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.
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.
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."
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - -
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.