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1 AndesMountains
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.
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
2 South American Plate and the oceanic Nazca Plate—gave rise to the orogenic (mountainbuilding) 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.
3 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
4 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.
6 7 The Andes’ Mou
untain
nous P
Paradoox
By DeLeene Beeland
(from Na
atural Historry, http://ww
ww.naturalhisstorymag.coom/partner/thhe-andes-moountainousparadox, accessed 5-25-2012)
When assked if moun
ntains grow
w slowly and steadily verrsus in rapid spurts, mostt people
intuitivelly gravitate to
t the “slow and steady”” model. Mouuntains, we are taught, take an
incompreehensively lo
ong time to build
b
up theiir scads of booulders, jaggged peaks annd high-altituude
plateaus.
m known mountain
m
bu
uilding proceesses do requuire large am
mounts of tim
me to compleete
In fact, most
their skyw
ward climb. But for everry rule theree is an excepttion. Considder the Himaalaya and Anndes
mountain
ns—despite their
t
relativee geologic yo
outh, these m
mountain belts rank amoong the world’s
tallest peeaks. And th
herein lies th
he mountaino
ous paradox:: How do geologically young mounttains
grow extremely tall in
i extremely
y short time periods?
p
Conventiional geolog
gy tells us thaat as the eartth’s tectonic plates collidde and dive bbeneath onee
another, and these acctions cause the
t earth’s skin
s
to crumpple and fold.. For a superrficial visuall
effect, pinch togetherr an inch or two
t of your forearm skinn. Just as yoour skin crum
mples into peeaks
and valleeys under preessure from your fingerss, deformingg tectonic preessures causee the earth’ss
crust to shorten
s
and thicken
t
into crenulationss and folds, w
which alpiniists yearn to climb and
landscape photograph
hers strive to
o capture on
n film. But beelow the surrface, mountains have deeep
roots wheere dense maaterial accum
mulates overr time, often from the action of one ttectonic platte
diving beeneath anoth
her whereby material is scraped
s
off oof one onto tthe other. It w
was previously
thought that
t a graduaal erosion off this root by the more pllastic asthenoosphere resuulted in the
gradual rise
r of the cru
ust (see figu
ure right).
8 But a new
w study track
king the upliift of a centrral portion off the massivve Andes Moountains in S
South
America shows that mountain
m
bu
uilding—whaat geologistss term “oroggeny”—may actually occcur in
much fasster fits and spurts
s
than previously
p
reealized due tto the rapid lloss of large amounts off
material from the mo
ountain’s roo
ot.
While cconventionaal theory wouuld predict thhat
the Anddes Mountaiins rose graddually and inn
sync w
with the scrunnching of thee Nazca platte
beneathh the South A
American pllate, which
scientissts know hass caused dennse material to
accumuulate millennnia after milllennia up to 70
kilomeeters below S
South Ameriica’s westernn
coast, F
Florida Museum of Natuural History
paleonttologist Brucce MacFaddden said that this
is not w
what happenned after all. MacFaddenn is a
co-authhor of the stuudy publisheed June 6 in the
journall Science.
“Insteaad of the Altiiplano risingg little by litttle
each yeear, we founnd two phasees of spasmoodic
or puncctuated uplifft intersperseed by millionns of
years oof stability,” MacFaddenn said.
The auuthors assert that as the ccrustal layer, or
lithosp here (whichh floats abovee the mantlee)
was squueezed undeer deformingg pressures, eearth
processses caused laarge parts off the accretedd
dense m
material to pplummet dow
wnwards intoo the
more pplastic upper mantle layeer, also know
wn as
the atheenosphere. T
This looseninng of the rooot
load caaused the surrface crust laayer to rise,
buoyedd upward likke a released cork, by thee
excisioon of massive extra weigght below.
“Our find
dings will fo
orce geologissts to acknow
wledge that rremoval of llower lithospphere materiial
could be an importan
nt process thaat causes rap
pid surface uuplift in diffeerent mountaain belts
worldwid
de and over geologic
g
tim
me,” said lead
d author Carrmala Garzioone, a geologgist at the
Universitty of Rochesster. “The su
ubduction pro
ocess may c ause shortenning and thicckening of thhe
mantle lithosphere an
nd dense low
wer crust thatt accumulatees at depth uuntil that dennse material is
removed rapidly—either by downward dripp
ping, which iis a convectiive process, or by anotheer
c
delam
mination.”
process called
9 micalclues
Geochem
The reesearchers foound that upllift began
betweeen 30 millionn and 20 milllion years aago,
then leeveled off innto relative sttability untill “a
pulse oof rapid upliift” occurredd between 100
millionn and 6 milliion years ago when the
landsccape rose bettween 1.5 annd 3.5 kilomeeters
in a maassive upwaards spurt. Too reconstrucct the
Altiplaano’s sequenntial rise, thee researcherss
examinned several llines of proxxy evidence
includiing two diffe
ferent types oof stable
isotopees, fossil plaants and anciient magnetiicbearingg deposits.
