Popocatapetl Volcano and Colima Volcano continue to erupt

 Mexico's geography in the Press, Updates to Geo-Mexico  Comments Off on Popocatapetl Volcano and Colima Volcano continue to erupt
Mar 302013
 

In our series of brief updates on topics featured in previous Geo-Mexico posts, we look this week at the continuing eruption of two major volcanoes: Popocatapetl Volcano (between Mexico City and Puebla) and Colima Volcano (on the Jalisco-Colima state border in western Mexico).

Popocatepetl, 30 July 2012

Popocatepetl, 30 July 2012

Since our previous post, about a year ago, entitled Alert level rises as Popocatepetl volcano starts to erupt, Popocatapetl Volcano (photo) has continued to be active, with up to 250 activity events a day. The alert level has been reduced slightly to Yellow Phase 2, the fourth highest level. This level indicates intermediate scale explosive activity and possible expulsion of lava, explosions of increasing intensity and wind-blown ash falling on nearby villages. The volcano is monitored daily, and updates from CENAPRED  (in Spanish and English) are issued every 24 hours.

The report issued on 27 March is typical of recent months. In the previous 24 hours, there were 83 low intensity events with emissions of gas, water vapor and ash. The two largest events sent material rising 1000 meters and 600 meters into the atmosphere respectively, before the wind blew the material north eastwards (away from Mexico City).

Colima Volcano

In January 2013, we reported how Colima Volcano erupts, destroying lava dome first created in 2007. The volcano has continued to erupt in the ten weeks since then. The experts monitoring the volcano have reported up to 200 eruptive events a day, with numerous minor emissions of lava. Local villagers have been asked to remain on alert, though the experts are not yet calling for any villages to be evacuated.

The image below (source: Nasa Earth Observatory) shows Colima Volcano in 2010, part way into its current eruptive phase which is expected to last several years. The image shows the evidence at that time of four different types of volcanic activity:

  • lava dome growth
  • explosive eruptions
  • flank collapse
  • lava flows.

(Note that the 2013 eruptions have significantly altered the top of the volcano since this image was taken).

Nasa Earth Observatory)

Colima Volcano in 2010 (Nasa Earth Observatory)

Related posts:

Paricutin Volcano in Mexico celebrates its 70th birthday

 Other  Comments Off on Paricutin Volcano in Mexico celebrates its 70th birthday
Feb 202013
 

Today (20 February 2013) marks the 70th anniversary of the first eruption of Paricutín Volcano in the state of Michoacán in western Mexico.

The landscape around the volcano, which suddenly started erupting in the middle of a farmer’s field in 1943 and which stopped equally abruptly in 1952, is some of the finest, most easily accessible volcanic scenery in the world. A short distance northwest of Uruapan, this is a geographic “must see”, even if you can only spare a few hours.

Paricutín Volcano, 16 July 1943

Paricutín Volcano, 16 July 1943

What makes Paricutín so special is that in all of recorded history, scientists have had the opportunity to study very few completely new volcanoes in continental areas (whereas new oceanic island volcanoes are comparatively common). The first two new volcanoes formed in the Americas in historic times are just one hundred kilometers apart. The first was Jorullo, which erupted in 1759, and the second is Paricutín.

Dominating the valley where Paricutín now exists is the peak of Tancítaro, the highest point in the state of Michoacán at 3845 meters (12,615 feet), and sometimes snow-capped in winter. In 1943, the local Purépechan Indians inhabited a series of small villages and towns spread across the valley floor. The villages included Angahuan, which still exists today, Paricutín, where all 500 people lost their homes, and San Juan Parangaricutiro. The latter was once famous for hand-woven bedspreads and quilts and consequently known as San Juan de las Colchas (bedspreads). Its church, begun in 1555, had never been finished and only ever had a single tower.

On 20 February 1943, a local campesino named Dionisio Pulido was tending his crops with his wife Paula, their son and a friend. At about 4:00pm, they noticed a small crack and felt the ground shaking under their feet. While they watched, the ground rose more than 2 meters and smoke rose into the air, accompanied by whistling noises and the smell of sulfur. Sparks set fire to a nearby pine tree. Not surprisingly, they fled!

Legend has it that Dionisio first tried to smother the emerging volcano with loose rocks and afterwards was of the opinion that the volcano would never have erupted if he hadn’t plowed his field, but such reports are almost certainly pure fiction.

The volcano grew rapidly, providing onlookers, visiting vulcanologists and residents alike, with spectacular fireworks displays. The month of March was a particularly noisy time in Paricutín’s history—explosions were heard as far away as Guanajuato and ash and sand were blown as far as Mexico City and Guadalajara.

After one week, the volcano was 140 meters high, and after six weeks 165 meters.

In April 1943 there were major lava flows, originating from about 10 kilometers underground. These lavas were basaltic; their chemistry suggested temperatures inside the volcano of between 960o and 1020oC. More than 30 million metric tons of lava flowed from the volcano between April and June 1943, raising the volcano’s height to more than 400 meters before its first birthday.

