canyon system. The volcanic activity in this area was associated with the subduction of the small (now destroyed) Farallon Plate which was pushed beneath the North American Plate by the expanding Pacific Plate. The line between the lower, older series and the higher, newer one is very irregular, indicating intensive erosional activity in the period between their times of formation.

Stage 2. A break in activity allowing erosion to take place.

The major lull in volcanic action, between 45 and 34 million years ago, may have been due to a change in the inclination of the subducting Farallon plate.

Stage 3. The upper rocks.

After this break in activity, there was a sudden resumption of vulcanism. The upper series is the most extensive cover of ignimbrite known anywhere in the world, covering an area which is 250 km wide, and 1200 km long from NW to SE. It stretches as far north as the southern USA. To the south, it disappears beneath the newer volcanic rocks of Mexico’s Volcanic Axis in Jalisco and Michoacán. The ignimbrites are rhyolitic and rhyo‑dacitic in composition, generally approximately horizontal, or slightly tilted, and with ages between 34 and 27 million years. In places, these ignimbrites are more than 1000 m thick (Demant & Robin, 1965).

It is unclear precisely where all these volcanic rocks originated. One estimate is that for such large volumes of rock to have been formed, there would have been between 200 and 400 volcanic outlets, some up to 40 km across. An alternative hypothesis (proposed by Aguirre-Díaz & Labarthe-Hernández) is that large bodies of magma (molten rock underground) reached shallow parts of the crust and then partially erupted, explosively, along the fault lines of the existing basin and range structures.

The common rock types in the Copper Canyon region

Volcanic Ash is unconsolidated fragments <2 mm in diameter. Volcanic ash commonly contains larger (up to 64 mm) fragments called lapilli. Ash may be composed of crystalline rock (eg. rhyolitic and andesitic ashes), of glassy fragments (vitric ash), or of individual crystals (crystal ash). In general, the size of individual particles comprising the ash diminishes as distance increases from the volcano where it originated.

Tuff is consolidated volcanic ash.

Ignimbrites are essentially pieces of light, vesicular, pumice, in a matrix of glassy fragments. Ignimbrites are often layered and sometimes split into vertical columns. They are deposited from ash flows that included large volumes of hot, expanding gases and incandescent glass fragments.

Lavas (molten rock on the surface). When lava cools and solidifies, it produces massive (as opposed to fragmentary) rocks, generally crystalline but of variable chemical composition (andesite, rhyolite, basalt).

The major landforms of the Copper Canyon region

These layers of igneous rocks were uplifted, forming a plateau with an average elevation of 2275 m (almost 7500 feet). Rivers have carved deep gashes, up to 1400 m deep, into the plateau surface, forming a series of steep-walled canyons, separated by giant blocks, remnants of the original, continuous plateau.

Incised meanders near Umira, Chihuahua

Incised meanders near Umira, Chihuahua

Since some of the rivers exhibit superb examples of incised meanders [see photo], some, possibly most, of these rivers already existed prior to the main periods of uplift. They were meandering across a gently sloping flood plain prior to the tectonic upheavals. Then, as the landscape slowly rose around them, they carved these giant canyons. These antecedent rivers retained their courses; the meanders were incised into the landscape.

Centuries of erosion by the various rivers, including the Urique river and its tributaries, have resulted in the present-day landscape of structurally-guided plateau remnants, termed mesas, buttes and pinnacles (depending on their size). There are many examples of these distinctive landforms in the Copper Canyon region.

Landforms resulting from dissection of a plateau

Landforms resulting from the dissection of a plateau

It is likely that some of the many waterfalls in the region were formed in places where the downward vertical erosion of rivers was insufficiently powerful to counteract the forces of uplift. Other waterfalls are more likely to have resulted from differences in the relative resistance of different rocks. The effects of differential erosion are noticeable in many smaller-scale features in this landscape, such as perched “mushroom” rocks.

