Chapter 1

Mexico's Place in the World: The Physical Setting

Soon after the formation of the earth some 4.5 billion years ago, NASA scientists tell us that an asteroid -- probably about the size of Mars -- passed so close to it that its gravitational pull wrenched a massive, vaporized glob of its semi-molten surface spinning off into space. Unable to escape the earth's gravitation, this spinning glob, long-since cold and dead, continues to orbit our planet as the Moon. The gaping, water-filled wound it left behind now forms the Pacific Ocean, which, for over 3500 miles (5600 km), defines the jagged and seismically unstable shoreline of the west coast of Mexico.

         As recently as 65 million years ago, another asteroid -- this one hardly more than 110 mi. (180 km) in diameter -- glanced off the northern tip of the Yucatán Peninsula and slammed into the Gulf of Mexico.  The cataclysm which resulted from this collision not only caused a major global climatic change by shutting out the rays of the sun for many years afterward but likewise led to the extinction of much of the world's existing plant and animal life, including the dinosaurs.

         To be sure, these drastic extraterrestrial deformations of the earth's crust on the perimeters of present-day Mexico were only the most dramatic of the processes that have continued to shape the landscapes of modern Mexico.   Ever since the very origins of the Earth itself, a much slower and less violent process of landform creation and destruction has been going on due to the convective movement of molten rock within the earth's gradually-cooling mantle.  Where these currents converge, they move upward, causing the crustal plates above them to bulge, separate, and begin drifting apart.  Along such zones of contact -- a primary one lying on the floor of the middle of the Atlantic Ocean (the Mid-Atlantic Ridge) -- the upwelling lava oozed out through great fissures or built up long rows of volcanic cones.  A second such zone of sea-floor spreading is found in the eastern Pacific (the East Pacific Rise), extending from the west coast of Mexico to the edges of Antarctica.  The vast expanse of the Earth's crust lying to the west of this ridge makes up the Pacific Plate, while the part found to its east is divided between two smaller plates, the Cocos Plate in the north and the Nazca Plate in the south.  The Caribbean Plate, which is jammed into the very middle of the much larger American Plate, borders the Cocos Plate on its east.  The almost continuous motion of these plates is constantly generating friction along their edges, and where the subterranean currents collide head-on, the leading edges of the plates are destroyed in a fiery fury of volcanic and seismic activity as the currents are forced downward into the earth's depths.  It is in these so-called subduction zones that the deepest trenches in the ocean are found, that off the west coast of Mexico dropping off about 20,000 feet (6000 m) while that bordering the north coast of Puerto Rico falls over 27,000 feet (8300 m) in depth.  Due to Mexico's location on or near the junction of four separate tectonic plates, it ranks as one of the most volcanically and seismically active countries on Earth.

 

The Hidden Volcano

         In late March and early April 1982, a small volcano in Chiapas state known as El Chichón suddenly erupted after having been dormant for centuries. It spewed out over 20 million tons of sulfur-rich dust into the atmosphere, much of which circled the globe for several years afterward.  Locally it killed over 2,000 persons and the heavy fall of ash caused severe damage as far away as San Cristóbal de Las Casas, some 50 km to the southeast. Until its totally unexpected outburst, it was scarcely recognized as a topographic feature of any consequence and, because it was so isolated from other volcanoes, it had never seemed to pose any threat at all. But then, one has only to recall that in February 1943 a fissure opened in a cornfield in Michoacán and by the time its hiccups of lava and ash had terminated, a volcanic cone 2000 feet in elevation had been created. Mexico is no stranger to the inner convulsions of the earth and with increasing apprehension the residents of its capital city continue to eye the rumblings of nearby Popocatépetl.

