9.1: Natural Environments

Learning Objectives

1. Explain the tectonic activity of Western Middle and South America and identify some of the resulting physical features.
2. Explain the physical processes driving the earthquake and hurricane hazards of Middle America.
3. Describe the biomes of Middle and South America and their shaping mechanisms and geographic distributions.
4. Explain the characteristics and significant of rainforests in Middle and South America.

 

Plate Tectonics

Seen from space, the Americas can be seen as two large landmasses stretching from the northern and southern hemisphere. They are connected by the Isthmus of Panama, a thin strip of land that bridged the Americas at around 15 million years go. The Isthmus is a biproduct of the collision of two tectonic plates. Scientists believe the formation of the Isthmus of Panama is one of the most important geologic events to happen on Earth in the last 60 million years. Even though it is only a tiny sliver of land relative to the sizes of continents, the Isthmus of Panama had an enormous impact on Earth’s climate and its environments. By shutting down the flow of water between the two oceans, the land bridge re-routed currents in both the Atlantic and Pacific Oceans. Atlantic currents were forced northward, eventually establishing an oceanic current that we call the Gulf Stream. With the warm Caribbean waters flowing toward the northeast Atlantic, the climate of northwestern Europe grew warmer. The Atlantic, no longer mingling with the Pacific, also grew saltier. Each of these changes helped establish the global ocean circulation patterns we see today.

The tectonic processes that have joined the Americas through the Isthmus of Panama shaped many of its landforms over millions of years. Middle and South America today experience the dynamics of seven interacting tectonic plates. We can synthesize this tectonic activity into two general tectonic regions: the active subduction zones of the western mountain system of Middle and South America and the transverse boundaries of North/South America and the Caribbean.

 

Western Mountain System

 

Figure \(\PageIndex{1}\): Map of major tectonic plates of Middle and South America and their general direction of movement. A subduction zone dominates the west coast and is responsible for the volcanic mountains and earthquakes of the western spine of the region - the Sierra Madres, Volcanic Axis, and the Andes are part of this western mountain system. The Caribbean plate has a combination of boundary types - in its northern boundary, it slips past the North American plate causing frequent earthquakes in the region (CC-BY-SA 3.0; adapted from Eric Gaba author via Wikimedia Commons).

 

The West Coast of the Americas is part of a highly active tectonic region. In Middle and South America, most of the west coast is a long subduction zone where plates are subducting. In Middle America, this occurs with the Coco’s Plate, a fast-moving plate headed east and in direct collision with the Caribbean Plate. Having a dense oceanic crust, the Cocos plate subducts underneath the Caribbean Plate. This is the physical process beneath a strip of volcanic mountains in Central America dotting lands from Guatemala to Costa Rica, called the Central American Volcanic Axis. Guatemala is known as the country with the highest density of active volcanoes in the world, a place where one can experience naturally volatile but astonishing landscapes surrounded by volcanic mountains. For years, the towering Volcán de Fuego near Guatemala City, for example, has puffed continuously, punctuated by occasional episodes of explosive activity.

In its northern edge, the Coco’s Plate also forms a subduction zone with the North American Plate, an important geologic force in Mexico. Many active volcanoes are found along the Trans-Mexican Volcanic Belt, also known as Eje Volcánico Transversal. The volcanic belt lines the southern part of the Mexican Plateau, a highland that sits as high as 8,000ft tucked between Mexico’s largest mountain ranges: the Sierra Madre Oriental (Eastern) and Sierra Madre Occidental (Western). Mexico’s largest city, Mexico City, lies on the southern portion of this plateau. It is regularly rattled by earthquakes, at the foot of various volcanoes like Popocatépetl (17, 802ft) and Iztaccihuatl (17,159ft). Aztec legend holds that the origins of the two volcanoes is rooted in the love between a princess (Iztaccihuatl) and a warrior (Popocatépetl), both immortalized when the Gods turned them into mountains and blanketed them in snow. Despite the proximity of the legendary lovers, their similar age, and geologic character, Iztaccihuatl has not erupted in historic times. This has encouraged the establishment of numerous agricultural fields around it. Named a “smoking mountain” the Aztec language, Nahuatl, Popocatépetl remains one of the most active volcanoes in Mexico.

