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:

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  • 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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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).

The Teziutlán disaster of 5 October 1999, a case study of vulnerability

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Oct 042012
 

Today marks the 13th anniversary of a major disaster that struck Teziutlán (current population about 65,000), a small city in the Eastern Sierra Madre, in the northeast corner of the state of Puebla, close to the border with Veracruz. The city is noteworthy as the birthplace of two prominent twentieth-century politicians: Manuel Ávila Camacho (served as President, 1940–1946) and Vicente Lombardo Toledano, who founded the Confederación de Trabajadores de México (CTM), Mexico’s largest confederation of labor unions.

The town’s name means “place of the hailstones”. But in 1999, it was not hailstones but torrential rain that triggered the major disaster of 4/5 October, with parts of the city destroyed by a series of landslides and mudflows. More than 80 municipalities were affected to some degree by this tremendous storm. Hundreds of landslides occurred in Hidalgo, Veracruz and Puebla states, causing an estimated US$457 million worth of damage, and at least 260 deaths.

This post focuses only on the consequences for Teziutlán where several hundred homes were completely destroyed and almost a thousand homes suffered partial damage. More than 100 Teziutlán residents lost their life. The local infrastructure, roads, housing, schools and farming were all severely impacted.

The worst damage was in the La Aurrora district on the eastern side of the city, where a landslide on a 23-degree slope buried more than 130 people. In another district, La Gloria, in the western part of the city, several more slips, flows and slides damaged homes, but without any fatalities.

With the benefit of hindsight, this disaster offers a good case study of the factors which made the inhabitants of Teziutlán particularly vulnerable to such an event. The diagram suggests one general classification of the multitude of factors that can affect vulnerability. In the case of Teziutlán, the discussion that follows suggests that the physical factors were probably the most significant.

factors affecting vulnerability

Factors affecting vulnerability (Geo-Mexico, Figure 7.2) All rights reserved.

Physical factors

1. Relief and geology. The area ranges in elevation from 300-2,280 meters above sea level and is drained by the El Calvario, Xóloatl and Xoloco rivers. The city is located at the southern limit of the Eastern Sierra Madre (Sierra Madre Oriental), very close to where it is truncated by the geologically more recent Volcanic Axis. The local geology includes a series of loosely compacted, pumice-rich pyroclastic flows, most of which are thought to be associated with the Los Humeros caldera. These deposits are interlaced with palaeosoils rich in clay which are impermeable and restrict the infiltration of rainwater, and overlie older folded rocks. The combination of steep slopes and impermeable, unconsolidated layers increases the risk of landslides and other forms of mass movement.

2. Climate.  This mountainous region is one of the most humid and foggiest in Mexico, averaging 280 days of mist or fog each year. Teziutlán has an average precipitation of 1600 mm/yr, though totals of over 2000mm are not that uncommon. Most rain falls between July and October.On 4 October 1999, a moist tropical depression off the coast of Veracruz was prevented from moving by a cold front. This led to an increase in humidity followed  by torrential downpours (over 300 mm of rain) over Teziutlán and the surrounding area. The storm continued the next day when a further 360 mm of rain fell.  The rain that fell on just those two days was equivalent to about 40% of Teziutlán’s usual total for an entire year.

3. Earthquake A few days prior to the storm, on 30 September 1999, a 7.4-magnitude earthquake occurred off the coast of Puerto Escondido (Oaxaca). This earthquake did cause  minor cracks in some homes in Teziutlán, and it possibly played a (minor) contributory role in the severity of the storm’s impacts.

Environmental

Deforestation, as a consequence of unplanned urban growth, was also important. Natural and secondary woodlands were steadily being cleared for construction and agriculture. This had an adverse effect on infiltration rates and the capacity of the land to absorb rainwater. However, given the extreme magnitude of the rainfall event, it is unlikely that the area would have escaped unscathed, even if the natural forest had remained.

In the La Aurrora district, where a landslide/mudflow buried more than 130 people, the construction of  a cemetery on a hill above La Aurrora may have played a part, since it appears that a cemetery wall held rainwater back, allowing more of it to seep into the underlying slope, increasing its susceptibility to a serious slide.