         Not too surprisingly, about one-third of Mexico's bedrock is made up of igneous rocks formed by volcanism.  These are likewise among the youngest of Mexico's rocks, for this process continues apace even today.  Igneous rock constitutes the core of four of the country's principal mountain ranges: the Sierra Madre Occidental, the Volcanic Axis (which crosses the center of the country from east to west), the Sierra Madre del Sur, and the Sierra Madre de Chiapas.  There are also isolated outliers of igneous rock in Baja California, as well as in the eastern states of Coahuila, Tamaulipas, San Luis Potosí, and Veracruz, so there are few parts of the country which have escaped volcanism at some time in its geologic past.  Although earthquakes show a strong spatial correlation with volcanism, they can occur quite independently as well, whereas eruptions invariably are accompanied by seismic tremors.  The most active seismic zone in Mexico is found along and just off the Pacific coast.  When earthquakes occur in the offshore trench, they often produce sizable tsunamis, or shockwaves, some of which have had heights as great as 40 ft. (12 m) as they have crashed into the adjacent shore.  Because the frequency of earthquakes declines toward the north and east, the Yucatán Peninsula is the only region of Mexico that has never recorded an earthquake.

         The Volcanic Axis of Mexico, which closely follows the 19th parallel of latitude across the middle of the country, has demonstrated great instability for much of the last 65 million years, with the primary center of action gradually shifting from east to west.  Most of the highest mountains in Mexico are, in fact, the cones of stratovolcanoes, including Orizaba, or Citlaltépetl (18,700 ft. or 5700 m); Cofre de Perote, or Nauhcampatépetl (14,048 ft. or 4,283 m); Popocatépetl (17,887 ft. or 5,453 m); Ixtaccíhuatl (17,343 ft. or 5,287 m); Nevado de Toluca (14,409 ft., or 4,393 m); Tancítaro (12,660 ft. or 3,860 m); Nevado de Colima (13,993 ft. or 4,266 m); and Volcán de Colima (12,620 ft. or 3,848 m).   Dotting the highlands of Michoacán are more than 250 cinder cones, most only a few hundreds of feet or meters in elevation, one of whose most recent additions is Paricutín.

         Younger sedimentary rocks such as limestone, sandstone, and shale underlie by far the largest part of the nation's territory (about 64%). Such formations predominate along the Gulf coast, varying in width from some 900 mi (1500 km) in the north to about one-tenth that in central Mexico and broadening out once again in the south to embrace the Yucatán peninsula.  In the Sierra Madre Oriental, which forms the eastern edge of the vast uplift known as the Mexican Plateau, crumpled limestone and sandstone ridges reach elevations of nearly 12,500 ft. or 3,810 m, while the highest point in the Sierra Madre de Oaxaca is found in Zempoaltepec (11,138 ft. or 3,396 m).   Folded and faulted limestone ridges likewise make up the Meseta Central of Chiapas, reaching elevations over 9100 ft. (2874 m) near San Cristóbal de las Casas.  Along the Pacific, narrow patches of sedimentary rock underlie the coast from the U.S. frontier south to San Blas, as well as parts of Jalisco and Colima states, and again from the Isthmus of Tehuantepec south to the Guatemalan border.

         On the other hand, ancient rock formations composed of gneisses and schists occupy scarcely 4 per cent of Mexico's total area, the core region being located chiefly in the state of Oaxaca, eastern Guerrero, and southern Puebla.  Similar outcrops are also found along short stretches of the Pacific coast in Chiapas, Guerrero, Michoacán, Sinaloa, Sonora, and the northern part of Baja California.

         The most extensive areas of lowland in Mexico border the Gulf of Mexico.  Composed chiefly of alluvial deposits, they are primarily the creation of the large rivers that debouch along this coast.  These include the Rio Bravo (Rio Grande), the Soto de la Marina, the Pánuco, the Tuxpan, and the Tecolutla, all to the north of the Volcanic Axis; the Papaloapan and the San Juan in central Veracruz; and the Coatzacoalcos, Uspanapa, Zanapa, Grijalva, Usumacinta, and Candelaria, all to the east of the volcanic massif of the Tuxtlas.    About mid-way along the Sierra Madre de Oaxaca, the Rio Santo Domingo forces it way through this imposing mountain wall of up-folded limestones to allow the waters of the inner valleys of Oaxaca to join the mighty Papaloapan and discharge into the Gulf of Mexico.  However, farther to the east the porous, flat-lying limestone platform of the Yucatán has no surface drainage whatsoever, so the first actively flowing river one encounters after the Candelaria is the Hondo, which forms the border between Mexico and the country of Belize.