In South America, the Nazca Plate functions similarly to the Cocos Plate, heading east and subducting underneath the South American Plate. This is the physical process responsible for the world’s longest mountain range, the Andes Mountains, which extend from the northernmost to the southernmost parts of the South American continent. Given its wide latitudinal range, the Andes display diverse environments, towering over tropical, sub-tropical, and temperate regions with many peaks high enough to hold glaciers. The Andes have the highest mountain peaks outside Asia, including the highest peak in the western hemisphere, Mt. Aconcagua (22,841ft), and hundreds of active volcanoes.

Together, the subductions of the Cocos and Nazca plates form the geologically active west coast of Middle and South America. It results in a western system of nearly continuous mountain ranges running through many degrees of latitude. This western spine is also part of the greater Ring of Fire, a ring of volcanic ranges that encircles the shores of the Pacific Ocean. It is here that many of the world’s volcanoes and largest earthquakes occur as a byproduct of subduction.

 

Sacred Towers: Las Madres and Apus

Mexico’s prominent ranges bear the name Sierra Madres – Spanish for “Mother Mountains.” This naming is rooted in how Indigenous people perceive their mountains. In many Indigenous cultures mountains are believed to be dwellings of the Gods, sacred sites of pilgrimage and ritual. Tlaloc, for example, an Aztec deity of rain, is believed to roam the mountains of the Trans-Mexican Volcanic Range and reside in Mt. Tlaloc, signaled by the presence of rain clouds frequently hover over the mountain peak. The mythical connection of the mountains with Tlaloc as a deity of water, sustenance, lightning, and storms suits the capricious characteristics of highland climates where conditions can change drastically within a day. By controlling water, Tlaloc is a central deity in the Aztec pantheon, one responsible for sustenance as rains influence agricultural and natural cycles.^[1]

In South America, the Andes are the center of the Inca universe, the believed realm of deities. The Inca named their tallest snow-capped mountains apus – a Quechua word meaning “God.” Innumerous pilgrims hike these mountains in religious rituals seeking a connection with the divine. The antiquity of this spiritual connection is supported by archeological findings of human offerings and sacrifice in various Andean peaks. A prominent example is the infamous Juanita, an excavated mummy of a young noble Inca girl found in Mt. Apato (Peru), believed to have been sacrificed for the Inca Gods five hundred years ago. Juanita and other recovered mummies had been frozen in the ice caps of the mountains, only made accessible by a volcanic discharge that sent ice blocks rolling down the slopes. Juanita stunned archeologists with the well-preserved features, accessories, and the fine wool that wrapped her body, signs of a prestigious offering to the Inca Gods. The location of Machu Pichu, a famous Inca sacred site, atop steep Andean cliffs is another example of the role of mountains in the Inca spiritual universe.

As geographers, we interpret this ascribed sacredness to a deep understanding of life-supporting environmental services. The mountain ranges that surround Aztec and Inca territories are central to the ecology and the hydrology that have supported civilizations for millennia. The ice caps of the Andes, for example, are water storage systems that replenish rivers during dry seasons, supporting people and ecosystems. In addition to their hydrological importance, volcanic mountains line the land with nutrient-rich volcanic soils that have enabled agriculture and sustained civilizations. Mountains also capture moisture carrying clouds and provide habitat for a wide array of species that find home in their ecological zones. Indigenous reverence for these natural towers reflects an understanding of the vital functions the Mesoamerican and Andean mountains provide.

Figure \(\PageIndex{2}\): [top] This satellite map of Mexico situates its prominent mountain ranges: Sierra de California, Sierra Madre Occidental (Western Sierra Madre), Sierra Madre Oriental (Eastern Sierra Madre), and Sierra Madre del Sur (Souther Sierra Madres). The Eje Volcanico Transversal (Trans-Mexican Volcanic Belt) crosses the country diagonally from the Pacific Ocean to the Caribbean at the southern portion of the Mexican Plateau (Public Domain; NASA via Wikimedia Commons). This satellite image illustrates some of the iconic volcanoes of the Trans-Mexican Volcanic belt, south of Mexico City. To the right of it, a replicated drawing of Tlaloc, the deity of rain thought to dwell among these ranges and reside in Mount Tlaloc (This graphic was created by Aline Gregorio using Google Earth; permitted use. Tlaloc as depicted in the Codex Magliabechiano via Wikimedia Commons, Public Domain).