La Aurrora, October 1999. Credit: Periódico Sierra Norte

La Aurrora, October 1999. Credit: Periódico Sierra Norte

Educational

The town had suffered severe mass movements during prior storms. For example, in 1955, the rains that accompanied Hurricane Janet provoked numerous mass movements resulting in the disruption of transport systems, including the main highway, but with no loss of life. However, in general, it is clear that these prior events did not increase Teziutlán’s preparedness for a similar event in the future. In particular, prior events did not lead to building regulations being enforced or prevent buildings from being erected in high-risk areas.

Social/Demographic

In the period following the last major event (in 1955), the population of Teziutlán had increased rapidly, leading to the equally rapid expansion of the urban area. This was uncontrolled and included construction on steep slopes with insufficient attention to stability or possible mass movement mitigation measures being taken. It is worth noting that the population has continued to increase rapidly since the disaster, too.

Economic

Home owners in Mexico do not generally carry insurance on their properties, and even when they do, it often specifically excludes major meteorological events. It is unlikely that any of the residents of Teziutlán were able to make insurance claims. Many of the those affected would not have had savings and would have been forced to rely on family, friends and emergency hand-outs to survive. As a 1999 BBC News article emphasizes, government help was slow to arrive.

Want to see more?

There are several Youtube videos with images of the disaster. Perhaps the most interesting is TEZIUTLAN 1999 – 10 años Después del desastre  because it includes some clips from an investigative 1995 TV program aired in 1995 (four years prior to the landslide) that highlighted the extreme risk of constructing unauthorized buildings on the steep slopes of the town along the main highway. This video includes many excellent photos [warning: some graphic images] of the landslide and its aftermath, with a commentary [in Spanish].

Other valuable Youtube resources include Teziutlan Desastre 1999 which has additional photos, plus some eyewitness memories of the event [in Spanish], and TEZIUTLAN historia y tragedia which has many photos of the disaster, accompanied by music only (no commentary) making it a good choice for English-speaking classes.

Sources:

Alcántara-Ayala, I.  Flowing Mountains in Mexico. Mountain Research and Development, Vol 24, No. 1, Feb. 2004: 10-13.

Flores Lorenzo, Pablo & Irasema Alcántara Ayala. Cartografía morfogenética e identificación de procesos de ladera en Teziutlán, Puebla. Boletín del Instituto de Geografía, UNAM. #49, 2002, pp. 7-26. [pdf file]

 

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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The seven main canyons in the Copper Canyon region

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Jul 302012
 

The Copper Canyon region in Mexico is the informal name for the area, in the south-west part of Chihuahua state, where several deep canyons bisect the Sierra Tarahumara. The 10,000 km2 area, part of the Western Sierra Madre, is home to about 50,000 Tarahumara Indians, one of the largest native Indian groups in North America. While generally referred to in English as the Tarahumara, the people’s own name for themselves is Raramuri“, literally “the light‑footed ones” or “footrunners”.

Location of Barrancas del Cobre (Copper Canyon region)

Location of Barrancas del Cobre (Copper Canyon region)

While the Tarahumara have so far succeeded in keeping many aspects of their distinctive culture relatively unadulterated, the pressures on them have increased considerably in recent years as improving highway links have made the region more accessible, not only to tourists, but also to developers looking to exploit the region’s forest and mineral resources.

Spanish-speakers usually refer to this region as the “Barrancas del Cobre” (Copper Canyons, plural). The table shows the seven main canyons, only one of which, strictly speaking, is the Copper Canyon. The precise number of canyons depends on whether they are defined by rivers or by local names since different stretches of canyon along a single river have sometimes been given different names.

Canyon Elevation at the rim (meters / feet a.s.l.) Elevation of stream in canyon floor (meters / feet) Depth (meters/feet)
Canyons south and east of railroad
Urique (south of Urique village) 2370 / 7775 500 / 1640 1870 / 6135
Sinforosa (Río Verde) 2530 / 8300 700 / 2300 1830 / 6000
Batopilas 2500 / 8200 700 / 2300 1800 / 5900
Urique (mid-point, aka Copper Canyon) 2300 /7545 1000 / 3280 1300 / 4265
Canyons north and west of railroad
Candameña (below Basaseachi Falls) 2540 / 8330 900 / 2950 1640 /5380
Chinipas 2000 / 6560 400 / 1310 1600 / 5250
Oteros 2220 / 7280 700 / 2300 1520 / 5980

The major canyon is the Urique Canyon. This is the one seen by most tourists because it is the closest to the railway line that traverses the region. Both the Urique River and the Batopilas River flow into the River Fuerte, which enters the Gulf of Mexico near Los Mochis.

