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.

Mexico’s geomorphosites: The Piedras Bola (Stone Balls) of the Sierra de Ameca, Jalisco

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Sep 272011

The Sierra de Ameca is a range of hills a short distance west of Guadalajara. The area was important in colonial times for gold and silver mining. One of the mines is called Piedra Bola (Stone Ball). The landscape immediately around this mine is so distinctive and unusual that it featured on the cover of the August 1969 edition of National Geographic.

In the middle of the forest surrounding the Piedra Bola mine are about a hundred strange stone balls. They are almost perfectly spherical and range in diameter from about sixty centimeters to more than ten meters. These symmetrical boulders are unusually large. Nothing quite like them exists elsewhere in Mexico and few similar examples are known anywhere in the world.

Piedras Bola

Piedras Bola

Some are buried, others partly or fully exposed. In some places, erosion of the surrounding rocks has left a sphere perched precariously atop narrow columns of softer rock, seemingly ready to topple in the next strong wind. These “hoodoos” or earth pillars have been formed as a result of water erosion and they may survive for centuries until the processes of sub-aerial weathering and erosion finally cause them to fall.

Piedra Bola atop an earth pillar

Piedra Bola atop an earth pillar

How were the Piedras Bola formed?

This summary of the most likely explanation of the origin of the stone spheres is based on that offered by Dr. Robert Smith of the U.S. Geological Survey in the original National Geographic article.

During the Tertiary geological era, 10-12 million years ago, a local volcano erupted, causing a deluge of glassy fragments of molten lava and ash, together with large quantities of volcanic gas trapped in the mixture. The mixture was very hot, probably between 550 and 800̊C. The deluge of material partially filled an existing valley, burying the former surface.

As the mixture cooled down, the existing glassy fragments formed nuclei around which much of the remainder of the material crystallized. Spherical balls began to form, their size depending on how long the crystallization process continued uninterrupted. The longer the time, the bigger the ball…. The most perfect balls were formed near the previous ground level, inside the hot mass of ashes, where the cooling would have occurred more evenly than in the bulk of the matrix material. The crystallized material is a kind of rhyolite which has an identical chemical composition to the fragments of glassy obsidian also found in the area.

The remainder of the ashes cooled down and became a consolidated accumulation of ashes and glassy fragments or tuff, without clearly defined spheres. This tuff is weaker, and has a lower density than the stone balls within it. During succeeding millenia, the combined processes of physical and chemical weathering weakened the surrounding tuff, and water (rain and rivulets) then eroded away this loose material, exposing some of the rhyolitic boulders completely and others partially.  As these processes continue, so more of the boulders will be exhumed from beneath their cover of tuff, and be revealed to us.


The Jalisco State government has developed a small park around the Piedras Bola, including decent trails, some signposts and an amphitheater. There are even (reportedly) two ziplines, though I haven’t yet had the dubious pleasure of seeing them for myself. Increasing the number of visitors to  geomorphosites is not a bad idea, but some basic education and protection is needed if these and other geomorphological sites are going to be preserved intact for future generations. In the case of the Piedras Bola, graffiti now mar many of the exposed stone spheres and some of the spheres have been dynamited, apparently in the mistaken belief that the center of the sphere contained gold.

picture of piedras bolaHow to get there:

The entrance road to the Piedras Bola (formerly only a hiking trail) begins from km. 13 of the paved road that crosses the mountains from Ahualulco to Ameca. For anyone who does not have time for the hike, but still wants to see what these extraordinary stone spheres look like, the locals have thoughtfully rolled one down the mountain and onto Ahualulco’s main plaza.

Want to read more?

For more images and details, see John Pint’s article, Las Piedras Bola: the great stone balls of Ahualulco, on MexConnect, together with his outstanding gallery of photos.

How were the Piedras Encimadas (Stacked Rocks) in Puebla, Mexico, formed?