Perched block near San Ignacio

Perched block near San Ignacio. Photo: Tony Burton; all rights reserved

Examples of many of these landforms can be seen by anyone driving along the Creel-Batopilas road. At km 5 (from Creel) is the entrance to the Mission village of San Ignacio, near which are strange tor‑like rock formations, including “mushroom” rocks, where a more resistant capstone sits perched atop weaker rocks that are slowly being eroded away.  At km 20 in  Cusárare, a short walk south of the road through woods and along the Cusárare river leads to the very pretty 30-metre high Cusárare waterfall. At km 44 (Basihuare), there are fine views of a mesa of pink and white rocks that overlooks the road. This is Cerro el Pastel (“Cake Mountain”) with its pinnacles. At km 56, near  Umirá (or Humirá) are several spectacular incised meanders formed when the river’s course was preserved while the surrounding land was undergoing relatively rapid uplift.

Small wonder that the Copper Canyon region is one of Mexico’s most important geomorphosites!

Sources:

  1. Aguirre-Díaz, Gerardo J. & Guillermo Labarthe-Hernández. 2003. Fissure ignimbrites: Fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting. Geology September, 2003 v. 31, no. 9, p. 773-776
  2. Gajdusek, D.C. (1953) “The Sierra Tarahumara” in Geographical Review, New York. 43: 15‑38
  3. Demant, A & Robin, C (1975) “Las Fases del vulcanismo en Mexico” Revista Instituto de Geologia, UNAM, Mexico City. 75 (1) pp 70‑83
  4. Schmidt, R.H. (1973) A Geographical Survey of Chihuahua, monograph #37 Texas Western Press.
  5. McDowell, F.W. & Clabaugh, S.E. (1979) “Ignimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico” in “Ash flow tuffs” edited by Chapin, C.E. & Elston, W.E., Geol. Soc. of America special paper # 180.

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Rocks and relief fieldtrip: Tequisquiapan and the Peña de Bernal

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Jun 212012
 

There is a close connection between geology and relief in many parts of Mexico. In this post we describe a one-day fieldwork excursion in the Tequisquiapan area of the central state of Querétaro that looks at this connection. The fieldwork is suitable for high school students but could easily be extended to provide challenges for college/university students.

The fieldwork starts with a fieldsketch from near Tequisquiapan. Any suitable vantage point will do, provided it offers a clear view northwards to the very distinctive Peña de Bernal (seen in the background of the photo below). At this point, a simple line sketch should suffice to help students identify the following four different kinds of terrain:

  • flat or gently sloping plain, used for cultivation
  • low hills, with gently sloping sides, which look to be covered in bushes and cacti [scrub vegetation]
  • high mountains, with steeper slopes, also with no obvious signs of cultivation
  • the Peña de Bernal itself, a distinctive monolith with exceptionally steep sides

There is no need to identify any rocks or use any geological terms (students can add appropriate labels later!). Engage the students in a discussion about why there might be four different kinds of relief visible in this area, and how their ideas or hypotheses could be investigated further. Conclude the discussion by explaining that they are now going to look for evidence related to the idea that these four different kinds of relief are connected to significant differences in geology.

View looking north from "Las Cruces" near Tequisquiapan. Photo: Tony Burton; all rights reserved.

View looking north from "Las Cruces" near Tequisquiapan. Photo: Tony Burton; all rights reserved.

The next stop is a small roadside quarry on the flat area. The most accessible quarry many years ago was located a short distance south of Tequisquiapan on the east side of the highway, but any quarry on the flat land will serve to reveal the rocks that form the plain.

Roadside quarry, near Tequisquiapan. Photo: Tony Burton; all rights reserved

Roadside quarry, near Tequisquiapan. Photo: Tony Burton; all rights reserved

[Warning: Ensure that you park off the highway; when examining the rock in the quarry, avoid any overhanging sections, and do not do anything to cause slope instability or collapse].

The rocks in the quarry are in layers (sedimentary) and very distinctive. The individual particles of the rock are rough to the touch and sand-sized, so this is some kind of sandstone.