The reesearchers cooaxed geochemical cluess in
the forrm of oxygenn isotopes frrom ancient soil
nodulees made of ccalcium carboonate. The
nodulees were samppled from laayered soil
deposiits between ffive million and 28 milliion
years oold. Oxygenn isotopes serrve as reliable
proxy indicators foor the actuall temperaturees in
which they formedd—so the ressearchers used
mperature
them tto reconstrucct ancient tem
recordds, and then llinked these records to
knownn temperaturre clines assoociated with
verticaal elevation ggain. They aalso analyzedd
magmaa and sedim
ment as additiional proxiess.
“Carm
mie’s ability tto put this sttudy togetherr
shows her brilliancce,” MacFaddden said. “S
She’s
synthesizzed research in theoreticaal geophysiccs, geochemiistry, and paaleontology aand made a
strong caase for the tim
ming and consequences of the Altipllano’s rise.”
A professsor of earth and environm
mental scien
nces at Lehiggh Universitty who also rresearches
ancient elevations
e
said that whilee weaknesses were inherrent when sinngle proxy m
methods werre
used, the multiple meethods used in this study
y made the reesults robustt.
“Remark
kably, the rap
pid recent up
plift scenario
o presented hhere is similaar to what I found for thhe
Colorado
o Plateau,” Dork
D
Sahagiaan said.
In 2005, Sahagian, who
w is also th
he director of Lehigh’s E
Environmenttal Initiative,, organized a
national workshop
w
to
o refine and strengthen
s
paleoelevatio
p
on techniques. Garzione attended andd
presented
d her work, but
b Sahagian
n said her An
ndean projecct was just bbeginning at that time.
10 “The greatest novelty
y in their stu
udy is the num
mber of proxxies they broought to beaar on the
problem,” Sahagian said.
s
“This is the right way
w to go aboout it.”
Reconsstructingan
ncienttopog
graphies,
climattes
MacFaadden, who hhas spent nearly three
decadees collectingg and studyinng fossil
mamm
mals from Boolivia, contriibuted by leaading
the ressearch team tto several keey fossil sitees in
the Alttiplano wherre he had esttablished
geologgical age seqquences in yeears prior. W
While
Garzioone’s interestt was groundded in the
geologgy, MacFaddden’s interest in the projeect
lay in uunderstandinng how the A
Andes’ birthh
affecteed South Am
merica’s anciient climate and
animalls. But in ordder for mounntains to beggin
drivingg climatic chhanges, they have to reacch a
certainn size.
“The bbig-picture qquestion is: W
When did thee
Andes grow high eenough to beecome driverrs of
the Souuth Americaan climatic regime? Becaause
this evvent obviouslly had cascaading effects
upon pplant and aniimal life acrooss the
contineent,” MacFaadden said.
Based on their finddings, MacF
Fadden said tthis
likely hhappened arround 10 milllion years aago.
Today , the massive Andes Moountain belt
snakess 4,400 miless along the ccontinent’s
westerrn edge and iis the longesst unbroken
terrestriaal chain on th
he planet, wiith peaks soaaring to 22,8841 feet. Thee world’s driiest desert, thhe
Atacamaa, stretches between the Andes’
A
centrral western ffoothills andd the Pacific Ocean. Six
hundred miles to the east, across the Bolivian
n bulge at thhe Andes’ wiidest point, tthe world’s
largest co
ollection of wetlands
w
forrm the Pantaanal.
“If we co
ould rewind a video of th
he Andes’ fo
ormation,” M
MacFadden ssaid, “we’d ssee how theyy
grew into
o an immensse force, affeecting the disstribution annd abundance of moisturre across largge
portions of South Am
merica.”
While wee may not haave a real-tim
me video, wee now have a much clear
arer picture oof how the A
Andes
climbed skyward
s
in a geologicallly short amo
ount of time, thanks to thhe efforts of Garzione,
MacFadd
den and the rest
r of the stu
udy’s collab
borators.
11 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.
12