 

Middle America’s Turbulent Land and Atmosphere

Middle America showcases turbulent geological processes and the friction zones in all borders of the Caribbean Plate. To the east, the tectonic collision of the Caribbean plate and the North American plate formed the Caribbean archipelago, or island chain. Many of these islands are the tops of underwater mountains. The islands of the Caribbean are divided into the Greater Antilles and the Lesser Antilles. The Greater Antilles include the larger islands of Cuba, Jamaica, Hispaniola, Puerto Rico and the Cayman Islands. The Lesser Antilles are smaller volcanic islands arching eastern edge of the Caribbean plate. On the northern edge of the Caribbean Plate, various islands are on a seismic ride as the plate heads east and slides past the North American Plate. This lateral slipping forms a transverse boundary, a friction zone that has resulted in several severely destructive earthquakes. Haiti, for example, sits on a series of faults that have been the epicenter of destructive earthquakes – the 2010 earthquake had a 7.0 magnitude and subsequent aftershocks that destroyed buildings, killed over two hundred thousand people, and left over one million people immediately homeless.^[2] As the poorest country in the western hemisphere, the impacts of the earthquake have been enduring. Ten years later, Haiti still had not recovered the disaster, despite billions of dollars of international aid. It is an example of how natural hazards can be particularly destructive in poor and politically unstable countries.

Countries in Middle America are also highly susceptible to hurricanes, tropical storms with winds of 74 miles per hour or more.  They tend to develop over the warm waters (about 80°F or higher) of the Atlantic Ocean and often escalate into hurricanes headed towards the Caribbean and Gulf of Mexico in the fall (between August and October). Hurricane Irma, for example, hit the Caribbean in September 2017, leaving a trail of devastation. Irma’s winds approached 180 miles per hour, the most powerful Atlantic Hurricane ever recorded. Satellite images reveal green islands turned brown as winds stripped trees and damaged infrastructure. Islands like San Martin and Barbuda faced near total devastation while Puerto Rico, Dominican Republic, and Haiti experienced extensive electricity loss and infrastructural damage. Scientists warn that as ocean waters are becoming warmer due to global warming, destructive hurricanes like Irma are more likely to occur.

 

Figure \(\PageIndex{3}\): Between August and October, the Caribbean and tropical Atlantic waters are warm enough to fuel powerful hurricanes. This satellite image captures hurricane Irma (to the left) and hurricane Jose (to the right) in the Atlantic Ocean on September 7, 2017 (CC BY 2.0; NOAA National Environmental Satellite via Wikimedia Commons). These are two of the many destructive hurricanes that have battered the region resulting in the loss of life and infrastructure.

 

 

Middle & South America’s Biomes

Middle and South America display immense environmental diversity. Atop the dynamics churning beneath the Earth lies a vast region with great topographical variability. Mountains, plateaus, and lowlands make for distinctive environmental conditions. Regions in higher altitude have cooler temperatures and less evaporation and display colder environments than lowlands. Furthermore, the wide latitudinal range of Middle and South America situates its vast territory within various global climate regions. In this section, we will explore how topography, latitude, and situation to airmasses shape natural environments in the region.

 

The ITCZ

Most of Middle and South America territories extend through a wide latitudinal range, through tropical and subtropical regions. As discussed in Chapter 1, latitude establishes a relationship between the Earth and the Sun, shaping global pressure systems and air circulation patterns to a large extent. Tropical latitudes receive the most direct solar radiation, which warms up the lands and oceans and the adjacent air. Warm air rises, as it rises it expands and cools, and as it cools the air is less able to hold moisture. Thus, the tropics are marked by the presence of an enveloping cloud belt, a rising air mass that generates abundant rainfall in tropical regions. This low-pressure system (of rising air and rain) is called the Intertropical Convergence Zone (ITCZ). The ITCZ is a latitudinal belt that shifts north and south of the equator with the seasons: in July (northern hemispheric summer) it shifts towards the north and in December (southern hemispheric summer) it shifts towards the south. The presence and seasonality of the ITCZ shapes the distribution of the tropical rainforests and savannas in Middle and South America and other parts of the world.

 

Figure \(\PageIndex{4}\): The clouds of the Intertropical Convergence Zone are visible along the equator and the tropical Pacific Ocean. The intense sun and warm water of the equator heats the air in the ITCZ, raising its humidity and causing it to rise. As the air rises it cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms. Variation in the location of the ITCZ drastically affects rainfall in many equatorial nations (Public Domain; NASA via Wikimedia Commons).

 

Tropical Rainforests

 

Figure \(\PageIndex{5}\): This satellite image shows the geographic distribution of tropical rainforests in Middle and South America. Notice the Mesoamerican rainforests extending from Mexico's Yucatan Peninsula south to Panama. In South America, much of the Amazon rainforest lies in the Amazon Basin, the lowlands of the Amazon River. Forests near the river and tributaries are adapted to flooding (Public Domain; NASA Observatory).

 

Middle and South America is home to vast tracts of rainforest, a lush biome with an immense biodiversity. Tropical rainforests experience warm temperatures (mean about 80°F) and abundant rains (over 80in) per year, a biproduct of the tropical latitude and the ITCZ overhead. These biomes are often viewed in simplistic terms (warm, wet, and green) when in reality they are complex ecosystems intertwined with myriad lifeforms, many of which are yet to be described by scientists. While tropical rainforests makeup for only six percent of the Earth's landmass, they are home to about half of all species on Earth. They are known for their unparalleled high levels of terrestrial biodiversity and endemic species, species that are found nowhere else on Earth. Rainforests in Middle and South America have over 30,000 species of vascular plants, more than three times as many as Africa and Asia.^[3] Six of the world’s most biodiverse countries are within Middle and South America: Brazil, Colombia, Ecuador, Mexico, Peru, and Venezuela.^[4]

Tropical rainforests are characterized by their vertical arrangement of three canopy layers that are home to most life forms occupying different stories of the forest. The top canopy contains giant trees that act as an inverse umbrella catching rain and sunlight and reducing access for species near the ground. Given the dense foliage and fierce competition, very little sunlight reaches the forest floor and most species live in the trees. Examples include countless colorful birds, sloths, monkeys, snakes, and jaguars. By contrast, the forest floor is dark, inhabited by shade loving species and fairly sparce of vegetation and animal life compared to the densely inhabited vegetation layers above.

Tropical plants play a fundamental role in their local climates. Trees contribute to the local precipitation by releasing moisture back into the atmosphere in a process called evapotranspiration. It is estimated that as much as half of the precipitation that falls on the Amazon Basin comes from the moisture returned to the atmosphere from the trees themselves. This self-sufficiency is also evident in how tropical trees manage nutrients. It may be surprising that the soils beneath so much life are acidic and leached of nutrients from the heavy rains. These nutrient-poor reddish soils are called oxisols and utisols, and they provide a challenging foundation for canopy giants to grow from. As an adaptation to this low soil fertility, tropical trees have shallow roots which are better equipped to access nutrients from the rapidly decomposing leaves on the moist forest floor. The scent of wood and decaying vegetation typical of rainforests is a sensory indicator of the ongoing nutrient cycling of this dynamic ecosystem.

While most of the tropical rainforests in Middle and especially South America are found in lowlands, topographical differences make for unique rainforest ecosystems. The Amazon Basin is a low-lying and relatively flat area of over 2.7 million square miles of a vast drainage system of the Amazon River, the most voluminous river in the world. Along the course of over 2,000 miles, it joins the waters of over a thousand tributaries and discharges into the Atlantic Ocean. It represents about one fifth of all river flow in the world. The Amazon River expands in the wet season and submerges vast areas of rainforest along its floodplain. These floodplain rainforests, also called várzeas, are composed of vegetation adapted to remain underwater for months out of the year (December – May). Fruit trees form symbiotic relationships with fish that swim through the forest floor, eat their fruits, and disperse their seeds. Sloths and freshwater pink dolphins roam the waters of flooded forests, a symbol of a seasonal fusion of riverine and terrestrial worlds.