Some layers are more or less horizontal, but in places successive layers are laid down at a much steeper angle. This is “current bedding” and indicates that the rocks were formed by water, perhaps where a river entered a lake. The individual particles in the rock are not well-rounded, so have not traveled all that far.

Within each major layer, the material shows signs of sorting, with fine material sitting on top of coarse material. In some places, the sandstone contains small pebbles, so this rock is a sandstone conglomerate. Small casts of fossil shells can be seen in places, further suggesting it is a sedimentary rock deposited in a former lake.

A very thin white layer is present (at about head height in the photo). This layer is totally different to the sandstone conglomerate. It is fine material that has been compacted. Given the volcanic history of central Mexico, this is almost certainly a thin layer of volcanic ash that covered older rocks before being covered in turn by the next layer of sediments.

With some guidance, students should be able to work most of this out for themselves! The last stage at this stop is to ask why this rock forms the flat land in the area, rather than the hills. (Answer: softer, weaker, less resistant, easier to erode, etc).

The next stop is to take a look at the rock forming the low hills. The highway between Tequisquiapan and Ezequiel Montes (see map) conveniently cuts through a low ridge at San Agustín. This affords an opportunity to take a close look at the rock forming that ridge. [Warning: Ensure that you find a safe parking spot, and take every precaution, since traffic along this highway can be heavy and very fast-moving]

The rock at San Agustín is darker and much harder than the rock in the quarry. It has clear crystals in it, apparently arranged haphazardly. From its color (grey) and grainsize (fine), it is a rhyolite [a fine grained, acid igneous rock].  It is far more difficult to erode than the sandstone on the plain, so it forms upstanding ridges and low hills in the landscape.

From San Agustín, drive through Ezequiel Montes and on to the town of Bernal, one of Mexico’s “Magic Towns“. The next part is the most physically-challenging part of the excursion since it is necessary to climb at least part-way up the Peña de Bernal! [Warning: this is very steep in places, and climbing beyond the mid-way “chapel” is definitely not recommended]. Examining the rocks of the Peña de Bernal reveals that they are lighter in color than the rhyolite and fine-grained, but with larger crystals (phenocrysts) in some places. This rock must have cooled very slowly (or the phenocrysts would not have had chance to form) and this is an intrusive igneous rock known as microgranite. Eagle-eyed students should also find some other rocks while climbing the Peña de Bernal. In places, it is possible to find good specimens of a very hard, banded metamorphic rock that was formed when heat and/or pressure transformed pre-existing rocks. The banded rock is a gneiss [pronounced “nice”].

The presence of intrusive igneous rocks (formed underground) together with metamorphic rocks strongly suggest that the Peña de Bernal is an example of a volcanic plug. It represents the central part (and all that now remains) of a former volcano, whose sides, presumably composed of ashes and lava, have long since eroded away.

Conclusion:

After students have had chance to work most of this out for themselves, a look at the local geological map should confirm that their deductions are reasonable. As can be seen on the map below, the flat area is indeed an alluvial plain (sandstone), with low rhyolite hills and ridges in places, and higher rhyolite mountains in the background, with the distinctive Peña de Bernal made of igneous and metamorphic rocks at the northern edge of the map.

In this particular part of Mexico, as in many other areas, the link between geology and relief is very strong! Happy exploring!

Sketch map, Geology of the Tequisquiapan area

Sketch map, Geology of the Tequisquiapan area; click to enlarge

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Mexico’s highest volcanoes

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Apr 302012
 

In a previous post, we saw how most of Mexico’s volcanoes are located in a broad band that crosses central Mexico known as the Volcanic Axis (Eje neovolcánico). In this post, we provide brief descriptions of some of the major volcanoes in Mexico.

Starting in the west, the first active volcanoes are Everman and Barcenas in the Revillagigedo Islands. Two of the westernmost volcanoes on the mainland are near Colima. At 4260 m (13,976 ft), the inactive Nevado of Colima, Mexico’s sixth-highest peak, is as tall as the highest mountains in the contiguous USA. Its younger brother, Colima Volcano (or Volcán de Fuego) is lower (3820 m) but highly active and considered potentially very dangerous. It has erupted in cycles for several hundred years, and is capped by a dacitic plug characteristic of a silica-rich Pelean volcano. Such volcanoes have the potential to erupt suddenly, not emitting vast quantities of molten lava, but shooting out less spectacular, but far more devastating, clouds of red‑hot asphyxiating gasses.

