The geography of Perú

The geography of Perú

This section begins with a discussion of the physical forces which have shaped Perú. It then offers a detailed description of the primary mountain ranges.

There are three dominating facts about Perú's geography: plate subduction and the resulting Andes, the Humboldt current and the proximity of Northern Perú to the equator. The Andes cut the country into three completely different regions. The current keeps the coast a desert, but provides abundant marine life. The equatorial position makes much of the country a humid jungle.

The Humboldt current sweeps up the coast of South America from Antarctica. It is a rising current, and so brings with it nutrients from the sea bed, making the ocean off Perú extremely fertile. Vast shoals of anchovies used to be caught from it before over-fishing made them virtually extinct. It is also a sharply cold current, making the sea temperature only 4°C. It usually does not quite brave the equator, sweeping off to the West. The steady flow of the Humboldt is occasionally disrupted by inflows of hot water from the central Pacific, creating the el Niño phenomenon that affects the climate world wide. We discuss this further elsewhere.

The ocean offshore shelves very steeply to a deep trench, where the Pacific plate dives under South America. This makes for high waves and strong currents. On-shore, the land is flat for 10-80 km, being a desert made from finely divided, wind-milled sand. Cold fogs blow in off the ocean, condensing on the higher land masses (lomos) where condensation allows a fairly complex ecology to exist. There is a national park just North of Lima which preserves this near-unique ecosystem. Much of the coastline receives no rainfall at all, and the official driest place on Earth is just South of the border, in Chile.

Numerous rivers run down from the Andes and across this desert. These have been the centres of settlement in Perú for at least 10,000 years. Today, they are welcome patches of green in the beige desert. The irrigation systems that were established by the Inca are sometimes still in use, but many are dry due to the changing tilt of the ground. The Andes are rising, and what was once downhill is now up. In some locations, it is possible to see fans of channels, cut successively over hundreds of years to combat this phenomenon. Southern canals are often constructed underground and may runs for tens of kilometres to reach the fields.

The Andes rise abruptly from these plains. Indeed, the sandy coastline is appended on the rock face which rises from the base of the trench offshore to the face peak of the cordillera. The youngest mountains - often heavily modified by both volcanism and erosion - open onto a high plain, the altiplano or puna. Lower hills at the Eastern flank drop swiftly down to the jungle region beyond. The Northern Perúvian Andes lack much of the puna, and are thus more striking when viewed as mountains than those of the South, where peaks rise from an already-high plain.

Basalt over Andesite. Note the Inca irrigation canal.

The Andes are a geologically young system. They have been created by the Pacific or Nazca plate pushing itself under South America. The range were first created in the Cretaceous and Tertiary periods. The plate is an active one, and areas on the coast such as Nazca are affected by almost continuous tremors and the occasional major earthquake. Volcanoes are common, although those in Perú are confined to the South of the country.

The Andes are shaped by subduction and volcanism Perú's active volcanoes are clustered in the South.
The subduction diagram does not adequately capture the sheer scale of the system that is at work. The trench that it digs in the ocean bed averages 5000m deep, with the coast line dropping almost sheer from the top of the Andes to the depths of the trough. The Perúvian coast is a piled-up afterthought on this.

The image shown opposite colour codes the Perú-Chile coastline for its depth below or height above sea level. The deep trench can be seen running to the left of the land mass. The high Andes (red) and the altiplano plain (blue and purple) can also be seen.

The general texture of the Andes is a layered, piled-up one, often with extremely complex geology. The rapid weathering of these systems led to the concentration of metal ores which, in their turn, provided the impetus for the wars of conquest fought over Perúvian soil.

The complexity of the geology can be seen from the admittedly crude map. The impact of volcanism is evident, with large areas of Andesite overlain with volcanic layers. Many river valleys and cliff faces offer a layer cake of basalt, limestone, andesite, more basalt, shale and so forth, all piled up, folded and rifted like a rock salad.

The Andes are the world's longest range, and they support mountains and volcanic peaks which are amongst the world's highest. Aconcagua in Chile is the highest Andean mountain, at 6,960 m. It is also the highest mountain of the Western Hemisphere.

Perú's highest peak is Huascarán, at 6768m. It is a part of the Northern Cordillera Blanca. Other peaks rise over 600m in the central Cordilleras of Huayhuash and Raura. There are also a number of 6000 m peaks in the South around Arequipa: Nevadas Coropuña and Ampato.

