Andes

The Andes are a classic example of mountains formed by the movement of a continental plate beneath another. They are a modern replication of many ancient mountain belts. Earthquakes and volcanic eruptions indicate that the Andes are still rising. Because of their geologic setting, the Andes contain abundant minerals, oil, and natural gas.

Location and Characteristics

The Andes are some of the most impressive mountains on Earth, not only for their grandeur but also for their geologic importance. Their evolution over the past 200 million years can be explained by the subduction, or descent, of an oceanic plate below a continent. Subduction is basic to the theory of plate tectonics, which is the fundamental paradigm of the earth sciences. The ease with which plate tectonics explained the Andes convinced numerous scientists of the theory's validity and established the mountains as the classic example of the active, or Pacific, type of continental margin. Further, many researchers consider the Andes a modern analog for the Sierra Nevada range of California and the Appalachians of the eastern United States, which are said to have formed along Andean-type margins more than 100 million and 450 million years ago.

The Andes rise 7,000 meters and stretch 7,500 kilometers along the western edge of South America, from Venezuela to southern Chile. The rugged terrain and the topographic relief attest to the youth of the mountains. Elevation drops rapidly from the high peaks of the Andes to the floor of the Pacific Ocean, a few hundred kilometers west, where the Peru-Chile Trench (also called the Atacama Trench), an oceanic trench 7,000 meters deep, lies parallel to the continental margin. The mountains and trench together are known as the Andean arc and are active today. Indeed, the periodic volcanic eruptions are consequences of subduction at the Peru-Chile Trench and are reminders that the Andes are still forming.

The Andes can be divided into three morphological provinces that parallel the South American continental margin. From west to east, these are the western cordillera (meaning “mountain system”), the altiplano, and the eastern cordillera. Each is distinct physiographically and geologically. The provinces are easily delineated in the Andes of southern Peru, Bolivia, and northern Chile, where the features are related exclusively to subduction. North and south of this region, the Andes are more complex, and their geologic evolution is not as straightforward. The descriptions of the provinces, therefore, pertain to the central Andes.

The western cordillera begins 100 kilometers inland from the coast. It rises 6,000 meters in 50 kilometers and contains Mesozoic and Tertiary volcanic rocks, whose compositions are so distinct that they are called andesites after the mountains in which they lie. The eastern flank of the western cordillera descends 1,000 meters in fifteen kilometers to the altiplano, a high plateau (3,800–4,500 meters high) greater than 150 kilometers wide. Beneath the altiplano is a basin filled with ten kilometers of Tertiary sedimentary and volcanic rocks.

Superimposed on the Mesozoic and Tertiary rocks of the western cordillera and the altiplano is a chain of huge volcanoes (7,000 meters) that began forming less than fifteen million years ago and continue to erupt today. The eastern boundary of the altiplano rises abruptly to altitudes near 6,000 meters and marks the transition to the thrust belt of the eastern cordillera. Rocks of the east cordillera were deposited along the South American continental margin between 450 million and 250 million years ago before the onset of subduction. During that time, the west coast of South America looked like the present east coast of North America. These ancient continental margin rocks now lie more than 250 kilometers inland from the present continental margin. East of the eastern cordillera is the Brazilian Shield, a piece of craton, or old continental crust, next to which the Andes have grown.

Evolution

Before the evolution of the Andes can be described, the theory of plate tectonics must be summarized. The theory states that about twelve rigid plates define the Earth's surface. The plates, which are either continental or oceanic, ride on a partially molten layer and move relative to each other at speeds between two and ten centimeters per year. Plates form at mid-ocean ridges, where magma (molten rock) from deep in the Earth ascends to the surface and solidifies into new seafloor. To make room for the new material, old pieces of sea floor on opposite sides of the ridge move away from each other. Thus, the mid-ocean ridge is a divergent plate boundary. Across the oceanic plate from the mid-ocean ridge is a trench that marks the convergent boundary where the oceanic plate moves toward another plate, either oceanic or continental, and subducts, or descends, below it. (Because oceanic crust is denser than continental crust, it always subducts.) If the overriding plate is oceanic, a trench develops in the ocean basin, and a chain of volcanic islands (an island arc) grows on the overriding plate. If the overriding plate is continental, the trench sits along the continental margin, and the string of volcanoes forms on the continental plate (continental arc).