Tequila Volcano, overlooking the town where the beverage is distilled, is also in Jalisco. In neighboring Michoacán state, the most noteworthy volcanoes are Jorullo (which last erupted in 1759) and Paricutín, which began life in a farmer’s field in 1943 and ceased activity in 1952, but only after its lava had overwhelmed several small villages.

Closer to Mexico City, the Nevado of Toluca (4680 m) has a drive-in crater and is a favored destination for Mexico City families in winter to take their children to play in the snow. It is Mexico’s fourth highest peak (see table below).

VolcanoStatesHeight (meters)Height (feet)
Pico de OrizabaVeracruz; Puebla5 61018 406
PopocatapetlMéxico; Morelos; Puebla5 50018 045
IztaccihuatlMéxico; Puebla5 22017 126
Nevado of Toluca México 4 68015 354
MalincheTlaxcala; Puebla4 42014 501
Nevado of Colima Jalisco4 26013 976
Cofre de PeroteVeracruz 4 20013 780
TacanáChiapas 4 08013 386
TelapónMéxico 4 06013 320
El AjuscoFederal District3 93012 894
Colima VolcanoJalisco; Colima3 82012 533

Continuing eastwards, we reach several other volcanoes that are among Mexico’s highest volcanic peaks (and are also included in the table).

The most famous volcano in the Volcanic Axis is the still active Popocatepetl (“Popo”), which rises to 5500 meters (18,045 feet). Alongside Popocatepetl is the dormant volcanic peak of Iztaccihuatl (5220 m or 17,126 ft). On a smog-free day, both are clearly visible from Mexico City. The southern suburbs of Mexico City are overshadowed by a smaller active volcano, Ajusco, which reaches 3930 m (12,894 ft).

The Nevado de Toluca volcano

The Pico de Orizaba, a dormant volcano on the border between states of Veracruz and Puebla, is Mexico’s highest mountain. At 5610 m (18,406 ft) it is the third highest peak in North America. By way of contrast, not very far away, in the outskirts of the city of Puebla, is the world’s smallest volcano!

Only a few volcanoes appear to be located outside the Volcanic Axis and therefore in an anomalous location to the general pattern. They include two volcanoes in Chiapas which lie south of the Volcanic Axis: El Chichón (which erupted in 1982) and Tacaná (4080 m).

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Mexico’s Volcanic Axis

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Apr 192012
 

Mexico’s active seismic zones have created numerous volcanoes, many of which are still active. Virtually all the country’s active and recently dormant volcanoes are located in a broad belt of high relief which crosses Mexico from west to east: the Volcanic Axis (see map).

volcanic-axisAltitudes in this region vary from a few hundred to several thousand meters. The principal peaks are shown on the map. They include many of Mexico’s most famous mountains, such as Popocatepetl and Iztaccihuatl, near Mexico City; Pico de Orizaba, Mexico’s highest peak; Paricutín, the only completely new volcano in the Americas in recent times; and Colima, considered the most active at present. Many of the volcanoes are surprisingly young. For instance, a study using Carbon‑14 dating on the palaeosols (ancient soils) under 12 volcanoes in the Toluca area yielded ages ranging from 38,600 to 8400 years before present.

It is unclear precisely why this broad belt of Mexico should be so active. Elsewhere in the world all major tectonically active areas have been linked in terms of their location to the margins or meeting‑zones of tectonic plates. Some Mexican geologists believe that Mexico’s Volcanic Axis is a rare example of activity associated with a gently dipping plate margin, one where the edge of the Cocos plate is subsumed, but at only moderate gradient, beneath the North American plate.