This Southern region is, as already remarked, a centre for volcanic activity. Bolivia has two notable volcanoes, Illimaní and Illampú. In Perú, the South offers a national park dedicated to volcanoes, el Valle de los Volcanes. [See here].

One of the the most active is Sabancaya, at 5967 m. As the photograph shows, it is the middle and smallest of three glacier-topped stratovolcanoes, and it is younger than its neighbours because it has two recent lava flows on its Eastern side. It began to erupt in 1986.

The jungle region consists of recent erosion sediments from the Andes, overlaying continental shelf. It has hydrocarbon deposits - oil in the North and very substantial gas reserves in the South. The Camisea natural gas field is one of the largest unexploited reserves on Earth. (Coupled to its hydroelectricity potential, Perú is potentially an enormously energy-rich country. It has yet to find a use for this, however.)

The other distinguishing feature of the jungle is its rainfall, which is considerable. The Andes act as a trap to winds off the Brazilian land mass and precipitation follows. The resulting drainage basins and river systems are extensive, including the source(s) of the Amazon. Rain that falls has to flow 4000 km to reach the sea, and it is estimated that an average molecule will be transpired by trees, subject to condensation, fall as rain and then re-evaporate a dozen times before it reaches the sea. The very large rivers meander and change course, as they have done for millions of years. During the wettest season, forest may be flooded 10 or more metres deep. It is possible to sit on some stretches of the Amazon and see only water - perhaps with a distant tree fringe just perceptible - as if on an ocean. If one is operating from a dug-out wooden vessel with a spluttering, moody little engine, then this can be a sobering experience. A compass is a good accessory to such a trip.

The consequence of this rainfall has been heavy erosion of the Eastern Andes - as evidenced by the sediments in the jungle basin - and extensive secondary weathering of these. Much of the jungle is therefore nutrient-deficient, and in some areas the soils are weathered to near-Bauxite. Rivers that run from these areas are usually tea-coloured, and largely void of plant life. One often finds insect-eating plants in such a setting, where the aim is less to eat meat than to supplement the meager soil nutrients.


The main Perúvian Cordilleras

The Andes can be thought of as the southern end of a mountain chain that runs from Alaska, through USA, Central and down into South America. The cause is, of course, plate subduction from the Pacific. The Perúvian Andes consist of two structural folds, the Eastern and Western Andes, known as the Cordilleras Oriental and Occidental. The Northerly ranges are all in the Cordillera Occidental, with the Western range offering peaks in the southern parts of the country.

People who are interested in the geology of Perú are usually also interested in its mountains. What follows is a more detailed look at the individual cordilleras or ranges. Readers may note that some areas that are of great interest to tourists - such as the Cordillera Vilcanota - are not covered, partly because they are treated in detail elsewhere, and partly because they are factually too small to enter this list on equal terms. Most of Perú's cordilleras are barely touched by tourism.

Cordillera Occidental

Cordillera Blanca

The Cordillera Blanca is the name given to the East slopes of a valley overlooking the Santa river. It is also frequently called the "Callejón the Huaylas". The West side is known as the Cordillera Negra, and this is lower, much drier and usually without snow cover. The Cordillera Blanca is the most extensive series of snow peaks in the tropics. Despite millennium of deglaciation, it still carries the major ice concentration in Perú. It runs North-West for about 200 km between latitude 8°08' and 9°58'S. and longitude 77°00' and 77°52'W. There is a detailed trekking description here.

The Cordillera Blanca marks the continental divide, for the Río Santa drains into the Pacific Ocean, whilst Río Marañón does so into the Atlantic Ocean. It is a point of some amusement (to the author, at any rate) to urinate on one of the many saddles in the range and mutter to oneself - "This is for the Atlantic and this is for the Pacific.

The Cordillera Blanca is the mountain scenery showpiece of Perú, as it has five peaks that rise above 6,000 m crammed into a relatively small area. Please see here for a panorama. Nevado Huascarán rises to an elevation of 6,768 m, slight by the standards of the much younger Himalayas, but still an extremely impressive sight.

There are some 722 individual glaciers in the Cordillera Blanca, covering an area of 723 square kilometres. Rainfall comes in from the jungle on the West, and so these are focused on the Western slopes of the range, so that 530 Western glaciers cover more than 500 square kilometres. Glaciers terminate at around 4200m, but the Holocene retreat is rapid and people will tell you that the ice sheets stretched kilometres further when they were young. Around 890% of the glaciers are "mountain" types, having steep slopes and a short extent. There is a single ice cap, and a few "valley" glaciers. The Northern part of the range effectively begins with Aguja Nevada, Artesonraju, Chacraraju, and Nevados Huandoy.