         The Mexican Plateau, a vast region whose boundaries are defined by the Sierra Madre Oriental to the east, the Sierra Madre Occidental to the west, the Volcanic Axis on the south, and the United States border on the north makes up the northern interior of the country.  The Plateau itself can be divided into two parts:  a lower, northern section which averages about 3600 ft. (1100 m) in elevation and represents a continuation of the Basin and Range Province of the United States, and a higher southern section which averages some 6500 ft. (2000 m) in elevation.  A portion of the northern Plateau drains by way of the Rio Bravo (Grande) into the Gulf of Mexico, another small section finds an outlet through the Rio Mezquital into the Pacific, but some of it also ends up draining into the interior basin of Mapimí.  The loftier basins of the southern Plateau are similarly diverse in their drainage patterns, that of the Valley of Mexico (7200 ft. or 2200 m) draining into the so-called Lake of the Moon in its interior while the yet-higher basin of Toluca to the west (7500 ft. or 2300 m) found an outlet through the Lerma and Santiago rivers to the Pacific.

         Tucked in between the Volcanic Axis on the north, the Sierra Madre del Sur on the west, and the Sierra Madre de Oaxaca on the east, is the structural basin through which the Balsas River flows.  In its central portion its elevation averages some 3300 ft. (1000 m), falling gradually to about 650 ft (200 m) in its western reaches. Although this region originally appears to have drained eastward into the Gulf of Mexico, the tectonic up faulting of the Sierra Madre del Sur and the Volcanic Axis gradually diverted its waters westward to the Pacific.

         Farther south another structural depression forms the Central Valley of Chiapas.  Floored by essentially horizontal layers of limestone, it is drained northwestward by the Grijalva into the Gulf of Mexico through a spectacular canyon called El Sumidero, a dramatic example of an antecedent stream cutting through a mountain massif uplifted over 4000 ft. (1200 m) in its path.

         Elsewhere in Mexico the most extensive area of lowland is to be found in the northwest along the mainland coast of the Gulf of California.  Varying in width from about 150 mi. (250 km) in the north to 45 mi. (75 km) in the south, this gently rolling plain is punctuated here and there by clusters of heavily eroded small hills.  Southward from the Bahía de Banderas, on which Puerto Vallarta is situated, many areas of ancient rock have been exposed along the uplifted coast and the few bays that are encountered are all ancient submerged valleys.  Not until the Isthmus of Tehuantepec is reached does a rim of relatively continuous alluvial plain again front the sea, deriving most of its sediments from the erosion of the adjacent block-fault mountains of the Sierra Madre de Chiapas.

 

The Earthquake that Didn't Happen . . . and A Couple That Did

         In the spring of 1978, after teaching in an off-campus program in Puebla, I had left my wife and daughter at a beach resort on the Pacific while a student assistant and I continued on to South America for a month of research in the Andes. On the flight back from Lima to Guatemala, I was given an English-language newspaper, the first I had seen for several weeks, so I avidly began devouring it to learn what had happened in the world during my absence. Imagine my shock and surprise when I found on the front page a report that a major earthquake whose epicenter was located at the beach resort where my wife and daughter were staying was "predicted" for 4 P.M. that very afternoon -- by two seismologists from Rice University in Houston, no less! Of course, my first reaction was one of panic, but my second was one of total disbelief. No one had ever managed to predict an earthquake before, and certainly not define its epicenter in advance, or tell us down to the hour and day when it would take place. I wondered if my wife and daughter knew anything of this preposterous prediction, and what, if anything, they had done or were going to do about it.

         It wasn't until I rejoined them a few days later that I got the whole story. Yes, they had known about the prediction; in fact, there was probably nobody in Mexico that hadn't known about it. The town where they were staying had been totally evacuated by its terrified residents, leaving my wife and daughter alone in an abandoned hotel.  Although the governor of the state had tried to reassure the citizens that the prediction was unfounded by throwing a cocktail party at precisely 4 P.M. as close to the designated epicenter as he could get, only a few people -- mostly party-hacks who had no alternative -- accepted the governor's invitation.  On the other hand, a lot of beachfront property in the resort town changed hands during the last hectic days before the calamity was supposed to occur. As a result, my daughter -- who is a trained geologist herself -- assumed that local real estate entrepreneurs might themselves have been involved in helping to disseminate the pseudo-scientific rumor.