Almost all the volcanic activity in this zone has taken place in the last 25 million years, from the upper Oligocene period, through the Miocene and Pliocene and up to Recent. Two distinct periods of activity are recognized by some geologists. The first, in the late Oligocene and early Miocene, produced volcanic rocks often found today tightly folded by later earth movements. The second, responsible for all the major composite cones as well as dozens of ash and cinder cones, started in the Pliocene and continues today.

Erosion has had relatively little time to work on these “new” volcanic peaks, some of which are still developing. As a result, this region includes Mexico’s highest mountains, reaching over 5500 m or 18,000 ft.

Thick, lava-rich volcanic soils make this one of the most fertile areas in North America. Though the relief is very rugged, this area has supported relatively high population densities for hundreds of years, including the current large metropolitan areas of Mexico City, Guadalajara and Puebla. Legacies of previous volcanic activity are found in craters, mud‑volcanoes, geothermal activity, and the numerous hot springs (and spa towns) scattered throughout the Volcanic Axis.

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How has the movement of tectonic plates affected Mexico?

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Apr 122012
 

In a previous post, we identified the tectonic plates that affect Mexico. In this piece, we look at some of the major impacts of Mexico lying on or close to so many different plates.

To the east of Mexico, in the last 100 million years, outward expansion from the Mid-Atlantic Ridge (a divergent boundary) first pushed South America ever further apart from Africa, and then (slightly more recently) forced the North American plate (and Mexico) away from Eurasia. The Atlantic Ocean continues to widen, expanding the separation between the New World and the Old World, by about 2.5 cm (1 in) each year.

Mexico's position in relation to tectonic plates

Mexico's position in relation to tectonic plates.Map: Geo-Mexico.com; all rights reserved

Meanwhile, to the west of Mexico, an analogous situation is occurring in the Pacific Ocean, where the Cocos plate is being forced eastwards away from the massive Pacific plate, again as a result of mid-ocean activity. The Cocos plate is effectively caught in a gigantic vice, its western edge being forced ever further eastwards while its leading eastern edge smacks into the North American plate.

The junction between the Cocos and North American plates is a classic example of a convergent plate boundary. The collision zone is marked by a deep ocean trench, variously known as the Middle America trench or the Acapulco trench. Off the coast of Chiapas, this trench is a staggering 6662 m (21,857 ft) deep. The trench is formed where the Cocos plate is forced to dive beneath the North American plate.

As the Cocos plate is subducted, its leading edge fractures, breaks and is partly re-melted into the surrounding mantle. Any cracks in the overlying North American plate are exploited by the molten magma, which is under immense pressure, and as the magma is forced to the surface, volcanoes form. The movement of the plates also gives rise to earthquakes. The depth of these earthquakes will vary with distance from the deep ocean trench. Those close to the trench will be relatively shallow, whereas those occurring further away from the trench (where the subducting plate is deeper) will have deeper points of origin.

As the plates move together, sediments, washed by erosion from the continent, collect in the continental shallows before being crushed upwards into fold mountains as the plates continue to come together. A line of fold mountains stretches almost continuously along the west coast of the Americas from the Rocky Mountains in Canada past the Western and Southern Sierra Madres in Mexico to the Andes in South America. Almost all Mexico’s major mountain ranges—including the Western Sierra Madre, the Eastern Sierra Madre and the Southern Sierra Madre—formed as a result of these processes during the Mesozoic Era, from 245 to 65 million years ago.

However, no sooner had they formed than another momentous event shook Mexico. About 65 million years ago, a giant iridium-rich asteroid slammed into the Gulf of Mexico, close to the Yucatán Peninsula, causing the Chicxulub Crater, and probably hastening the demise of the dinosaurs. An estimated 200,000 cubic km of crust was pulverized; most of it was thrown into the air. The resulting dust cloud is thought to have contributed to the extinction of up to 50% of all the species then on Earth. Not only did this event have an enormous impact on all life forms on Earth, it also left a legacy in the Yucatán. The impact crater is about 200 km (125 mi) across. Its outer edge is marked by a ring of sinkholes (locally known as cenotes) and springs where the fractured crust provided easy access to ground water. These locations include the ria (drowned river valley) of Celestún (now a UNESCO Biosphere Reserve), where fresh water springs mingle with salt water to create an especially rich habitat for birdlife.

