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

 Other, Teaching ideas  Comments Off on Rocks and relief fieldtrip: Tequisquiapan and the Peña de Bernal
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

 Excerpts from Geo-Mexico  Comments Off on Mexico’s highest volcanoes
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

 Excerpts from Geo-Mexico  Comments Off on Mexico’s Volcanic Axis
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?

 Excerpts from Geo-Mexico  Comments Off on How has the movement of tectonic plates affected Mexico?
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?

 Excerpts from Geo-Mexico  Comments Off on Which tectonic plates affect Mexico?
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

 Excerpts from Geo-Mexico  Comments Off on The eruption of El Chichón volcano in 1982
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.” (Ref).

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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