         Speaking of earthquakes, when my student assistant and I had stopped in Guatemala City on our way to South America a few weeks earlier, a tremor of fairly violent proportions had caused us to flee our own hotel just as we were about to take a siesta. With pipes rattling against the walls, light fixtures swaying from the ceiling, and the beds themselves dancing across the room, we dashed out into the plaza where we waited almost an hour to see if there would be any aftershocks.   Finally, getting up our courage to re-enter the hotel, I remarked to the desk clerk, "That was quite a severe one". His face lit up with a smile and he shrugged his shoulders, replying only "Regular. Regular."

         The next day as the student and I were checking in for our flight to South America, I happened to look out of the large window in the airport concourse and noticed a volcano erupting at the end of the runway. Turning to a custodian who was mopping the floor nearby I said, "I suppose that earthquake yesterday got that volcano going, didn't it?" Hardly bothering to look up, he turned to me and replied, "No, no. Regular. Regular."

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         In September 1984 I was invited to participate in an archaeo-astronomy symposium in Mexico City.  Although the meetings were held at the University near the south edge of the city, I decided to stay at the venerable Hotel Regis on the Alameda, because of its centrality and its easy access to the Metro line. A year-to-the-day later, a major earthquake struck Mexico City causing thousands of casualties. The Hotel Regis collapsed, killing everyone in it.

 

 

         Mexico's rugged topography and its low-latitude location combine to produce a wide diversity of climates, ranging from tropical super-humid to cool-temperate arid.  Lying between the parallels of 15 and 33 degrees north latitude, Mexico comes primarily under the influence of the trade winds that bring moisture from the warm waters of the Caribbean and Gulf of Mexico onshore from the east.  Almost immediately these moisture-laden air masses are forced to rise over the mountain wall of the Sierra Madre Oriental, the Sierra Madre de Oaxaca, and the Meseta Central de Chiapas, cooling and condensing in the process and producing an almost continuous belt of lush cloud forest along the upper slopes of these ridges.  Once the air masses clear these heights, however, they flow down slope into the interior, warming and drying as they do so, leaving the western sides of the mountains in a "rain shadow".  Only when they encounter yet higher mountains farther to the west and are forced to rise again are they squeezed of yet more of their life-giving water.  Naturally, the lower the valleys are and the farther they lie from the Gulf, the more starved for moisture they are.  For this reason, the Balsas Depression is the hottest and driest region in all of Mexico.

         Fortunately for Mexico, this simple pattern of a trade wind soaked east and a parched rain-shadowed west is offset in part by a seasonal shift of winds that brings an inflow of air from the Pacific during the summer months.  As the landmass of the North American continent heats up, the air above it becomes light and buoyant and the air pressure falls.  Cooler, moister air starts streaming in from the Pacific bringing  rain, first into the south, later into the middle of the country, and finally into the far north.  Only as the sun retreats southward again do the winds die down and the rains subside.  Because this annual monsoon cycle is erratic both in its timing and its intensity, Mexico plays a continual game of "Russian roulette" with its climate.  By the same token, the country's native vegetation, soils, and animal life  all reflect this delicate balance between warmth and water need on the one hand and the duration and relative abundance of its moisture supply on the other.

         With every 1000 ft. rise in elevation, the temperature falls about 3.5º F. (2ºC.), so that, in a country as rugged as Mexico, climate is as much a product of height above sea level as it is of latitude.  As a result, the Spanish were quick to recognize that their preferred areas for settlement were those parts of the country that experienced temperatures most like their homeland.  This meant that below about 3300 ft. elevation (1000 m) they found the heat generally oppressive, and they referred to such regions as the tierra caliente ("hot country").  On the other hand, above that level they found the climate quite congenial up to elevations of about 8,000 ft. (2400 m), and they termed these regions the tierra templada, or "temperate country".  Beyond that elevation, however, the increasing challenges to both human comfort and economic livelihood made the so-called tierra fria, or "cold country", definitely less attractive to them.   Altogether, about seven percent of Mexico's area experiences temperatures that average below 10ºC. (50º F.) -- an isotherm that effectively demarcates the tree line in the higher mountains.