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Jun 072011

The Valle de las Piedras Encimadas (Valley of the Stacked Rocks) is 150 km from the city of Puebla in the northern part of the eponymous state. The Piedras Encimadas are rock outcrops occupying an area of about 4 square km (990 acres) centered on a small valley at an altitude of 2400 meters above sea level. The dominant natural vegetation is pine-oak forest. The main natural attraction of the area are the numerous, fascinating and photogenic”stacked rock” formations.

Piedras Encimadas, Zacatlán
Piedras Encimadas, Zacatlán, Sierra Norte, Puebla

The stacked rocks of the Piedras Encimadas can easily be likened to people (soldiers, sentries) and animals (dinosaurs, elephants, turtles), depending on the sensibilities of the observer. The shapes appear even more “fantastic” on the frequent occasions when clouds roll into the valley, enveloping the rocks in a thin mist.

According to geography researchers from the National University (UNAM), the volcanic rocks (rhyolites and andesites) forming the Piedras Encimadas date from the Tertiary period (60 million years BP).

The Piedras Encimadas look very similar to the much-studied granite “tors” found in the UK and elsewhere. Indeed, they may even have been formed in a similar way. However, geologists still debate precisely how tors were formed, and their uncertainties almost certainly apply equally to the Piedras Encimadas.

  • Theories for the formation of tors on Dartmoor, UK

Most theories of tor formation (see link)  involve the concept of “differential weathering”. This occurs when some parts of an area weather (disintegrate) more rapidly than others. Differences in weathering rates result from a variety of reasons, including differences in rock types and resistance within the same rock type, as well as localized changes in the climate, vegetation cover, aspect (direction the slope faces), altitude or exposure to air or water.

Tor formation (after Linton).
Tor formation (after Linton). Fig 3.5 of B.W. Sparks: Rocks and Relief (1971)

Over a long period of time, the weaker parts of the rock may have been weathered to greater depths than the more resistant parts. If subsequent erosion, most likely by river action in the context of Puebla, stripped away all the weathered rock, it would leave the more resistant rock as upstanding craggy outcrops (see sequence diagram above)

The shape of many of the blocks of rock forming the Piedras Encimadas does suggest that they were originally weathered deep underground from chemical reactions they underwent as water percolated slowly down towards the water table. Such a process would have acted more on the upper faces of each block, rather than the lower faces, producing a block that was rounded above, and almost flat below.

If the blocks had been modified by erosion (the other major possible interpretation), then it is more likely that both the upper and lower faces of each block would be equally rounded or that the lower face would be more eroded than the upper face.

Regardless of the details, it is almost certain that the curiously-shaped Piedras Encimadas were formed by a combination of volcanic action, differential weathering and erosion. The Piedras Encimadas offer lots of interesting possibilities for geography fieldwork.

Related posts:

How did the world’s deepest water-filled sinkhole form?

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Oct 112010

In an earlier post, we described the El Zacatón sinkhole, the deepest water-filled sinkhole known at present anywhere on the planet. Such a large sinkhole begs some important questions:

How did such a large sinkhole form?

Most major sinkholes form as a result of the collapse of the ceilings of underground cavities which have been formed by the gradual dissolution of limestone due to percolating acidic rainwater. However, according to Marcus Gary, of the Jackson School of Geosciences at the University of Texas at Austin, who has studied this area since the 1990s, the Zacatón pit is not a conventional sinkhole. He believes that this pit began to form in the Pleistocene period as a result of underground volcanic activity. Volcanism increased the acidity of water deep underground which then gradually ate away at the surrounding limestone in a process known as “hypogenic karstification“. As the underground caverns grew larger, the overlying rock would periodically collapse into them, eventually leaving giant pits extending to the surface above.

Is the sinkhole continuing to get deeper?

Equally interestingly, some of the sinkholes appear to be closing over. All the major sinkholes in Tamaulipas contain lakes and in several cases, they appear to be crusting over with travertine, a form of calcium carbonate which, in the right conditions, can be precipitated out of calcium bicarbonate-rich water. According to Marc Airhart, another researcher at the Jackson School of Geosciences, the process is probably an excruciatingly slow one, but at least one sinkhole (Poza Seca) has closed up entirely, sealing off an underwater lake. This travertine skin may also explain why El Zacatón has floating islands. It seems likely that pioneer species may have colonized small rafts of travertine, beginning the series of ecological processes that resulted in the grassy islands that can be seen there today.