The Andes are above a subduction zone, the Peru-Chile Trench, which is adjacent to the continental margin of South America between the latitudes of 4 degrees north and 40 degrees south. There, the Nazca plate, one of several plates in the Pacific Ocean, subducts below the South American continental plate at a rate of six centimeters per year. Subduction along the South American margin has been active for two hundred million years and has produced the three Andean provinces during this interval. Three parameters govern the physiography and geology of the provinces: the amount of sediment that descends with the subducting plate in the trench, the angle at which the subducting plate dives below the continent, and the quantity of magma generated by melting of the subducting and overriding plates that are added to the continental crust at shallow depths.

As an oceanic plate subducts, it bends below the overriding plate. This bend forms a huge depression, or trench, on the ocean floor. Sediments on the descending plate either remain attached and subduct or scrape against and accrete to the overriding plate. When the descending plate reaches a depth of 100 kilometers, the Earth's temperature is hot enough to melt small areas of either the subducting or the overriding plate. Because the magma generated during melting is less dense than the surrounding solid rock, it rises through the earth, either crystallizing at shallow depths or erupting on the surface.

In the Andes, the overriding plate is continental. Sediments remain attached to the subducting plate and travel to great depths in the Earth. These two factors explain the voluminous andesite in the western cordillera. If magma produced at a depth of 100 kilometers is contaminated either by sediments subducted with the oceanic plate or by continental crust, it yields volcanic rocks of andesitic composition.

The locus of volcanism on the surface is controlled by the angle at which the subducting plate descends below the overriding plate. If the angle is steep, the plate reaches a depth of 100 kilometers closer to the trench than if the angle is shallow. Changes in the angle, therefore, cause the volcanoes to migrate. In the modern Andes, the angle of subduction varies between 10 and 30 degrees. In the past, the angle was steeper. Thus, the recent volcanoes are at the boundary between the western cordillera and the altiplano, whereas the older ones were in the western cordillera closer to the trench.

The addition of magma at shallow depths in the crust pushes adjacent rocks out of the way. This process causes the surface to extend and rift (fracture) immediately adjacent to the volcanic arc and to shorten farther inland. The shortening is accommodated in a thrust belt that develops approximately 200 kilometers continentward of the volcanic arc. In the thrust belt, rocks that accumulated prior to subduction are deformed by faults and folds into mountains hundreds of kilometers long, tens of kilometers wide, and thousands of meters high. The thrust belt in the Andes is the eastern cordillera. The rifting, in turn, forms a trough, or basin, between the volcanic arc and the thrust belt. The basin fills quickly with debris eroded from the adjacent highlands. Lava flows and ash eruptions from the volcanic arc occasionally are large enough to cover the basin, resulting in the interleaving of volcanic and sedimentary rocks. This is precisely the setting of the altiplano, which is a trough between two highlands: the volcanic chain of the western cordillera and the thrust belt of the eastern cordillera.

Before subduction, the eastern cordillera was on the west coast of South America. The rise of the Andes implies that the width of South America has increased by 250 kilometers in 200 million years by forming a new continental crust. The new crust below the western cordillera has a thickness greater than seventy kilometers, compared with a thickness of continental crust of thirty kilometers below the Brazilian Shield. The large thickness of continental crust, called a root, is required by the principle of isostasy, which states that a mass excess on the surface must be compensated for by a mass deficiency at depth. Because the continental crust is lighter than the mantle (the layer of the Earth below the crust), the space taken up by the continental root (which should be taken up by the mantle) is less dense than it should be. Geophysical data indicate that the crust beneath the Andes is homogeneous and is composed of andesite. Subduction below continental margins, therefore, may be a fundamental mechanism by which continents grow.

