The Bolivian Altiplano: The High Plateau in the Mountains
The temperature is -17°C in the tent! Altitude: 4,700m, the highest campsite of the journey. The early morning sunlight does not succeed in heating up the frozen air of Sur Lípez, the southernmost region of Bolivia. The last noticeable human presence was a tourist 4x4 vehicle yesterday afternoon. Since then, nothing; the whole desert of the high Altiplano was ours. For the first time, we experienced true silence: no running water, no moving plant, no insect or bird, and no wind.
This vast emptiness was our anxiogenic sleeping cocoon, a pure feeling of isolation and solitude. After a tasteless oat-based breakfast, we extracted our creaking bodies out of the tent to embrace a stunning landscape: the Salar de Chalviri, nestled between mineral red-ochre and yellow snow-coated, eroded, volcanic edifices. The Bolivian Altiplano revealed in all its cruel beauty, so challenging to conquer. Ten exhausting days fighting against awful sandy and deep washboard tracks will be necessary to escape this hostile part of the world.
The Altiplano Plateau
The Altiplano is a high plateau that formed along with the tectonic uplift of the Andes mountains. Its average elevation is about 3,750m. It is located where the Andean range is the widest, bounded to the west by the peaks of the Western Cordillera, consisting of the modern volcanic arc, and to the east by the peaks of the Eastern Cordillera, of tectonic origin. This peculiar geological structural configuration makes the Altiplano an area of inland drainage (also called an endorheic basin), the lowest point of which is the Salar de Uyuni. The basin has an approximate area of 154,000 km2.
The existence of the plateau is the result of a gap in tectonic deformation in this segment of the Andes. Here, most of the tectonic shortening concentrates on the eastern boundary of the plateau, in the eastern Cordillera and the Sub-Andean ranges, whereas the plateau itself has not experienced much shortening since the Upper Miocene. The reason why the Altiplano Plateau is poorly affected by tectonic shortening is still debated. Its preservation as a high plateau is a result of the arid climate, since the very limited erosion did not erode the western and eastern boundaries of the endorheic basin enough to connect it to either the Pacific or the Atlantic.
Natural Sculptures and Wonderful Lakes
Volcanic rocks make up most of the relief of the southern end of the Bolivian Altiplano. Among these, extensive sheets of ignimbrite deposits resulting from gigantic volcanic explosions almost entirely covered this part of the plateau during the Oligocene and Miocene. In such a dry and windy climate, ignimbrite exhibits peculiar weathering patterns due to the abrasive effect of sand transported by the ferocious winds. These aeolian processes have sculpted astonishing natural features, as can be seen on the next page, which evoke the surrealist paintings of Salvador Dalí, which is why it became known as the Salvador Dalí Desert. The southern Altiplano Plateau hosts shallow lakes of wonderful colors, targets of photographers from all over the world. The southernmost of these lakes is the Laguna Verde (‘green lake’ in Spanish), close to the Chilean border. It nestles at the foot of Licancabur volcano, the shape of which is characteristic of Andean strato-volcanoes. Its green color is a result of mineral suspensions of arsenic and other minerals collected by the weathering of the neighboring volcanic rocks. Depending on the sediment input into the lake, the colors vary from turquoise to dark emerald green.
The other famous and picturesque lagoon is the salty Laguna Colorada (‘colored lagoon’ in Spanish). Extending over more than 60 km2 and less than one meter deep, the deep red ‘bloodcolored’ waters spectacularly contrast with shining white borax islands, the deep blue sky of the high altitudes and the snowy peaks surrounding the lake. Suspension of red fine sediments and pigments of certain types of algae provide this unique color. Because it contains high amounts of plankton, Laguna Colorada is a special nesting place for three species of flamingos, including the extremely rare James’ Flamingo.
The Salar de Uyuni
The Salar de Uyuni is the world’s largest salt flat, extending over an area of 10,000 km2 at an elevation of 3,656m. It is the lowest point of the endorheic Altiplano Basin and so collects all the running water. It is covered by a salt crust a few meters thick, although the salt deposit can locally reach up to 140m in places. This vast shining white plain is extraordinarily flat, with an average elevation change of less than one meter over its entire area.Natural sculptures of the Salvador Dalí Desert. © Olivier Galland and Caroline Sassier.