South of Huandoy lies the great peak of Huascarán. The whole region was shaken by a major earthquake in 1970 and the North peak of Huascarán shed a large fraction of its South-Western ice cover. The ice fell 3500m and moved about 12 km in a few minutes, bringing with it millions of tonnes of rubble. This completely destroyed the city of Yungay. The image above as taken days after the event,whilst that below was taken in 1981, eleven years after the earthquake. Yungay has now been rebuilt about a kilometre from the original site, which remains a sterile plain of rubble, building parts and other debris.

Cordillera Huallanca

The Cordillera Huallanca is a small but highly accessible range. It lies between latitude 9°52' and 10°03'S and longitude 76°58' and 77°04'W. It is about 20 km long, with some 20 square kilometres of snow cover. The highest peak attains 5,480m.

Cordillera Huayhuash

The Cordillera Huayhuash is described in detail elsewhere. It is effectively an extension of the Cordillera Blanca, lying between lat 10°11' and 10°26'S and long 76°50' and 77°00'W.Although only around 26 kilometres long, it offers nearly 90 square kilometres of snow cover. The highest peak is Cerro Yerupaja, 6,634m. Meltwater drains chiefly into the Pacific through the Río Pativilca, although it also connects through the Río Marañón to the Amazon basin and the Atlantic.

Cordillera Raura

The Cordillera Raura lies between latitude 10°21' and 10°31'S. and longitude 76°41' and 76°50'W, covering about 20 kilometres. It has about 57 square kilometres of snow cover, chiefly made up by mountain glaciers. The highest peak, Cerro Santa Rosa, rises to 5,727m above sea level. The Cordillera drains into the Pacific Ocean by Río Pativilca and Río Huaura and into the Atlantic Ocean by way of the Amazon, but is initially drained by the by Ríos Marañón and Huallaga.

Cordillera La Viuda

The Cordillera La Viuda is 130 kilometres long, with some 25-30 square kilometres of snow cover. It runs North-West between latitude 10°33' and 11°37'S, and longitude 76°07' and 76°42'W. It is a pleasing but unspectacular set of minor peaks in high altiplano, but it is little visited and the potential for relatively gentle trekking in a wild area is strong. Drainage to the Pacific Ocean occurs by way of the by Ríos Huaura, Chancay, Chillón, and Rímac - the water supply for Lima itself - and eastward to the Atlantic Ocean by means of the Río Huallaga and Mantaro. The highest peak in the range is the Nevado Alcoy at 5,780m above sea level.

Cordillera Central

The Cordillera Central is a large area which follows the trend of the Andes for 100 kilometres, offering about 175 square kilometres of snow cover clustered in two main fields. Unusually, this offers two valley glaciers. Its co-ordinates are latitude 11°37' and 12°26'S. and longitude 75°30' and 76°18'W. Like the Cordillera La Viuda, it drains to the Pacific Ocean by the Ríos Rímac and Cañete, and to the Atlantic Ocean by Río Mantaro. The highest peak is the Nevado Cotoni at 5,817 m above sea level.

Cordillera de Chonta

Here we encounter the first of the snow peaks in the Cordillera Oriental, the Western branch of the Andes. The Cordillera de Chonta offers segregated ice-peaks across a length of some 50 kilometers, between latitude 12°37' and 13°07'S and longitude 75°00' and 75°30'W. Altogether, it has about 50 square kilometres of snow cover. The Río Cañete drains the area to the Pacific and the Río Mantaro takes water away to the Amazon. The highest peak is Nevado Palomo at 5,305 m above sea level. (More on the Cordillera Oriental in a section which follows.)

Cordillera de Huanzo

Returning to the the Cordillera Occidental, the Cordillera de Huanzo runs for 57 kilometres between latitude 14°30' and 15°01'S. and longitude 72°50' and 73°15'W. Its highest peak is the Nevado Huaychahui at 5,445 m above sea level. This is a little-visited range - at least for purposes of walking- and it has at least 150 square kilometres of snow cover. It feeds the by Río Ocoña, which exits to the Pacific Ocean and the Río Apurimac, which runs to the jungle and ultimately the Atlantic Ocean.