Tarahumara place names

The Tarahumara have very few place-names. They do not usually have identifying names for specific mountains, streams, trails or landmarks , but do give names to every small settlement, even if it only consists of two or three homes.  These names serve to distinguish one family from another, but a single family may have several farms, each with a different name. The Tarahumara do have “a rather complete terminology for plants, animals, and birds.”  [Bennet & Zingg, 1935] The place-names for settlements are usually two-part names, consisting of a descriptive name plus a place suffix.

Examples include:

  • Aworítci, from aworíki, “cedar tree” (tci gives idea of a grove of trees).
  • Wisarótici, from wisaró, “poplar tree.”
  • Tcimétabo, from tcimáka, “leather money bag”, plus –tabo, place-ending.
  • Wagítali, from wagítci, “dead tree.”
  • Garitcí, from garíki, “house.”
  • Kusárare, from kusáka, “eagle.”

In future posts, we will delve further into the geography of the  Copper Canyon region and the lifestyle of the Tarahumara Indians.

Sources:

Bennett, W. and Zingg, R. (1935) The Tarahumara. Univ. of Chicago Press. Reprinted by Rio Grande Press, 1976. Classic anthropological work.

Gajdusek, D.C. (1953) “The Sierra Tarahumara” in Geographical Review, New York. 43: 15‑38

Schmidt, R.H. (1973) A Geographical Survey of Chihuahua, monograph #37 Texas Western Press.

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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

Related posts:

Jun 072012
 

The Peña de Bernal, in the central state of Querétaro, is one of Mexico’s most distinctive geomorphosites. Geomorphosites are “landforms that have acquired a scientific, cultural/historical, aesthetic and/or social/economic value due to human perception or exploitation” (Panizza M., 2001). See Geotourism and geomorphosites in Mexico for a brief introduction to the topic.

The Peña de Bernal is a dramatic sight, which only gets more imposing the closer you get. How high is the Peña de Bernal? We are unable to give you a definitive answer (it depends where you start measuring from) but claims of 350 meters (1150 feet) sound about right, assuming we start from the town.

Peña de Bernal. Photo: Tony Burton; all rights reserved

The Peña de Bernal. Photo: Tony Burton; all rights reserved.

According to its Wikipedia entry, this is the “third tallest monolith in the world”, apparently only exceeded by the Rock of Gibralter and Sugarloaf Mountain in Rio de Janeiro. Others, including Melville King, have described it as the “third largest rock in the world”. These claims may (or may not) be exaggerated, but in reality it is definitely a very steep and tiring climb, even to reach the small chapel that has been built half-way up! The photo below is taken from this chapel, looking out over Bernal and the local farmland and vineyards.

View from the Peña Bernal, with the town of Bernal in the foreground.

View from the Peña de Bernal over the small town of Bernal. Photo: Tony Burton; all rights reserved.

How was the Peña de Bernal formed?

The most likely explanation is that this monolith represents the hardened magma (molten rock) from the central vent of a former volcano. This rock was much more resistant to erosion that the layers of ash and/or lava that formed the volcano’s flanks. Centuries of erosion removed the sides, leaving the resistant core of the volcano exposed as a volcanic neck. We will examine this idea in slightly more detail in a future post.

The town of Bernal

The town of San Sebastián Bernal is also well worth visiting. Having become a magnet for New Age types, it now boasts several decent restaurants, good stores and a range of hotels including high quality “boutique” hotels. Bernal was designated one of Mexico’s “Magic Towns” in 2005. To learn more about the town of Bernal and see some fine photos, we highly recommend Jane Ammeson’s article “The magic of Bernal, Querétaro: wine, opals and historic charm.

At the Spring Equinox (March 21), the town is invaded by visitors “dressed in long, white robes or gowns, and red neckerchiefs” who come seeking “wisdom, unity, energy and new beginnings”. (Loretta Scott Miller writing in El Ojo del Lago, July 1997).

How to get there:

From Mexico City, take the Querétaro highway (Hwy 57D) north-west to San Juan del Río. Then take Highway 120 past Tequisquiapan as far as the small cross-roads town of Ezequiel Montes. Turn left for about 11 kilometers, then right… and you’re there! Taking this route gives you glimpses of the Peña de Bernal from afar. Allow 2.0 to 2.5 hours for the drive.

Other geomorphosites worth visiting:

Mexico has literally thousands of geomorphosites. Among those described in previous posts are:

 

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.