In the 65 million years since the asteroid impact (the Cenozoic period), the remainder of Mexico has been formed, including many of the plateaus and plains, and the noteworthy Volcanic Axis, which owes its origin to still-on-going tectonic activity at the junction of the North American and Cocos plates.

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Which tectonic plates affect Mexico?

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Apr 022012
 

The theory of plate tectonics suggests that the earth’s crust or lithosphere is from 5 to 65 km (3 to 40 mi) thick and divided into about a dozen large tectonic plates, tabular blocks that drift across the Earth in different directions and at various speeds (up to a few centimeters or inches per year), probably as a result of thermal convection currents in the Earth’s molten mantle. Most plates consist of a combination of both ocean floor and continent, though some are entirely ocean floor.

Each tectonic plate is moving relative to other plates. The movements are not independent because the plates smash into and scrape against one another. Areas in the center of tectonic plates, far from the boundaries, have relatively little seismic activity, but the boundaries between plates are seismically very active, creating earthquakes and volcanoes. The level of seismic activity depends on the relative speed and direction of the plates at the boundary.

There are three distinct kinds of boundaries between plates. At divergent boundaries, along mid-ocean ridges, plates are being steadily pushed apart, with new crust being added by volcanic activity to the rear of each plate as it moves. At convergent boundaries, plates collide and parts of the plates either buckle or fracture or are subducted back down into the molten mantle. The third kind of boundary is where plates are neither created nor destroyed but are moving side by side. The resulting friction as they rub against each other can produce large earthquakes.

Almost all of Mexico sits atop the south-west corner of the massive North American plate (see map). Immediately to the south is the much smaller Caribbean plate. The North American plate extends westwards from the Mid-Atlantic Ridge, which runs through Iceland and down the middle of the Atlantic Ocean, to the western edge of North America. In a north-south direction, it extends from close to the North Pole as far south as the Caribbean.

Mexico's position in relation to tectonic plates

Mexico's position in relation to tectonic plates.Map: Geo-Mexico.com; all rights reserved

While most of Mexico rests on the North American plate, it is also influenced by several other plates.

The Baja California Peninsula is on the gigantic Pacific plate, which is moving northwest and under the North American plate. The intersection of these plates under the Gulf of California causes parallel faults which are part of the famous San Andreas Fault system. Thus, the Gulf of California is an area of heavy seismic activity.

The small Rivera plate, between Puerto Vallarta and the southern tip of Baja California, is moving in a southeasterly direction and rubbing against the Pacific plate; it, too, is moving under the North American plate.

The Cocos plate and tiny Orozco plate are ocean crust plates located off the south coast of Mexico. The collision of the Cocos plate and the North American plate has had several far-reaching consequences, including both the disastrous 1985 earthquakes that caused such severe loss of life and damage in Mexico City and the much more recent 2012 earthquake that, fortunately, was far less destructive.

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The eruption of El Chichón volcano in 1982

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Mar 282012
 

Not all volcanoes give any warning of impending activity. Exactly thirty years ago, just before midnight 28/29 March 1982, the El Chichón volcano in Chiapas erupted completely without warning and with unexpected fury.  Two further eruptions followed in early April. The lack of warning caused heavy loss of life among local villagers who had been unable to evacuate their villages. About 2,000 people lost their lives as a result of the eruption.

Palenque covered in ash following the eruption of El Chichón

Palenque covered in ash following the eruption of El Chichón. Photo: Tony Burton; all rights reserved.

Ash from El Chichón fell over a wide area of southern Mexico. The nearby Mayan archaeological site of Palenque (set on the edge of what is normally a luxuriant, tropical-green jungle) was covered in ash (see photo above).

