Mountains loom large on the face of the planet. These rocky landforms, which tower over all others on Earth, are places of extreme temperatures and winds. Reaching high into the atmosphere, mountains form a barrier against moving air, robbing it of any precipitation. The tops of many mountains are laden with glossy caps of snow and ice. The summits of the highest mountains are often shrouded in mists and clouds.
Mountains also loom large in people's imaginations. Throughout human history, many people have regarded these mysterious places as the domain or home of supernatural beings or gods. Others have seen them as the ultimate in human adventure. Mountain climbing is viewed as an extreme test of human endurance and desire. Many climbers have succeeded in scaling the summits of the world's highest mountains; others have died trying.
The shape of the land
A mountain is any landmass on Earth's surface that rises abruptly to a great height in comparison to its surrounding landscape. By definition, a mountain rises 1,000 feet (305 meters) or more above its surroundings and has steep sides meeting in a summit that is much narrower in width than the mountain's base. Any highland that rises no higher than 1,000 feet (305 meters) above its surroundings, has a rounded top, and is less rugged in outline than a mountain is considered a hill. High hills at the base of mountains are known as foothills.
Mountains cover approximately one-fifth of Earth's land surface. Although rare, a mountain can exist singly, such as Mount Kilimanjaro in northeast Tanzania. Most mountains, however, occur as a group, called a mountain range. An example of a mountain range is the Sierra Nevada,
which extends for about 400 miles (643 kilometers) in eastern California. A group of mountain ranges that share a common origin and form is known as a mountain system. The Sierra Madre, which arises just south of the U.S. border and extends south, is Mexico's chief mountain system. A group of mountain systems is called a mountain chain. The Pyrenees forms a mountain chain in southwest Europe between Spain and France. Finally, a complex group of mountain ranges, systems, and chains is called a mountain belt or cordillera (pronounced kor-dee-YARE-ah). The North American Cordillera runs from Alaska to Guatemala and includes all of the mountains and elevated plateaus in that vast region.
Like everything else in the natural world, mountains go though a life cycle. They rise from a variety of causes and wear down over time at various rates. The building up of mountains takes millions of years, and the process has been occurring since Earth's beginning over 4.5 billion years ago. Yet as soon as their rocks are exposed to the erosive actions of water and wind, mountains begin to fracture and dissolve. This explains the high and rugged appearance of young mountains and the lower and smoother appearance of older mountains. Some mountains that once existed on the planet hundred of millions of years ago have long since eroded away.
Orogeny (pronounced o-RAH-je-nee) is the word scientists use to describe the process of mountain building. (Orogeny comes from the Greek words oro , meaning "mountain," and geneia , meaning "born.") There are several distinct types of mountains, each having formed through varying causes: volcanic mountains, upwarped mountains, folded mountains, and fault-block mountains.
Mountain: Words to Know
- The section of the mantle immediately beneath the lithosphere that is composed of partially melted rock.
- An upward-curving (convex) fold in rock that resembles an arch.
- Convection current:
- The circular movement of a gas or liquid between hot and cold areas.
- A complex group of mountain ranges, systems, and chains.
- The thin, solid outermost layer of Earth.
- The gradual wearing away of Earth surfaces through the action of wind and water.
- A crack or fracture in Earth's crust along which rock on one side has moved relative to rock on the other.
- A high hill at the base of a mountain.
- A block of Earth's crust dropped downward between faults.
- A highland that rises up to 1,000 feet (305 meters) above its surroundings, has a rounded top, and is less rugged in outline than a mountain.
- A block of Earth's crust forced upward between faults.
- The rigid uppermost section of the mantle combined with the crust.
- Molten rock containing particles of mineral grains and dissolved gas that forms deep within Earth.
- Magma chamber:
- A reservoir or cavity beneath Earth's surface containing magma that feeds a volcano.
- The thick, dense layer of rock that lies beneath Earth's crust.
- Large sections of Earth's lithosphere that are separated by deep fault zones.
- Plate tectonics:
- The geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions.
- A downward-curving (concave) fold in rock that resembles a trough.
- In geology, the slow upward movement of large parts of stable areas of Earth's crust.
Technically, a volcano is a vent or hole in Earth's surface through which magma and other molten matter escapes from underground. Many volcanoes are classified as mountains because the magma (called lava once it reaches Earth's surface) ejected through the vent often accumulates to form a cone around the vent reaching thousands of feet in height. The shape of the accumulated landform (also known as a volcano), with a summit much narrower than its base, also fits the definition of a mountain. (For further information on volcanic landforms, see the Volcano chapter.)