Study of the Andes

Techniques from various disciplines in the earth sciences, including geophysics, geology, and geochemistry, are used to study the Andes. Those from geophysics document subduction and a thick continental root below present-day South America, whereas those from geology reveal subduction in the past. Geochemical methods provide evidence for relationships between subduction, andesitic volcanism, and continental crust formation.

The most important geophysical tools are seismic reflection profiling and earthquake analysis. Seismic reflection profiling uses sound waves to allow scientists to determine the physical properties of the Earth several tens of kilometers below the surface, the most critical of which is density. Conceptually, seismic profiling is very simple: sound waves travel downward from a source and reflect back to sensors on the surface when they encounter a change in the physical properties at depth. Because the velocity of the waves depends on the density of the material in which they are traveling, the time the reflected waves take to reach the sensor conveys information about the depth of the change and the composition of the rock above it. From this technique, geophysicists learned that the thickness of the crust below the western cordillera and the altiplano in the central Andes is greater than that of the crust below the Pacific Ocean and the Brazilian Shield. They also discovered that the compositions of the crust at the surface and of the crust tens of kilometers below were similar.

The analysis of earthquakes involves finding their location, or epicenter. An earthquake occurs when the Earth's crust breaks along a fault, and the two pieces on either side of the rupture move past each other. The energy released by the faulting travels as waves through the Earth. A geophysicist finds the epicenter of an earthquake by measuring the different times at which the waves arrive at various places around the world. Epicenters of earthquakes in the Andes define a zone that extends at an angle from the Peru-Chile Trench to depths hundreds of kilometers below South America. This zone outlines the subducting Nazca plate and provides strong evidence that the west coast of South America is a convergent plate boundary. (These zones, called Benioff zones after the scientist who discovered them, occur along all convergent plate boundaries.)

Geologic fieldwork is necessary to determine the evolutionary history of the Andes. In the field, the geologist establishes the different types of rocks and their relationships to one another. The geologist carefully describes the rocks and documents the faults that may indicate that two adjacent sequences of rocks have had vastly different histories. In the Andes, such faults are absent, suggesting that all the rocks formed nearby. Volcanic rocks, most of which were Andesites, were the most abundant and were discovered to have erupted throughout the evolution of the Andes. Thus, scientists believe that the tectonic setting of the Andes has remained the same for the last 200 million years.

In addition to geophysics and geology, geochemistry has contributed to an understanding of the Andes. Geochemists in the laboratory melted pieces of basalt and granite, the primary constituents of oceanic and continental crust, respectively. From this experiment, the scientists determined that andesites solidify from magmas that contain significant amounts of continental crust. This led to the conclusion that large areas of the overriding continent melt where the temperature is high, at great depths in the Earth above subduction zones. The andesitic composition of the root below the central Andes indicated that substantial melting of the continental crust has occurred beneath the volcanic chain.

Natural Resources

Adventurers have long explored the Andes for their beauty and riches, both of which are direct consequences of subduction. The mountains tower above the landscape, hosting spectacularly large ore, oil, and natural gas deposits. In the first half of the nineteenth century, the earliest geological expeditions were hampered by the rugged and remote terrain. However, the technological advances of the twentieth century solved this problem and allowed the exploitation of natural resources. Chile is known for copper and Bolivia for tin.

The ore deposits of the Andes are hydrothermal, meaning they form where hot, mineral-rich fluids associated with magmas derived from deep in the Earth interact with cold rock at the surface. These fluids can contain dissolved iron, silver, tin, lead, manganese, molybdenum, zinc, tungsten, and copper. The drop in temperature at the surface forces the minerals to precipitate in the surrounding rock, where, given enough fluid, economically significant ore deposits will form. Large amounts of fluid require abundant magmatism. Because most magmatism on the Earth occurs in subduction zones, and the Andes formed by subduction, the mineral wealth of the Andes is no surprise. Predictably, most mines are found in the western cordillera and the altiplano, where magmatism tends to be most active.