In early 1944, another lava flow streamed in a gigantic arc reaching the outskirts of the town of San Juan Parangaricutiro. Fortunately, the town had already been abandoned following many earthquakes, some of which rang church bells as far away as Morelia! The lava flowed about 30 meters a day and went right through the church but, miraculously, left the main altar standing. The parts of the church which survived in “old” San Juan, including its altar, can still be visited today, though reaching them involves clambering over jagged blocks of lava.

The villagers who had abandoned the town were escorted to Uruapan for safety. Some of them later founded a new town called San Juan Nuevo. Other villagers moved away to live in Los Reyes, Uruapan, Angahuan, Morelia and Guadalajara.

Yet another lava flow buried the village of Paricutin which fortunately had also been evacuated in time. Nothing remains of this village, covered by lava which is more than 200 meters thick in places! A small cross atop the lava marks its approximate position. Besides lava, ashes and dust were also thrown out by the volcano. The ash was 1 millimeter thick in Guadalajara, 25 centimeters thick in Angahuan, and 12 meters thick near the cone. The volcano rose 410 meters above the original ground surface.

Suddenly, in February 1952, nine years after the volcano first erupted, the lava stopped flowing. In many places, plumes of hot steam still rise as fumaroles from the ground, ground that is still most distinctly warm to the touch.

Enjoying a full day at Paricutín has been made much easier since the construction of rustic tourist cabins on the edge of Angahuan village. The restaurant, which serves tasty local specialties, gives clients a panoramic view encompassing the lava and the half-buried church. A small permanent exhibition of maps, charts and photographs in one of the cabins, describing the volcano’s history and the surrounding area, was inaugurated in February 1993 to commemorate the 50th anniversary of the volcano.

Horses can be hired for a trip to the church or to the more distant cone of the volcano. The latter requires an early start since the last part involves clambering up the loose ashes and cinders which comprise the cone and scrambling onto the narrow rim of the truly magnificent crater. A marvelous view can be enjoyed from atop the crater rim and only then can the full extent of the area devastated by the volcano be fully appreciated. Nothing can quite have prepared you for this startling lunar-like landscape.

Note:

This post is a lightly edited extract from my “Western Mexico: A Traveler’s Treasury” (Sombrero Books, 2013). Chapter 35 describes Paricutín Volcano and its surrounding area, including the fascinating indigenous village of Angahuan, in much more detail. “Western Mexico: A Traveler’s Treasury” is also available as either a Kindle edition or Kobo ebook.

Related posts:

Colima Volcano erupts, destroying lava dome first created in 2007

 Mexico's geography in the Press  Comments Off on Colima Volcano erupts, destroying lava dome first created in 2007
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).

Related posts:

Update on the activity of Popocatepetl Volcano

 Mexico's geography in the Press  Comments Off on Update on the activity of Popocatepetl Volcano
May 102012
 

In the past few days, Popocatepetl Volcano has continued to emit gas, steam and ashes, periodically shooting ash-laden clouds high into the sky. The columns of ash have risen up to 2500 meters above the volcano before drifting downwind. Depending on the wind direction at the time, light falls of ash have been reported from Mexico City (especially the Milpa Alta and Iztapalapa districts) and the city of Puebla.

Ash cloud rises above Popocatepetl

Ash cloud rises above Popocatepetl Volcano

The National University’s Atmospheric Sciences Institute has developed atmospheric models taking account of the volcanic emissions and is releasing regular forecasts of where ash is likely to fall.

Ramón Espinasa Pereña, who heads the Geological Risks department in the National Disaster Prevention Centre (Centro Nacional de Prevención de Desastres, Cenapred) expressed concern recently that Popocatepetl Volcano could be headed towards much more significant activity in coming weeks.

In an interview with Mexico City daily Excelsior, Espinasa started by saying that that the current level of activity is less than that experienced in 2000 when the volcano’s heat caused the melting of the glacier then found on the northwest side of the mountain. However, he added, the situation today is quite different. The dome of lava inside the crater of Popocatepetl Volcano has been growing, increasing the risks of a significant and possibly explosive eruption. In 1994, prior to the 2000 eruption, the crater of the volcano was about 800 meters long, 600 meters wide and 100 meters deep. The activity in 2000 raised successive domes of lava in the crater to within 20 or 30 meters of the crater rim. So far this year, the depth of the crater has remained about the same, but only because almost all the new material being added to the existing domes is being blown into the air.

Experts are concerned that the high density of the magma beneath the volcano may lead to the existing vents being blocked. If this happens, pressure will build up underground and greatly increase the possibility of a violent eruption.

Evacuation plans have been in place since 1994, and they have been modified and updated regularly since. There are ten major evacuation routes (see map). The villages most at risk (inside the 12-kilometer radius “high risk” zone) include several in the states of Puebla (San Nicolás de los Ranchos, Santiago Xalitzintla, San Pedro Benito Juárez, San Baltazar Atlimeyaya and Tochimilco), Morelos (Ocuituco, Tetela del Volcán, Yecapixtla, Zacualpan de Amilpas and Temoac) and the State of México (Tepetlixpa, Ozumba, Atlautla, Ecatzingo and Amecameca).