Cordillera Chila

The Cordillera Chila consists of three blocks of mountain terrain between latitude 15°02' and 15°26'S and longitude 71°43' and 72°37'W. It runs in an Easterly direction for some 80 kilometres, offering about 50 square kilometres of snow peaks. The highest peak is the Nevado Mismi at 5,556 m above sea level. The Río Colca takes melt water into the into the Pacific Ocean and the Río Apurimac flows, once again, to the jungle. This is the official source of the Amazon, although in practice it has many, many tributaries. No pots of gold or fountains of youth were available when I visited it.

Cordillera Ampato

The South of the country is characterised by recent volcanic activity, and the Andes geology gives way to basalt, cinder layers and alluvium. The Cordillera Ampato consists of three massive volcanoes: Ampato, Coropuna, and Solimana. Each are regarded as a 'Nevado' in their own right, collectively offering about 100 square kilometres of snow cover. The group lies between latitude 15°24' and 15°51'S. and longitude 71°51' and 73°00'W, and covers about 140 square kilometres. The highest peak is the Nevado Coropuna, at 6,426 m above sea level. Detailed discussions of trekking in the region are available here.

Cordillera Volcánica

Continuing on this geological theme, the Cordillera Volcánica runs parallel to the Pacific coast for about 50 kilometres, between latitude 16°07' and 16°33'S and longitude 71°12' and 71°33'W. The highest peak is Nevado Chachani, at 6,100 m above sea level, but the best known and most photographed is the Volcán Misti, near to Arequipa.

Cordillera del Barroso

Close to the Bolivian and Chilean borders, in the deep South of the country, lies the seldom-visited Cordillera del Barroso. This, too, is volcanic in origin. It lies between latitude 16°51' and 17°37'S and longitude 69°45' and 70°30'W, running for about 110 kilometres, and offering about 20 square kilometres of snow cover. The Ríos Caplina, Sama, Locumba and Río de Ilo run to the Pacific; the Río Huenque feeds Lake Titicaca basin. The highest peak is Volcán Tutupaca, at 5,741 m above sea level.

Cordillera Oriental

Cordillera Huagaruncho

This ten kilometre range is drained by the Huallaga river. It lies latitude 10°14' and 10°19'S and longitude 75°57' and 76°03'W. Nevado Huagaruncho reaches 5880m and around 48 square kilometres is snow-covered. The Huallaga river valley is one of the world's centres for narcotics production and expeditions should be aware of this.

Cordillera Urubamba

The Urubamba river is, of course, known for its associations with Cuzco, Machu Picchu and related attractions. It does not, however, drain this Cordillera, which in fact uses the Ríos Vilcanota and Yanatile. It lies between latitude 13°08' and 13°17'S and longitude 71°58' and 72°16'W and runs for around 30 kilometres, offering about 20 square kilometres of snow cover and glaciers. The Nevado Veronica reaches a height of 5,750 m.

Cordillera de Vilcanota

This region is the subject of a detailed walking guide, here. It is second only to the Callejon de Huayllas as a concentration of snow cover in Perú. It runs around 80 kilometres North-South and around 40 kilometres East-West, between latitude 13° 39' and 14°29'S and longitude 70°31' and 71°20'W. Nearly 540 square kilometres are snow-covered or glaciated. It drains into the selva through the Ríos Vilcanota, Paucartambo, Inambari, and Río Madre de Dios. Nevado Auzangate reaches 6,384 m, nearly the same as Huascaran in Huayllas.

Quelccaya, the Western part of the range, is seldom trekked despite offering the largest single glacier in Perú, and probably in the Southern hemisphere. This sheet is 54 square kilometres in extent, and reaches 5,650 m. The edges are retreating sharply and present steep ice cliffs, but once ascended, the area offers good walking to the experienced (and wary.)

Cordillera de Carabaya

This is a 75 kilometre long Cordillera located between latitude 14°00' and 14°22'S and longitude 69°38' and 70°19'W, and offering around 100 square kilometres of ice cover. It drains into Lake Titicaca basin and to the jungle through the Río Inambari. Nevado Allincapac attains 5,780 m.

Cordillera Apolobamba

This Cordillera is essentially a Bolivian range, the end of which projects into Perú. This section is only around 35 kilometres long, between latitude 14°35' and 14°45'S and longitude 69°14' and 69°34'W. It is, however, heavily clad with snow, offering about 100 square kilometres of lonely peaks. The highest peak, Nevado Ananea, stands at 5,850 m.