The vast oil and natural gas fields of the Andes lie east of the mineral deposits in the thrust belt of the eastern cordillera, where hot fluids or magmas have not heated the rocks. The location of the fields reflects the low temperatures and thick sedimentary sequences necessary for transforming organic matter into oil and natural gas. Faulting in the thrust belt created traps that collected the oil and gas in economically viable deposits.

Volcanic and Seismic Hazards

The plethora of natural resources makes the Andes one of the world's greatest mountain systems. The Andes, however, are important for another reason: They yield insight into the potential for destruction above subduction zones.

Subduction zones are the locus of most of the large earthquakes and violent volcanic eruptions that presently occur. Investigations of the Andes may prevent the loss of life and property that often accompanies these natural events. First, geophysical instruments can monitor a volcano to determine when an eruption is imminent. (The number of small earthquakes increases when magma is moving toward the surface.) This technique successfully predicted an eruption at Mount St. Helens, an andesitic volcano in the western United States. Second, detailed examinations of volcanoes and the surrounding area may reveal the preferred slope for the descent of mudslides often triggered by eruptions and earthquakes. Through the centuries, mudslides have killed hundreds of people and destroyed countless villages. Towns in the paths of slides can be moved or evacuated. Finally, ground motion analysis during a major earthquake may help civil engineers design buildings that can withstand the shaking.

The ultimate goal is to predict earthquakes and volcanic eruptions far enough in advance that adequate precautions can be taken to minimize the loss of life and property. If prediction is successful in the Andes, disasters along other convergent boundaries may be avoided.

The Andes and Climate Change

The Andes region is one of the world’s most important and diverse crop producers. However, the increasing temperatures and unpredictable weather patterns caused by climate change are impacting the Andes’s thousands of kilometers of glaciers, altering the flow of nearby rivers and modifying the ecosystems in the mountains and valleys below. As these changes occur, the region’s farmers must adapt, moving their farms to areas suitable for their crops, leading to increased deforestation, while others have stopped farming altogether. The Andes region is home to some of the world’s most important and unique crops that grow in 85 of the world’s 110 climate zones. Potatoes, the fourth largest crop in the world, have been cultivated in the Andes for over seven thousand years, and nowhere else are potato crops as high in protein, pest-resistant, or varied in flavor. Other important food sources from the Andes include quinoa, oca, lima beans, lupines, and tomatoes. Though the region is well-known for its sustainable agricultural practices and crops that are resilient in extreme conditions, its agrobiodiversity is increasingly threatened by droughts, hailstorms, and frosts caused by climate change. Protecting and studying this region provides insight into climate change’s impact on the food system. Because the region is experiencing climate change extremes at an amplified rate compared to most other regions, the data gathered in the Andes is critical for preparing and preventing future, more widespread changes in humans, the food supply, and the economy.

Principal Terms

continental margin: the edge of a continent that is the transition to the ocean basin; continental margins are said to be active if subduction occurs along them

crust: the outermost layer of the Earth; continental crust is thirty to thirty-five kilometers thick, and oceanic crust is five to ten kilometers thick; the greater density of oceanic crust relative to continental crust forces it to subduct

faulting: the process of fracturing the Earth such that rocks on opposite sides of the fracture move relative to each other; faults are the structures produced during the process

folding: the process of bending initially horizontal layers of rock so that they dip; folds are the features produced by folding and can be as small as millimeters and as big as kilometers long

Mesozoic: the era of the geologic time scale that preceded the Cenozoic era; it represents the time between about 245 million and sixty-five million years ago

sedimentary rock: rock that was deposited by the settling of grains through either air or water

tertiary: a period in the Cenozoic era of the geologic time scale; it encompasses the time span between about sixty-five million and two million years ago

thrust belt: a linear belt of rocks that have been deformed by thrust faults; thrust faults emplace older rocks above younger and generally move mass uphill

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