Tropical mountain environments are also associated with high levels of biodiversity largely due to the variations of temperature and moisture at different elevations. In a small country like Ecuador, for example, the intersection of the Andes mountains, the Equator, and ITCZ at Yasuni National Park form an ecological epicenter with a remarkable diversity of plants, mammals, birds, and amphibians. It may be the most biodiverse place on Earth.^[5] Some rainforests in higher elevations of mountain slopes, in Ecuador and beyond, become consistently submerged by clouds. Known as cloud forests, these misty environments are cooler and almost always humid. Thus, they contain lifeforms specifically adapted to these conditions. In Central America, an emblematic example of a cloud forest species is the quetzal, an iridescent turquoise bird highly revered by Mesoamerican Indigenous cultures for its beauty. Guatemala is home to one of the largest unbroken extents of cloud forest in Mesoamerica containing the largest remaining habitat of the quetzal, an endemic species of the region. Other species that reside here include the endangered tapir, howler monkey, jaguar, and harpy eagle. Like the quetzal, many species face an uncertain future due to habitat loss. The causes and consequences of tropical rainforest destruction will be discussed later in this chapter (In Section 9.5).  

 

Figure \(\PageIndex{6}\): This collection of images illustrates floodplain rainforests of Brazil and cloud forests of Costa Rica. [top left] The Amazon rainforest on the Urubu river, a tributary of the Amazon River running through Amazonas State, Brazil (CC BY 2.0; Deak via Wikimedia Commons). [top right] Aerial view of the Amazon Rainforest, near Manaus, the capital of the Brazilian state of Amazonas (CC BY-NC-ND; Palmer via Flickr). [bottom left] Tropical trees covered in mist at the cloudforest of Monteverde, Costa Rica (CC BY-NC-SA 2.0; Crisp via Flickr). The iconic quetzal, an endemic species of Mesoamerican cloudforests (CC BY-SA 4.0; Harleybroker via Wikimedia Commons).

Tepuis, the sky islands of the northern Amazon

The Guiana Highlands are known for the pronounced flat-top mountains called tepuis, sandstone and quartzite rock features that are remnants of a once-continuous plateau that eroded over millions of years. Tepuis are fascinating due to their prominence in the landscape, their antiquity, and their internal structures. Many tepuis contain extensive cave systems, rivers, and tunnels within them.  There are about one hundred tepuis in South America, mostly in Venezuela but also in northern Brazil and western Guiana. One of the most well-known is the heart shaped Auyán Tepui in Venezuela. It is home to the world’s highest waterfall, Angel Falls, which drops nearly 1,000 meters (3,300 feet)—19 times farther than Niagara Falls. 

Tepuis can be described as sky islands. They tower above the forest disconnected from each other and the environments beneath them. While tepuis have been minimally explored by scientists, it is estimated that many unknown endemic species inhabit their cliffs. Tepuis host many mammals on their lower slopes, including monkeys, jaguars, pumas, and sloths, and several bat species. Reptiles, such as snakes, iguanas, and lizards, are found on the summits and slopes. The Pemón, the local indigenous people of the region, have mythologies connected to nearby rivers, waterfalls, and rock formations. To the Pemón, the tepuis are sacred, the home of the Gods, and forbidden to humans.^[6]

 

 

Figure \(\PageIndex{7}\): A panoramic photo of Angel Falls, Venezuela, the world's tallest waterfall. Its waters cascade from the Auyán-tepui mountain in the Canaima National Park, a UNESCO World Heritage. The rock cliffs tower ver a canyon, Cañon del Diablo, and a meandering river, Río Churun surrounded by lush tropical forest (CC BY-SA 3.0; Dezeo via Wikimedia Commons).