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!

Jun 162010

Thousands of dinosaur bones have been found in northern Mexico.

Bones literally litter the ground. Here’s a femur; there’s a tibia; vertebrae, ribs, skulls…

Dozens of dinosaurs have been unearthed in a broad belt across northern Mexico, from Baja California and Sonora in the west, through Chihuahua, and Coahuila to Nuevo León and Tamaulipas in the east. Dinosaurs lived from around 230 to 65 millions years ago. Most of the dinosaurs dug up in northern states date back about 70 million years, though those found in the Huizachal canyon in Tamaulipas include the oldest known dinosaur bones in Mexico, from the Jurassic period 180 million years ago.

Earlier this year, the discovery of a new species of dinosaur, Coahuilaceratops magnacuerna, was reported from Coahuila. It is 6.7 meters long, 1.8 meters tall, and weighed 4.5 tonnes, with two 1.2-meter-long horns, longer than any other dinosaur.

Dinosaur area in Coahuila

There are two prime locations for dinosaurs in the state of Coahuila in northern Mexico. It is hard to imagine today, but millions of years ago the southern part of the state was a vast river delta bordering the ocean. It appears that literally hundreds of dinosaurs floundered in the mud while trying to forage for food. Conditions changed and fossilization preserved the remains of these beasts for prosperity.

Coahuila’s most famous dinosaur was discovered at Presa San Antonio, some seventy kilometers west of the state capital Saltillo. Between 1988 and 1992, excavations on ejido land, led by Dr. René Hernández of the National University (UNAM), succeeded in recovering more than 70% of the bones of a herbivorous Kritosaurus.

The world’s cheapest dinosaur

When reassembled, the creature was 7 meters (23 feet) long and stood 3.8 meters (12.5 feet) tall. It also became the cheapest dinosaur in the world. Excluding Dr. Hernandez’s salary, its excavation cost just 15,000 pesos, equivalent at the time to 5,000 dollars. Making a replica for the Institute of Geology Museum in Mexico City cost a whopping 40,000 pesos. The replica (218 bones) is named “Isauria”, apparently because one of the first high school students to view it was overheard to say that it looked just like Isaura, one of their fellow students!

The second location is even closer to Saltillo. The Rincón Colorado ejido is 43.5 kilometers west of the state capital along federal highway 40, the road to Torreón. Here, a small village, located three kilometers north of the road, boasts Mexico’s only “on-site” paleontology museum, complete with lots of dinosaur bones and imprints.

The displays in the Rincón Colorado museum reveal the wide diversity of fauna that once thrived in this area, including fish, turtles, crocodiles, sharks, bivalves, gastropods, ammonites as well as a considerable variety of flora, as shown by tell-tale signs of at least nine different kinds of fruit. Today’s vegetation is much less varied, consisting primarily of plants like agaves, prickly pear cacti, creosote bushes and acacias that can survive in the harsh semi-arid conditions.

Dinosaur museum in Coahuila

Visitors to the museum can also obtain permission to visit the dinosaur dig, located a short distance away on arid, red-tinted, scrub-covered hillsides, one of which, formerly called Cerro de la Virgen, is now known as Cerro de los Dinosaurios. Bones litter the ground, and more than 20 dinosaurs, dating from the Late Cretaceous period, have so far been found in an area of 25 square kilometers. They include specimens of duck-billed hadrosaurs, one of which still sports its toenails, horned ceratopids (three rhinoceros-like horns on the head), carnivorous tyrannosaurids and dromaeosaurids. One triceratopid still had some skin attached to its bones, making it a very rare find. Hadrosaurs are particularly common and may have been amphibious.

See also:

  • Dinosaur Bones in Mexico on MexConnect.
  • Fosiles de Dinosaurios en Coahuila. Gobierno de Coahuila (SEPC) (no date)
  • Hernández H., René Dinosaurios. Gobierno de Coahuila (SEPC) (no date)