Andes Mountains

M. Tulio Velásquez & Norman R. Stewart

(from http://www.history.com/topics/andes‐mountains, accessed 5‐23‐2012)

Introduction

The Andes consist of a vast series of extremely high plateaus surmounted by even higher peaks that form an unbroken rampart over a distance of some 5,500 miles (8,900 kilometres)—from the southern tip of South America to the continent's northernmost coast on the Caribbean. They separate a narrow western coastal area from the rest of the continent, affecting deeply the conditions of life within the ranges themselves and in surrounding areas. The Andes contain the highest peaks in the Western Hemisphere. The highest of them is Mount Aconcagua (22,831 feet [6,959 metres]) on the border of Argentina and Chile.

The Andes are not a single line of formidable peaks but rather a succession of parallel and transverse mountain ranges, or cordilleras, and of intervening plateaus and depressions. Distinct eastern and western ranges—respectively named the Cordillera Oriental and the Cordillera Occidental—are characteristic of most of the system. The directional trend of both the cordilleras generally is north-south, but in several places the Cordillera Oriental bulges eastward to form either isolated peninsula-like ranges or such high intermontane plateau regions as the Altiplano (Spanish: “High Plateau”), occupying adjoining parts of Argentina, Chile, Bolivia, and Peru.

Some historians believe the name Andes comes from the Quechuan word anti (“east”); others suggest it is derived from the Quechuan anta (“copper”). It perhaps is more reasonable to ascribe it to the anta of the older Aymara language, which connotes copper colour generally.

Physical features

There is no universal agreement about the major north-south subdivisions of the Andes system. For the purposes of this discussion, the system is divided into three broad categories. From south to north these are the Southern Andes, consisting of the Chilean, Fuegian, and Patagonian cordilleras; the Central Andes, including the Peruvian cordilleras; and the Northern Andes, encompassing the Ecuadorian, Colombian, and Venezuelan (or Caribbean) cordilleras.

Geology

The Andean mountain system is the result of global plate-tectonic forces during the Cenozoic Era (roughly the past 65 million years) that built upon earlier geologic activity. About 250 million years ago the crustal plates constituting the Earth's landmass were joined together into the supercontinent Pangaea. The subsequent breakup of Pangaea and of its southern portion, Gondwana, dispersed these plates outward, where they began to take the form and position of the present-day continents. The collision (or convergence) of two of these plates—the continental

South American Plate and the oceanic Nazca Plate—gave rise to the orogenic (mountain- building) activity that produced the Andes.

Many of the rocks comprising the present-day cordilleras are of great age. They began as sediments eroded from the Amazonia craton (or Brazilian shield)—the ancient granitic continental fragment that constitutes much of Brazil—and deposited between about 450 and 250 million years ago on the craton's western flank. The weight of these deposits forced a subsidence (downwarping) of the crust, and the resulting pressure and heat metamorphosed the deposits into more resistant rocks; thus, sandstone, siltstone, and limestone were transformed, respectively, into quartzite, shale, and marble.

Approximately 170 million years ago this complex geologic matrix began to be uplifted as the eastern edge of the Nazca Plate was forced under the western edge of the South American Plate (i.e., the Nazca Plate was subducted), the result of the latter plate's westward movement in response to the opening of the Atlantic Ocean to the east. This subduction-uplift process was accompanied by the intrusion of considerable quantities of magma from the mantle, first in the form of a volcanic arc along the western edge of the South American Plate and later by the injection of hot solutions into surrounding continental rocks; the latter process created numerous dikes and veins containing concentrations of economically valuable minerals that later were to play a critical role in the human occupation of the Andes.

The intensity of this activity increased during the Cenozoic Era, and the present shape of the cordilleras emerged. The accepted time period for their rise had been from about 15 million to 6 million years ago. However, through the use of more advanced techniques, researchers in the early 21st century were able to determine that the uplift started much earlier, about 25 million years ago. The resultant mountain system exhibits an extraordinary vertical differential of more than 40,000 feet between the bottom of the Peru-Chile (Atacama) Trench off the Pacific coast of the continent and the peaks of the high mountains within a horizontal distance of less than 200 miles. The tectonic processes that created the Andes have continued to the present day. The system—part of the larger circum-Pacific volcanic chain that often is called the Ring of Fire— remains volcanically active and is subject to devastating earthquakes.

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Physiography of the Northern Andes

A rough and eroded high mass of mountains called the Loja Knot (4° S) in southern Ecuador marks the transition between the Peruvian cordilleras and the Ecuadorian Andes. The Ecuadorian system consists of a long, narrow plateau running from south to north bordered by two mountain chains containing numerous high volcanoes. To the west, in the geologically recent and relatively low Cordillera Occidental, stands a line of 19 volcanoes, 7 of them exceeding 15,000 feet in elevation. The eastern border is the higher and older Cordillera Central, capped by a line of 20 volcanoes; some of these, such as Chimborazu Volcano (20,702 feet), have permanent snowcaps.

The outpouring of lava from these volcanoes has divided the central plateau into 10 major basins that are strung in beadlike fashion between the two cordilleras. These basins and their adjacent slopes, which are intensively cultivated, contain roughly half of Ecuador's population.

A third cordillera has been identified in the eastern jungle of Ecuador and has been named the Cordillera Oriental. The range appears to be an ancient alluvial formation that has been divided by rivers and heavy rainfall into a number of mountain masses. Such masses as the cordilleras of Guacamayo, Galeras, and Lumbaquí are isolated or form irregular short chains and are covered by luxuriant forest. Altitudes do not exceed 7,900 feet, except at Cordilleras del Cóndor (13,000 feet) and Mount Pax (11,000 feet).

North of the boundary with Colombia is a group of high, snowcapped volcanoes (Azufral, Cumbal, Chiles) known as the Huaca Knot. Farther to the north is the great massif of the Pasto Mountains (latitude 1°–2° N), which is the most important Colombian physiographic complex and the source of many of the country's rivers.

Three distinct ranges, the Cordilleras Occidental, Central, and Oriental, run northward. The Cordillera Occidental, parallel to the coast and moderately high, reaches an elevation of nearly 13,000 feet at Mount Paramillo before descending in three smaller ranges into the lowlands of northern Colombia. The Cordillera Central is the highest (average altitude of almost 10,000 feet) but also the shortest range of Colombian Andes, stretching some 400 miles before its most northerly spurs disappear at about latitude 8° N. Most of the volcanoes of the zone are in this range, including Mounts Tolima (17,105 feet), Ruiz (17,717 feet), and Huila (18,865 feet). At about latitude 6° N, the range widens into a plateau on which Medellín is situated.

Between the Cordilleras Central and Occidental is a great depression, the Patía-Cauca valley, divided into three longitudinal plains. The southernmost is the narrow valley of the Patía River, the waters of which flow to the Pacific. The middle plain is the highest in elevation (8,200 feet) and constitutes the divide of the other two. The northern plain, the largest (15 miles wide and 125 miles long), is the valley of Cauca River, which drains northward to the Magdalena River.

The Cordillera Oriental trends slightly to the northeast and is the widest and the longest of the three. The average altitude is 7,900 to 8,900 feet. North of latitude 3° N the cordillera widens and after a small depression rises into the Sumapaz Uplands, which range in elevation from 10,000 to 13,000 feet. North of the Sumapaz Upland the range divides into two, enclosing a large plain 125 miles wide and 200 miles long, often interrupted by small transverse chains that form several upland basins called sabanas that contain about a third of Colombia's population. The city of Bogotá is on the largest and most populated of these sabanas; other important cities on sabanas are Chiquinquirá, Tunja, and Sogamoso. East of Honda (5° N) the cordillera divides into a series of abrupt parallel chains running to the north-northeast; among them the Sierra Nevada del Cocuy (18,022 feet) is high enough to have snowcapped peaks.