 

Deserts

The vast territorial expanse of Middle and South America includes not only tropical rainforests but also pronounced deserts. The Atacama Desert is well known as the world’s driest desert, receiving only a few millimeters of rain per year, sometimes only in the form of coastal fog. Framed by the Andes Mountains in the east and the Pacific Ocean in the west, it extends from the northern coast of Chile to the south coast of Peru. Not so far from vast tropical regions, the Atacama Desert is situated at the foothills of the Andes, a formidable topographic barrier that traps the incoming humid air masses coming from the Amazon Basin. Moisture-laden clouds are carried west from the Atlantic to the Amazon Basin. When they reach the Andes, the moist air forced to rise. As it rises, it cools, condenses, and precipitates in the east side of the Andes leaving the west dry. The Atacama Desert is in the rain shadow of the Andes mountains. The desert is also shaped by the Peru Current (also known as the Humboldt Current), an oceanic current along Atacama’s coast that brings cold waters from Antarctic latitudes north. Since cold water does not evaporate, the adjacent air masses are dry and bear little to no moisture for the Atacama coast. The current is also associated with abundance along the Chilean coast, as it forces up cold water nutrients that sustain rich fisheries. Together, the Andes and the Peru Current are the major factors making the Atacama the driest desert in the world.

 

Figure \(\PageIndex{8}\): [top left, map A] The map illustrates the location of the Atacama Desert in South America. [top right, maps B and C] The Zoom of the white frame shown in map A depicts the masses of clouds at the eastern side of the Andes over Argentina and Bolivia. The general location of the Coastal Range (light blue), the hyperarid core Atacama region (Yellow), and the Andes Mountains and its foothills (green). [bottom left] This diagram shows the incoming air masses from the east being trapped by the Andes mountains, leaving the west side dry. The Atacama lies at the shore of the Pacific Ocean, dominated by the Humboldt Current, a cold oceanic current (CC BY; Azuza-Bustos et al. 2022, via Frontiers). [bottom right] An Atacama landscape showing the snowcapped mountains of Nevado Tres Cruces National Park, along the Atacama region in Chile (CC BY; Cadenas via Wikimedia Commons).

 

Reiterations of the rainshadow effect are evident throughout many landscapes in Middle and South America (and the world). For example, note the distribution of rainforests in the eastern slopes of the Central American Volcanic Axis. The moisture-carrying airmasses carried by the northeast trade winds bring precipitation to the east while leaving western portions of Central America drier. Similarly, along the southeastern coast of Brazil, the Atlantic Forest is elevated by the rolling hills of Serra do Mar, a coastal mountain system that captures incoming rain clouds that provide moisture for a continuous rainforest along the coast of São Paulo and Rio de Janeiro. Further south, the westerlies encounter the western portion of the Andes mountains, with Chile in the windward side and Argentina in the rainshadow of the mountain chain. Thus, the Argentinian Patagonia is a cold desert, not only because of the blockade of the Andes but also the presence of the cold oceanic current, the Falkland Current bringing Antarctic waters to the Argentinian coast. In short, the great mountains of Middle and South America provide for great environmental diversity based on their latitudinal and altitudinal range and interaction with circulating air masses.

 

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References:

[1] Popovici, C. (2022). The Ritual Ascent at Mount Tlaloc, Mexico. MAVCOR Journal 5 (2).

[2] Savard, J., Sael, E., and Clormeus, J. (Jan 9, 2020). A decade after the earthquake, Haiti still struggles to recover. The Conversation.

[3] Miller, S. W. (2007). An environmental history of Latin America. Cambridge University Press. pp 5.

[4] Miller, S. W. (2007). An environmental history of Latin America. Cambridge University Press. pp 5.

[5] Blitz, M. (May 15, 2022). This park in Ecuador is one of the most biodiverse places on Earth. Smithsonian Magazine.

[6] Sharpe, C. J., & Rodríguez, I. (1997, January). Discovering the lost world: Canaima National Park and world heritage site, Venezuela. In The George Wright Forum (pp. 15-23). 

 

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Attributions:

“Isthmus of Panama” is adapted from Isthmus that Changed the World by NASA, permitted use.

Parts of “Tropical rainforests” is adapted from Hope for Guatemala's national bird by the World Wildlife Fund (WWF), CC BY NC.

“Hurricane Irma” is adapted from Hurricane Irma Strengthens by NASA, permitted use.

“Tepuis” is adapted from Venezuela’s flat topped mountains by NASA, permitted use.

This page is also adapted from World Regional Geography by University of Minnesota and World Regional Geography by Caitlin Finlayson, both CC BY-NC-SA 4.0.