Popocatepetl Volcano: the planned evacuation routes

Popocatepetl Volcano: the planned evacuation routes

Last week, the evacuation system (that will only be put into effect if the risk level rises) was tested with a large-scale practice evacuation in which the Mexican Army assisted municipal and state officials and emergency response crews. The practice has enabled authorities to improve the forecasts of precisely how long it will take to evacuate all villagers from the likely danger zone, in the event that the risk level is raised.

Evacuation will not be easy. Some local inhabitants argue that the volcano has never caused them any harm, because, on the contrary, it is their “protector and guide”. They are unlikely to move voluntarily even if an eruption is imminent. They hold a festival each year on 12 March thanking the volcano for its rich soil, abundant rainfall and “to keep the volcano calm and happy.” The ceremonies include the placing of offerings part-way up the volcano, accompanied by folk dancing.

Related posts:

 

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).

Related posts:

 

Apr 222012
 

Popocatepetl is Mexico’s second largest volcano, after El Pico de Orizaba. Popocatepetl rises to a height of 5500 meters (18,045 feet) and is located approximately mid-way between Mexico City, about 80 kilometers (50 miles) to the northwest, and the city of Puebla, a similar distance to the east.

In the past week, Popocatepetl (aka “Popo” or “Don Goyo”) has sprung back into life, blowing off steam and ash in a series of minor eruptions, accompanied by minor earth tremors, many of which registered between 3 and 4 on the Richter scale. Incandescent rocks (“volcanic bombs”) have been thrown up to 1000 meters (3000 feet) from the crater down the slopes of volcano, and water vapor, gasses and ash have formed towering clouds, up to 2000 meters high, rising above the iconic volcano.

Popocatapetl lets off steam. Credit: Victor Hugo Rojas (Universal)

Popocatepetl lets off steam. Credit: Victor Hugo Rojas (Universal)

The latest eruptions of Popocatepetl come from an estimated sixty different vents that are connected to a magma chamber 10 km (6 miles) beneath the volcano that is thought to hold upwards of 1,000,000 cubic meters (36,000,000 cubic feet) of magma. On the one hand, the small eruptions are good news, since they relieve the pressure building up underground, at least temporarily. On the other hand, they may presage a much more serious and major eruption.

Thousands of families live in the farming villages on the lower slopes of the volcano; some 25 million people live within a 100 kilometer (60 mile) radius. In the event of a major eruption, and depending on wind directions, airborne ash could fall on Mexico City, interrupting normal activities and Mexico City’s busy international airport, or on the Metropolitan area of Puebla (population 2.7 million), an important industrial center, where Volkswagen has its main vehicle factory.

Even though geophysicists are unable to say whether or when a major eruption will actually occur, authorities have raised the threat level and taken steps to ensure that local residents can be safely evacuated, if necessary, to emergency shelters in nearby public buildings such as schools. Almost 200 temporary shelters have been prepared in nearby villages to house any people that are forced to leave their homes.

Mexico’s National Disaster Prevention Centre (El Centro Nacional de Prevención de Desastres, Cenapred) has raised the alert level to “yellow stage three” – the third highest level. This level indicates that a magma expulsion is possible and that the intensity of explosions is likely to increase.

In the event that the alert is raised still further, into the red zone, villages within a 12-km zone will be immediately evacuated, and the exclusion zone may be extended still further if this is deemed a prudent safety measure. During the last major evacuation, in 2000, nearly 50,000 residents in three states were moved into temporary shelters.

As in the case of previous volcanic eruptions in Mexico, such as Paricutín in the 1940s, the Mexican Army would take charge of ensuring that local residents are taken to safety.

Roads are being kept open, and emergency repaving is underway, in case an evacuation is required. Local villages are arranging to have sufficient buses standing by to ensure that their residents can be evacuated rapidly should the alert level be raised. Even so, it is unlikely that everyone would choose to leave, and it is thought that up to half the population might attempt to remain in their homes even if the alert level is raised.

Images of the volcano:

Health authorities have already distributed free face masks and bottles of water to families in the area. The cloth face masks are intended to filter out the fine ash released by the volcano, and reduce the likely increase in respiratory problems. Falling ash is also expected to lead to an increased incidence of allergic conjunctivitis.

Authorities in the city of Puebla have temporarily suspended open air activities until further notice since much of the city has received a thin layer of ash. Ash falls of about one centimeter have been reported in some districts of the city. Ash has also fallen over the nearby town of Cholula and as far away as Atlixco and Huejotzingo.

Puebla hoteliers, restauranteurs and merchants will be hoping that the city’s restriction on open air activities ends quickly, since the city is gearing up for the annual festivities associated with the 5 de Mayo (5 May) festival for which the city is famous. (The festival commemorates Mexico’s victory over French forces at the Battle of Puebla on 5 May 1862).

Related links:

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.

Related posts:

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.

Related posts:

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.

Related posts:

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.

Related posts: