1 

2  

AndesMountainsM. 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.

Physicalfeatures

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

3  

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.

*********

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.

4  

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

5  

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

6  

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.

 7 

 

The By DeLe(from Naparadox,

When asintuitivelincompreplateaus.

In fact, mtheir skywmountaintallest pegrow ext

Conventianother, effect, piand vallecrust to slandscaproots whediving bethought tgradual r

Andeene Beeland atural Histor

accessed 5-

sked if mounly gravitate tehensively lo

most known mward climb. ns—despite teaks. And thremely tall i

ional geologand these acnch together

eys under preshorten and te photographere dense maeneath anoththat a graduarise of the cru

s’ Mou

ry, http://ww25-2012)

ntains growto the “slow ong time to b

mountain buBut for evertheir relative

herein lies thin extremely

gy tells us thactions cause tr an inch or tessure from thicken into hers strive toaterial accum

her whereby al erosion of ust (see figu

untain

ww.naturalhis

w slowly and and steady”

build up thei

uilding procery rule theree geologic yo

he mountainoy short time p

at as the eartthe earth’s stwo of your your fingerscrenulationso capture onmulates overmaterial is s

f this root by ure right).

8 

nous P

storymag.co

steadily ver” model. Mouir scads of bo

esses do reque is an exceptouth, these mous paradox:periods?

th’s tectonicskin to crumpforearm skin

s, deforming s and folds, w

n film. But ber time, often scraped off o

the more pl

Parado

om/partner/th

rsus in rapid untains, we oulders, jagg

uire large amtion. Considmountain be: How do ge

plates collidple and fold.n. Just as yo

g tectonic prewhich alpinielow the surfrom the ac

of one onto tlastic astheno

ox

he-andes-mo

spurts, mostare taught, tged peaks an

mounts of timder the Himalts rank amoologically y

de and dive b. For a super

our skin crumessures causeists yearn to rface, mounttion of one t

the other. It wosphere resu

ountainous-

t people ake an nd high-altitu

me to complealaya and Anong the worloung mount

beneath onerficial visual

mples into pee the earth’sclimb and ains have detectonic platwas previouulted in the

ude

ete ndes d’s tains

e l eaks s

eep te sly

 

But a newAmerica much fasmaterial

“Our findcould be worldwidUniversitmantle liremoved process c

w study trackshows that m

ster fits and sfrom the mo

dings will foan importan

de and over gty of Rochesthosphere anrapidly—ei

called delam

king the uplimountain buspurts than pountain’s roo

orce geologisnt process thageologic timster. “The sund dense lowther by dow

mination.”

ift of a centruilding—whapreviously reot.

sts to acknowat causes rap

me,” said leadubduction prower crust thatnward dripp

9 

ral portion ofat geologistsealized due t

While cthe Andsync wbeneathscientisaccumukilomecoast, Fpaleontis not wco-authjournal

“Insteaeach yeor puncyears o

The aulithospwas squprocessdense mmore pthe atheload cabuoyedexcisio

wledge that rpid surface ud author Carocess may ct accumulate

ping, which i

f the massivs term “orogto the rapid l

conventionades Mountai

with the scrunh the South Asts know hasulate millenn

eters below SFlorida Mustologist Brucwhat happenhor of the stul Science.

ad of the Altiear, we founctuated uplifof stability,”

uthors assert here (which ueezed undeses caused lamaterial to pplastic upper enosphere. T

aused the surd upward likon of massiv

removal of luplift in differmala Garzioause shortenes at depth uis a convecti

ve Andes Mogeny”—may loss of large

al theory wouins rose gradnching of theAmerican pls caused dennia after milSouth Amerieum of Natuce MacFadd

ned after all. udy publishe

iplano risingnd two phaseft intersperseMacFadden

that as the ch floats aboveer deformingarge parts of

plummet dowmantle laye

This looseninrface crust la

ke a released e extra weig

lower lithosperent mountaone, a geologning and thicuntil that denive process,

ountains in Sactually occamounts of

uld predict thdually and ine Nazca platlate, which nse material llennia up to ica’s westernural History den said that MacFadden

ed June 6 in

g little by littes of spasmoed by millionn said.

crustal layer,e the mantleg pressures, ef the accretedwnwards intoer, also knowng of the rooayer to rise, cork, by the

ght below.

phere materiain belts gist at the ckening of thnse material or by anothe

South cur in f

hat n te

to 70 n

this n is a

the

tle odic ns of

or e) earth d o the

wn as ot

e

ial

he is er

 

Geochem

synthesizstrong ca

A professancient eused, the

“RemarkColorado

In 2005, national wpresented

micalclues

zed research ase for the tim

sor of earth elevations sa multiple me

kably, the rapo Plateau,” D

Sahagian, wworkshop tod her work, b

in theoreticaming and co

and environmid that whileethods used

pid recent upDork Sahagia

who is also tho refine and sbut Sahagian

al geophysicnsequences

mental sciene weaknessein this study

plift scenarioan said.

he director ostrengthen pn said her An

10 

The rebetweethen lepulse omillionlandscin a maAltiplaexaminincludiisotopebearing

The rethe fornodulenoduledeposiyears oproxy which them trecordknownverticamagma

“Carmshows

cs, geochemiof the Altipl

nces at Lehigs were inher

y made the re

o presented h

f Lehigh’s Epaleoelevationdean projec

esearchers foen 30 millioneveled off inof rapid uplin and 6 milli

cape rose betassive upwaano’s sequenned several ling two diffees, fossil plag deposits.

esearchers corm of oxygenes made of ces were sampits between fold. Oxygenindicators fothey formed

to reconstrucds, and then ln temperatural elevation ga and sedim

mie’s ability ther brillianc

istry, and palano’s rise.”

gh Universitrent when sinesults robust

here is simila

Environmenton techniquect was just b

ound that upln and 20 mil

nto relative stift” occurredion years agtween 1.5 anards spurt. Tontial rise, thelines of prox

ferent types oants and anci

oaxed geochn isotopes fr

calcium carbopled from lafive million

n isotopes seror the actuald—so the resct ancient temlinked these re clines assogain. They a

ment as additi

to put this stce,” MacFadaleontology a

ty who also rngle proxy mt.

ar to what I

tal Initiative,s. Garzione

beginning at

lift began llion years atability until

d between 10o when the

nd 3.5 kilomeo reconstruce researchersxy evidence of stable ient magneti

emical cluesrom ancient onate. The

ayered soil and 28 millirve as reliabl temperaturesearchers usmperature records to

ociated with also analyzedional proxies

tudy togetherdden said. “Sand made a

researches methods wer

found for th

, organized aattended andthat time.

ago, l “a 0

eters ct the s

ic-

s in soil

ion le es in ed

d s.

r She’s

re

he

a d

 

“The greproblem,

terrestriaAtacamahundred largest co

“If we cogrew intoportions

While weclimbed sMacFadd

atest novelty” Sahagian s

al chain on tha, stretches bmiles to the ollection of w

ould rewind o an immensof South Am

e may not haskyward in aden and the r

y in their stusaid. “This i

he planet, wietween the Aeast, across

wetlands for

a video of thse force, affemerica.”

ave a real-tima geologicallrest of the stu

udy is the nums the right w

ith peaks soaAndes’ centrthe Bolivian

rm the Panta

he Andes’ foecting the dis

me video, wely short amoudy’s collab

11 

mber of proxway to go abo

Reconsclimat

MacFadecademammthe resthe AltgeologGarziogeologlay in uaffecteanimaldrivingcertain

“The bAndes the Southis evupon pcontine

Based likely h

Todaysnakeswester

aring to 22,8ral western fn bulge at thanal.

ormation,” Mstribution an

e now have aount of time,borators.

xies they broout it.”

structingantes

adden, who hes collecting

mals from Bosearch team ttiplano wher

gical age seqone’s interestgy, MacFaddunderstandined South Amls. But in ordg climatic chn size.

big-picture qgrow high e

uth Americavent obviouslplant and anient,” MacFa

on their findhappened ar

, the massivs 4,400 milesrn edge and i841 feet. Thefoothills andhe Andes’ wi

MacFadden snd abundanc

a much clear thanks to th

ought to bea

ncienttopog

has spent neg and studyinolivia, contrito several kere he had est

quences in yet was ground

den’s interesng how the A

merica’s ancider for mounhanges, they

question is: Wenough to bean climatic rly had cascaimal life acroadden said.

dings, MacFround 10 mil

e Andes Mos along the cis the longese world’s drid the Pacific idest point, t

said, “we’d se of moistur

arer picture ohe efforts of

ar on the

graphies,

arly three ng fossil ibuted by leaey fossil sitetablished ears prior. Wded in the t in the projeAndes’ birthient climate ntains to beghave to reac

When did theecome driveregime? Beca

ading effectsoss the

Fadden said tllion years a

ountain belt continent’s st unbroken iest desert, thOcean. Six the world’s

see how theyre across larg

of how the AGarzione,

ading es in

While

ect h and

gin ch a

e rs of ause

this ago.

he

y ge

Andes

12  

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.