         Elsewhere in Mexico where the limits to human settlement are not set by warmth, the critical factor is, of course, moisture.  Apart from the small areas of super-humid cloud forests along the eastern front of the mountains, the bulk of the country experiences sub-humid conditions; 23% is classified as "warm sub-humid", and another 21% as "temperate sub-humid".  A further 28% qualifies as "semi-arid", and fully 21% as "arid", so inadequacy of moisture is a fact of life for most of the territory of Mexico.  Given the monsoon rainfall regime which influences most of the country, the water balance tends to oscillate between a high-sun, or summer, surplus and a low-sun, or winter, deficit, with the latter becoming longer and more critical the farther north one goes.

         There are, however, a couple of other seasonal variations in moisture patterns that deserve mention as well.  During the normally dry winter period, an occasional outburst of cold air from the Canadian Arctic may be strong enough to push southward into Mexico and even beyond into Central America.  If it spreads over the Plateau, it can bring freezing temperatures and even snow to the higher elevations, but neither of these is usually of long duration.  If it spreads down along the Gulf Coast instead, the northerly winds can drop temperatures markedly, bringing cloudy weather and short-lived rainsqualls as well.  Such winter outbursts are familiar in Texas as "northers" and in Mexico as "nortes".

         But far more violent and devastating are the tropical disturbances that sweep in from the Caribbean during the autumn months.  Although these primarily impact the Yucatán and the Gulf coast of the mainland, occasional tropical waves trigger off hurricanes in the warm waters off the Pacific coast as well.  Their effect may be felt from Chiapas up along the coast of Oaxaca but because these storms usually move out to sea as they progress northward, most of the rest of the Pacific coast of Mexico is spared their strong winds and heavy downpours.     

         Naturally, such marked changes in seasonal moisture supply also find their reflection in the patterns of native vegetation, soils, and animal life that are found within Mexico.   Depending on the level of detail one wishes to define, the native vegetation of the country may be classified in a number of different ways; for example, Rzedowski generalizes nine principal vegetation types, while INEGI (the Instituto Nacional de Estadística, Geografía, é Información) differentiates as many as thirty-five.  Here it will suffice with the more general classification, for that is adequate to reveal the critical differences that result from the combination of exposure, elevation, and moisture supply.

         Three types of tropical forest may be recognized within Mexico:  the most extensive is the "tropical rainforest" (also called the "tropical evergreen forest"), which originally covered about 11% of Mexico's area.  It is found along the Gulf Coastal plain from about 22º N. latitude stretching southward across the base of the Yucatán Peninsula.  Experiencing  an abundance of warmth and moisture throughout the year, it is without question the richest and most varied of all of Mexico's vegetation communities. 

         Arcing across the middle of the Yucatán is a small area of "tropical semi-deciduous forest" where the trees are not only lower and scrubbier but at least half of them also lose their leaves during the dry season.  Similar types of forest are found in narrow bands along the lower slopes of the mountains on the Pacific side of the country, in Chiapas, Oaxaca, and as far north as Jalisco and Nayarit.  Altogether, tropical semi-deciduous forests cover about 4% of the country's area.

         A yet-drier form of tropical forest in which the trees are markedly shorter and scrubbier and all of which lose their leaves during the dry season constitutes what Rzedowski calls the "tropical deciduous forest".  Such types of vegetation dominate the northwestern corner of the Yucatán, a part of northern coast of Veracruz and patches of coastal Tamaulipas, but are much more extensive on the Pacific slope of the country.  There such forests are found in the central valley of Chiapas, the lower slopes of the Sierra Madre de Sur in Oaxaca, over large parts of the eastern Balsas Depression, and along most of the lower foothills of Michoacán, Jalisco, the southeastern corner of Baja California, and the Sierra Madre Occidental all the way north into Sonora.  Its total area covers about 8% of Mexico's territory.

         A fourth type of forest is found both in tropical lowlands and in more temperate regions, sometimes even at elevations exceeding 2000 m (6500 ft.).  These are the "thorn forests", which are most extensive along the coasts of Sonora and Sinaloa, in the so-called "Bajío" region of the Mexican Plateau centered in southern Guanajuato state and the adjacent areas of Michoacán and Querétaro, and in the lower, western section of the Balsas Depression.  The southern coastal margins of the Isthmus of Tehuantepec are also clothed in thorn forest and there are also small outliers of thorn forest in coastal Jalisco and Colima, and on the east coast in northern Veracruz and southern Tamaulipas.  Taken together, these forests cover some 5% of the Mexican landscape.

         Grasslands in Mexico are most extensive along the lower eastern foothills of the Sierra Madre Occidental, extending all the way from the U.S. border in Sonora and Chihuahua as far south as Jalisco and Guanajuato.  Smaller patches of native grasslands are found in similar locations in Hidalgo and Puebla states, with isolated outliers both in interior Oaxaca to the south and in San Luis Potosí and Coahuila to the north.  Their total extent amounts to between 10 and 12% of the country's area.

         By far the most extensive of any vegetation type in Mexico is the "desert", made up of such plants as the cactus, creosote bush, agave, and yucca.   This is the dominant plant community over most of Baja California and Sonora in the west and over the entire Mexican Plateau from the U.S. border in the north to northern Oaxaca in the south.  Indeed, no less than one-fifth of Mexico's total area qualifies as a "desert".

         In the mountains where precipitation is heavier, trees once again take over from the grass and succulents that are typical of the lower, drier regions.  One of the hardiest trees is the oak, of which there are more than 150 species in Mexico and may number as many as 200.  They usually are found at elevations above 4500 ft. (1200 m) and occupy 5-6% of the country's total area.  They are often found in association with pine, of which some 35 species are found in the country, and together the oak and pine forests cover nearly 15% of Mexico's land area.  By themselves, pure stands of pine today clothe about 5% of the country's area, but no doubt their initial extent was from two to three times greater before the arrival of the Spaniards.   Mixed forests of conifers and oaks are found in all of the higher mountains of Mexico including the Sierra Madre Oriental, the Sierra Madre de Oaxaca, and the Meseta Central de Chiapas in the east, as well as the Sierra Madre de Chiapas, and the Sierra Madre del Sur in the south, and the Volcanic Axis in the center of the country.  However, the most extensive stands occur along the eastern slopes of the Sierra Madre Occidental, ranging from Sonora and Chihuahua in the north to Jalisco in the south.

         Lying near the top of the windward slopes of the highest mountains of Mexico is a narrow band of "cloud forest" composed of numerous species of oak as well as of sweet gum (Liquidámbar).  Found generally at elevations from 2000 to 6500 ft. (600-2000 m), these forests everywhere receive more than 40 inches (1000 mm) of precipitation annually, and, in most places up to three times that amount of moisture.  Not surprisingly, the longest continuous band of cloud forest is found on the east side of the Sierra Madre Oriental and the Sierra Madre de Oaxaca, extending from San Luis Potosí in the north to the Isthmus of Tehuantepec in the south.  East of the Tehuantepec Gap, another small outlier of cloud forest is found along the northern edge of the Meseta Central de Chiapas, and on the Pacific slope of Mexico isolated bands of cloud forest are found below the crests of the Sierra Madre de Chiapas and the Sierra Madre del Sur.  Even when taken together, however, these areas of cloud forest cover less than 1% of the country's total area.

         Finally, the areas of Mexico that support aquatic or sub-aquatic forms of vegetation are primarily limited to the vast, poorly drained coastal marshes of Tabasco and southern Campeche on the Gulf coast and to the alluvial lowlands of southern Sinaloa and northern Nayarit on the Pacific coast.  Mangrove thickets also border many coastal inlets where streams debouch into the sea and the shorelines of many interior highland lakes likewise demonstrate specialized forms of hygrophytic vegetation, but the total area they occupy is very small.

         Mexico's isthmian location between the landmasses of North and South America makes it a frontier between the Nearctic and Neotropical faunal realms, with species of both the high-latitudes and the tropics finding representation there.  The loftier areas of the Mexican Plateau form the southernmost habitat of such species as the mountain lion, bear, and white-tailed deer, while the lower coastal regions constitute the northernmost habitats of such creatures as the alligator, monkey, jaguar, and tapir.  It is in this physically diverse environment of rugged mountains and broad alluvial plains, of lush tropical rainforest and parched desert scrub that the long and colorful human drama of Mexico has played itself out, and it is to that subject that we next turn our attention.                

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