Salars are the natural products of endorheic basins in an arid climate. Their formation results from a similar process to that of salt accumulation in the oceans. The rain falling on the endorheic basin reacts and weathers the exposed rocks, dissolving ions. These running waters collect into rivers, which converge toward the lowest point of the basin, in this case the Salar de Uyuni, where the dissolved salty elements transported by the rivers accumulate. The arid climate conditions then trigger water evaporation, leaving the salty elements behind. Continued inflow of water from rivers brings more salty elements, which again are deposited after water evaporation. This process continues through time and leads to high concentrations and large volumes of salts; the subsurface of the Salar de Uyuni hosts salt-saturated brines. The drained area which collects at the Salar de Uyuni is so vast that when, during the wet season, Lake Titicaca further north overflows and discharges into other lakes and salars, the waters eventually reach Salar de Uyuni. When flooded, the Salar de Uyuni can be described as the ‘world’s largest mirror’.
One of the salts contained in the brines of the Salar de Uyuni is subject to worldwide interest: lithium chloride (LiCl). The so-called ‘white gold’ is a key player in the industrial boom of electric car batteries, among others. Exploration campaigns estimate that the Salar de Uyuni hosts more than half of the world’s reserves of lithium. Together with Chile and Argentina, Bolivia is part of the Lithium Triangle, which is believed to host 75% of the known lithium reserves! One reason for the high abundance of lithium in this part of the world is that the rocks surrounding the salars are young volcanics, containing significant amounts of the mineral.
The production of lithium consists of pumping out the subsurface brines, which are stored in large evaporation pools. In Uyuni, the lithium concentration in the brine is about 0.3%, i.e. almost ten times lower than in the Salar de Atacama. During the evaporation process, several salts successively precipitate, including halite (NaCl), sylvite (KCl), magnesium chloride (MgCl2) and eventually lithium salt. However, because of the high altitude of the Salar de Uyuni, the relatively abundant precipitations, moderate temperatures and brine concentrations, the salt separation process from evaporation in Uyuni takes about six months, making the production costs high with respect to other deposits.
Cerro Rico de Potosí
The Eldorado legend in South America probably takes its origin from the infamous Cerro Rico de Potosí. This mountain, in the Bolivian Eastern Cordillera, east of the Salar de Uyuni, hosts the world’s largest silver deposit. The massive 4,782m-high red mountain dominates the incredible city of Potosí, which has many stunning Spanish imperial-style edifices. The Cerro Rico was the richest source of silver in the history of mankind.
The silver ore was known long before the Incas, but its mass production started with Spanish conquistadores, back in the mid-16th century, to fulfill their thirst for shiny metals. Their discovery of the silver deposit revolutionized this part of the Andes. In just 70 years, what used to be a high altitude, unpopulated Incan shepherd area became a city of 170,000 inhabitants, the fourth largest in the Christian world, bigger than London, Milan and Seville put together. The activity boom associated with the silver rush attracted artists, architects and intellectuals. However, the downside of this was the need for manual workers in the mines, so that thousands of native Peruvians and African slaves were deported to Potosí to extract the silver ore. Many of them perished in the mine, and the Cerro Rico became ‘the mountain that eats men’. The mountain’s silver production was so large that it changed the world forever, facilitating the exchange of slaves, fabrics, spices and other goods across the globe: it is considered that this initiated the premises of capitalism.
The Cerro Rico de Potosí ore deposit belongs to the Bolivian tin belt, which is a mineral-rich region in the Eastern Cordillera of Bolivia with voluminous tin, tungsten and silver deposits. The mineralizations of the belt formed as a result of fluids expelled from peraluminous magmas, which usually form by partial melting of the continental crust. The Cerro Rico itself is an extruded dacite volcanic dome, the age of which is estimated to be 13.8 ± 0.2 Ma. The intense hydrothermal alteration associated with the volcanic dome made the mountain red, recognizable far away from all over the area.
Today, small-scale artisanal mining activity remains, mostly to extract zinc and tin. The working conditions are worthy of Europe’s 19th-century mining, and life expectation among the miners is about 40 years. It is possible to visit the miners in the galleries, after buying peculiar gifts for them at the local market: coca leaves, 96% alcohol, and dynamite sticks for half a euro. A real geological adventure!