Concerned about the potential for the ash to combine with rainfall and form an acidic solution that might erase delicate and intricately carved stones, workers at the site engaged in a major clean up, even before all the ash had stopped falling. The second photo (below) shows a worker on top of one of Palenque’s distinctive roof combs sweeping the recently-fallen ash off the structure.

Sweeping ash off Palenque following the eruption of El Chichón

Sweeping ash off Palenque following the eruption of El Chichón. Photo: Tony Burton; all rights reserved.

Vulcanologists later worked out that the last previous eruption of El Chichón had been 1200 years earlier. The eruption “left behind a brooding, sulfuric, acidic lake that formed when the dome collapsed into a crater and filled with water.”

Aztec glyph for a “hill that smokes”

El Chichón forced more than 7 million tons of sulfur dioxide and 20 million tons of particulate material into the stratosphere. The resultant cloud of volcanic gases circled the Earth in three weeks and was still dissipating three years later. It was expected that the additional particulates in the atmosphere would reduce the solar radiation reaching the earth and cause the following summer to be cooler than usual. However, in an unlikely coincidence, an El Niño event began that same year, negating any significant cooling effect from the volcano’s particulates.

The El Chichón eruption was one of the largest volcanic eruptions of the 20th century, exceeded only by the 1991 Mt. Pinatubo eruption in the Philippines in terms of the amount of volcanic gases and particulates entering the stratosphere. Ash fell over a wide area, from Campeche to San Cristóbal de las Casas in Chiapas.

By the time the eruption was over, the volcano, whose summit had been 1260 m (4134 ft) prior to the eruption, had lost 200 m in height. The Chiapanecan Volcanic Arc, which includes El Chichón, falls outside Mexico’s Volcanic Axis (the location of almost all Mexico’s volcanoes) and is thought to be related to the subduction of the edge of the Cocos Plate underneath the North American plate.

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Mexico’s geomorphosites: the volcanic spine of Tequila Volcano, Jalisco

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Oct 062011
 

The small town of Tequila, the center of production of Mexico’s national drink, lies in the shadow of an imposing 2700-meter (8860-ft) volcano. Most visitors to the town visit the National Tequila Museum, take a distillery tour, and then sample one or two of the many world-famous brands of tequila made in the area.

The spine of Tequila Volcano

The spine of Tequila Volcano. Drawing by Mark Eager (Western Mexico, A Traveler’s Treasury); all rights reserved.

Tequila Volcano, which overlooks the rolling fields of blue agaves required to make the liquor, is the home of one of Mexico’s most distinctive geomorphosites. From the rim of its crater, the most arresting thing about the view is not the green, tree-covered crater itself but the giant monolith with almost vertical sides rising perpendicularly from the middle of the crater floor.

This well-preserved central spine, known locally as la tetilla (“the nipple”) is quite unusual. It represents the hardened lava which cooled in the central vent of the volcano and which, solid and unyielding, was later pushed upwards by tremendous subterranean pressure.

Few such good examples exist anywhere in the world. The example most often quoted in geography texts is the spine that was pushed up by Mont Pelée on the island of Martinique in the West Indies in October 1902, immediately prior to that volcano’s disastrous eruption which cost 32,000 lives.

How to get there

A cobblestone road begins near the railway station in the town of Tequila and winds up Tequila Volcano towards the short-wave communications tower on its rim. It is about 20 kilometers from the town to the rim. The hike or drive up to the rim affords glorious views over the surrounding countryside. As you gain altitude, so the vegetation changes, becoming luxuriant pine-oak forest well before you reach the rim. Looking across the crater, on a day when clouds slowly drift across and partially obscure the view, is like watching a silent movie of ancient Chinese landscape drawings.

Want to read more?

For a fuller description of a visit to Tequila Volcano and a climb up the volcanic spine, see John and Susy Pint’s Outdoors in Western Mexico (2nd edition 2011).

For a description of Tequila Volcano and the varied villages and sights in its vicinity, see chapters 9 and 10 of my “Western Mexico: A Traveler’s Treasury” (Sombrero Books, 2013), also available in a Kindle edition.

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