Farther north the central ranges of the Cordillera Central come to an end, but the flanking chains continue and diverge to the north and northeast. The westernmost of these chains is the Sierra de Ocaña, which on its northeastern side includes the Sierra de Perijá; the latter range forms a portion of the boundary between Colombia and Venezuela and extends as far north as latitude

11° N in La Guajira Peninsula. The eastern chain bends to the east and enters Venezuela as the Cordillera de Mérida. On the Caribbean coast just west of the Sierra de Perijá stands the isolated, triangular Santa Marta Massif, which rises abruptly from the coast to snowcapped peaks of 18,947 feet; geologically, however, the Santa Marta Massif is not part of the Andes.

The Venezuelan Andes are represented by the Cordillera de Mérida (280 miles long, 50 to 90 miles wide, and about 10,000 feet in elevation), which extends in a northeasterly direction to the city of Barquisimeto, where it ends. The cordillera is a great uplifted axis where erosion has uncovered granite and gneiss rocks but where the northwestern and southeastern flanks remain covered by sediments; it consists of numerous chains with snow-covered summits separated by longitudinal and transverse depressions—Sierras Tovar, Nevada, Santo Domingo, de la Culata, Trujillo, and others. The range forms the northwestern limit of the Orinoco River basin, beyond which water flows to the Caribbean. North of Barquisimeto, the Sierra Falcón and Cordillera del Litoral (called in Venezuela the Sistema Andino) do not belong to the Andes but rather to the Guiana system.

Soils

The complex interchange between climate, parent material, topography, and biology that determines soil types and their condition is deeply affected by altitude in the Andes. In general, Andean soils are relatively young and are subject to great erosion by water and winds because of the steep gradients of much of the land.

In the Fuegian and southern Patagonian Andes, the formation of soils is difficult; the actions of glaciers and of strong winds have left nearly bare rock in many places. Peat bogs, podzols, and meadow soils, all with thick horizons (layers) of humus, are found; drainage is poor. Volcanic soils that are rich in organic material and are well drained occur in the region of lakes. North of latitude 45° S, soils are formed directly on weathered rocks at higher elevations, and reddish brown soils with gravel and quartz are found in the lower zones; erosion is heavy.

North of 37° S the Atacama Desert is covered with heavily eroded desertic soils that are low in moisture and organic material and high in mineral salts. This soil type, with few differences, extends along the Cordillera Occidental to north of Peru.

From Bolivia to Colombia the soils of the plateau and the east side of the eastern cordilleras show characteristics closely related to altitude. In the Andean páramo embryonic soils black with organic material are found. At altitudes between 6,000 and 12,000 feet, red, brown, and chernozem soils occur on moderate slopes and on basin floors. In more poorly drained locations, soils with a permeable sandy horizon are relatively fertile; these soils are the most economically important in Bolivia, Peru, and Ecuador. The sabana soils of Colombia are gray-brown, with an impermeable claypan in certain levels, resulting in poor drainage.

At high elevations soils are thin and stony. On the east side of the eastern cordilleras, descending to the Amazon basin, thin, poorly developed humid soils are subject to considerable erosion. Intrazonal soils (those with weakly developed horizons) include humic clay and solonetz (dark

5 alkaline soils) types found close to lakes and lagoons. Also included in this group are soils formed from volcanic ash in the Cordillera Occidental from Chile to Ecuador.

The azonal soils—alluvials (soils incompletely evolved and stratified without definite profile) and lithosols (shallow soils consisting of imperfectly weathered rock fragments)—occupy much of the Andean massif. In Colombia, sandy yellow-brown azonal soils on slopes and in gorges are the base of the large coffee plantations.

Climate

In general, temperature increases northward from Tierra del Fuego to the Equator, but such factors as altitude, proximity to the sea, the cold Peru (Humboldt) Current, rainfall, and topographic barriers to the wind contribute to a wide variety of climatic conditions. The hottest rain forests and deserts often are separated from tundralike puna by a few miles. There also is considerable climatic disparity between the external slopes (i.e., those facing the Pacific or the Amazon basin) and the internal slopes of the cordilleras; the external slopes are under the influence of either the ocean or the Amazon basin. As mentioned above, the line of permanent snow varies greatly. It increases from 2,600 feet at the Strait of Magellan, to 20,000 feet at latitude 27° S, after which it begins descending again until it reaches 15,000 feet in the Colombian Andes.

Precipitation varies widely. South of latitude 38° S, annual precipitation exceeds 20 inches, whereas to the north it diminishes considerably and becomes markedly seasonal. Farther north— on the Altiplano of Bolivia, the Peruvian plateau, and in the valleys of Ecuador and the sabanas of Colombia—rainfall is moderate, though amounts are highly variable. It rains only in very small amounts on the west side of the Peruvian Cordillera Occidental but considerably more in Ecuador and Colombia. On the east (Amazonian) side of the Cordilleras Orientales, rainfall usually is seasonal and heavy.

Temperature varies greatly with altitude. In the Peruvian and Ecuadorian Andes, for example, the climate is tropical up to an altitude of 4,900 feet, becoming subtropical up to 8,200 feet; hot temperatures prevail during the day, and nights are mildly warm. Between 8,200 and 11,500 feet daytime temperatures are mild, but there are marked differences between night and day; this zone constitutes the most hospitable area of the Andes. From 11,500 to 14,800 feet it generally is cold—with great differences between day and night and between sunshine and shadow—and temperatures are below freezing at night. Between about 13,500 and 15,700 feet (the puna), the climate of the páramo is found, with constant subfreezing temperatures. Finally, above 15,700 feet, the climate of the peaks and high ridges is polar with extremely low temperatures and icy winds.

As in other mountainous areas of the world, a wide variety of microclimates (highly localized climatic conditions) exist because of the interplay of aspect, exposure to winds, latitude, length of day, and other factors. Peru, in particular, has one of the world's most complex arrays of habitats because of its numerous microclimates.

The Andes’ Mountainous Paradox

By DeLene Beeland (from Natural History, http://www.naturalhistorymag.com/partner/the-andes-mountainous- paradox, accessed 5-25-2012)

When asked if mountains grow slowly and steadily versus in rapid spurts, most people intuitively gravitate to the “slow and steady” model. Mountains, we are taught, take an incomprehensively long time to build up their scads of booulders, jagged peaks and high-altitude plateaus.

In fact, most known mountain building processes do require large amounts of time to complete their skyward climb. But for every rule there is an excepttion. Consider the Himalaya and Andes mountains—despite their relative geologic youth, these mountain belts rank among the world’s tallest peaks. And therein lies the mountainous paradox: How do geologically young mountains grow extremely tall in extremely short time periods?

Conventional geology tells us that as the earth’s tectonic plates collide and dive beneath onee another, and these actions cause the earth’s skin to crumple and fold. For a superficial visual effect, pinch together an inch or two of your forearm skin. Just as your skin crumples into peaks and valleys under pressure from your fingers, deformingg tectonic pressures cause the earth’s crust to shorten and thicken into crenulations and folds, which alpinists yearn to climb and landscape photographers strive to capture on film. But below the surface, mountains have deep roots where dense material accumulates over time, often from the action of one tectonic plate diving beneath another whereby material is scraped off of one onto the other. It was previously thought that a gradual erosion of this root by the more plastic asthenosphere resulted in the gradual rise of the crust (see figure right).

But a new study tracking the uplift of a central portion of the massivve Andes Mountains in South America shows that mountain building—what geologists term “orogeny”—may actually occur in much faster fits and spurts than previously realized due to the rapid loss of large amounts of material from the mountain’s root.

While cconventional theory would predict thhat the Anddes Mountains rose gradually and in sync with the scrunching of the Nazca plate beneath the South American plate, which scientists know has caused dense material to accumulate millennia after millennia up to 70 kilometers below South America’s western coast, Florida Museum of Natural History paleontologist Brucce MacFadden said that this is not what happened after all. MacFadden is a co-author of the study published June 6 in the journal Science.

“Instead of the Altiplano rising little by little each year, we found two phases of spasmodic or punctuated uplift interspersed by millions of years of stability,” MacFadden said.

The authors assert that as the crustal layer, or lithosphere (whichh floats above the mantle) was squeezed under deforming pressures, earth processes caused large parts of the accreted dense material to plummet downwards into the more plastic upper mantle layer, also known as the atheenosphere. This loosening of the root load caused the surface crust layer to rise, buoyed upward like a released cork, by the excision of massive extra weight below.

“Our findings will force geologists to acknowledge that removal of lower lithosphere material could be an important process that causes rapid surface uplift in different mountain belts worldwide and over geologic time,” said lead author Carmala Garzione, a geologist at the University of Rochester. “The subduction process may cause shortening and thickening of the mantle lithosphere and dense lower crust that accumulates at depth until that dense material is removed rapidly—either by downward dripping, which is a convective process, or by another process called delamination.”

Geochemical clues

The researchers found that uplift began between 30 million and 20 million years ago, then leveled off into relative stability until “a pulse of rapid upliift” occurred between 10 million and 6 million years ago when the landsccape rose between 1.5 and 3.5 kilometers in a maassive upwards spurt. To reconstruct the Altiplano’s sequential rise, the researchers examined several lines of proxy evidence includiing two different types of stable isotopes, fossil plants and ancient magnetic- bearing deposits.

The researchers coaxed geochemical clues in the form of oxygen isotopes from ancient soil nodules made of calcium carboonate. The nodules were sampled from layered soil deposits between five million and 28 million years old. Oxygen isotopes serve as reliable proxy indicators for the actual temperatures in which they formed—so the researchers used them to reconstruct ancient temperature records, and then linked these records to known temperature clines associated with vertical elevation gain. They also analyzed magma and sedimment as additional proxies.

“Carmie’s ability to put this study together shows her brilliance,” MacFadden said. “She’s synthesized research in theoretical geophysics, geochemistry, and paleontology and made a strong case for the timing and consequences of the Altiplano’s rise.”

A professor of earth and environmental sciences at Lehigh University who also researches ancient elevations said that while weaknesses were inherent when single proxy methods were used, the multiple methods used in this study made the results robust.

“Remarkably, the rapid recent uplift scenario presented here is similar to what I found for the Colorado Plateau,” Dork Sahagian said.

In 2005, Sahagian, who is also the director of Lehigh’s Environmental Initiative, organized a national workshop to refine and strengthen paleoelevation techniques. Garzione attended and presented her work, but Sahagian said her Andean project was just beginning at that time.

“The greatest novelty in their study is the number of proxies they brought to bear on the problem,” Sahagian said. “This is the right way to go about it.”

Reconstructing ancient topographies, climates

MacFadden, who has spent nearly three decades collecting and studying fossil mammals from Bolivia, contributed by leading the research team to several key fossil sitees in the Altiplano where he had established geological age sequences in years prior. While Garzione’s interest was grounded in the geology, MacFadden’s interest in the project lay in understanding how the Andes’ birth affected South America’s ancient climate and animals. But in order for mountains to begin driving climatic changes, they have to reach a certain size.

“The big-picture question is: When did the Andes grow high enough to become drivers of the South American climatic regime? Because this event obviouslly had cascading effects upon plant and animal life across the continent,” MacFadden said.

Based on their findings, MacFadden said this likely happened around 10 million years ago.

Today, the massive Andes Mountain belt snakes 4,400 miles along the continent’s western edge and is the longest unbroken terrestrial chain on the planet, with peaks soaring to 22,841 feet. The world’s driest desert, the Atacama, stretches between the Andes’ central western foothills and the Pacific Ocean. Six hundred miles to the east, across the Bolivian bulge at the Andes’ widest point, the world’s largest collection of wetlands form the Pantanal.

“If we could rewind a video of the Andes’ formation,” MacFadden said, “we’d see how they grew into an immense force, affecting the distribution and abundance of moisture across large portions of South America.”

While we may not have a real-time video, we now have a much cleaarrer picture of how the Andes climbed skyward in a geologically short amount of time, thanks to the efforts of Garzione, MacFadden and the rest of the study’s collaborators.

Additional study co-authors include: Gregory Hoke, University of Rochester; Julie Libarkin and Saunia Withers, Michigan State University; John Eiler, California Institute of Technology; Prosenjit Ghosh, Center for Atmospheric and Oceanic Science; and Andreas Mulch, Universität Hanover in Germany.