An example of a volcanic mountain in North America is Mount Rainier in the state of Washington. Part of the Cascade Range mountain chain, it rises 14,410 feet (4,392 meters) in elevation. Mauna Loa on the island of Hawaii is the world's largest volcano, rising 13,680 feet (4,170 meters) above sea level. Since it also extends more than 18,000 feet (5,544 meters) to the floor of the Pacific Ocean, Mauna Loa measures about 32,000 feet (9,754 meters) from its base to its summit. This makes it the tallest mountain on the planet (Mount Everest in the Himalayan Mountains is the tallest on land).
Most of the world's volcanic mountains lie not on land but underwater. The global mid-ocean ridge system is an undersea mountain system that snakes its way between the continents, encircling the planet like the seams on a baseball. It extends more than 40,000 miles (64,000 kilometers) in length. At the mid-ocean ridge, the seafloor splits apart and lava from below wells up into the crack or rift, solidifying and forming new seafloor. On either side of the rift lie tall volcanic mountains. The peaks of some of these mountains rise above the surface of the ocean to form islands, such as Iceland and the Azores. (For further information on oceanic landforms, see the Ocean basin chapter.)
Upwarped mountains are also formed by the action of rising magma. In this process, instead of passing through Earth's surface, such as it does in the formation of a volcanic mountain, magma remains underground, exerting pressure on the crust (the thin, solid outermost layer of Earth). This pressure gently uplifts a broad area of the crust, sometimes in the shape of a blisterlike dome. As the crust is uplifted, softer material on top may also be eroded or worn away by rivers or other flowing water, leaving sharp peaks and ridges. Examples of upwarped mountains in North America include the Adirondack Mountains in northeast New York and the Black Hills in western South Dakota and northeast Wyoming.
Fault-block and folded mountains are formed when stresses on Earth's crust cause it to crack and uplift or buckle and fold. As its name indicates, a fault-block mountain forms along a fault. A fault is a crack or fracture in Earth's crust along which rock on one side has moved relative to rock on the other. (For further information on faults, see the Fault chapter.) In the formation of a fault-block mountain, a section of the crust on one side of the fault is forced upward. The resulting mountains in the range may have steep clifflike faces on one side and gentler inclines on the other. The Teton Range of Wyoming is an example of fault-block mountains.
Folded mountains are the most common type on land. They are created when forces within Earth push adjacent sections of the crust into each other. When the sections collide, their edges along the line of collision buckle and fold in a wavelike pattern like a wrinkled rug. As the sections continue to push into each other, their leading edges are thrust higher and higher. This process has created some of the world's highest, and most famous, mountain ranges and systems. Folded mountains include those of the Appalachian Mountain system in eastern North America, the Alps mountain system in southern-central Europe, and the Himalayan mountain system in southwest Asia.
The Literary Landscape
"Straddling the top of the world, one foot in China and the other in Nepal, I cleared the ice from my oxygen mask, hunched a shoulder against the wind, and stared absently down at the vastness of Tibet. I understood on some dim, detached level that the sweep of earth beneath my feet was a spectacular sight. I'd been fantasizing about this moment, and the release of emotion that would accompany it, for many months. But now that I was finally here, actually standing on the summit of Mount Everest, I just couldn't summon the energy to care."
—Jon Krakauer, Into Thin Air , 1997.
Forces and changes: Construction and destruction
Mountains form mainly as a result of movements of sections of Earth's crust in response to heat and pressure within the planet. Prior to the 1960s, geologists lacked a clear scientific explanation as to what moved continents and other sections of crust across the surface of the planet. The theory of plate
tectonics, developed at that time, provided the answer. It explains not only how mountains are built, but also how and why volcanoes erupt, why earthquakes occur, why the seafloor spreads, and how many other topographic features (physical features on Earth's surface) are formed. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology.
Earth's heated interior
At Earth's center, the inner core spins at a rate slightly faster than the planet. A blisteringly hot mass of iron, the inner core has a temperature exceeding 9,900°F (5,482°C). Around this solid inner portion is a molten, or melted, outer portion. Above the two-layered core is a large section of very dense rock called the mantle that extends upward to the crust.
The mantle itself is separated into two distinct layers: a rigid upper layer and a partially melted lower layer. The crust and the uppermost layer together make up what geologists call the lithosphere (pronounced LITH-uh-sfeer). The part of the mantle immediately beneath the lithosphere is called the asthenosphere (pronounced as-THEN-uh-sfeer). This layer is composed of partially melted rock that has the consistency of soft putty. The lithosphere is broken into many large slabs or plates that "float" on the soft asthenosphere. In constant contact with each other, these plates fit together like a jigsaw puzzle.
It is the intense heat energy created by the extreme temperatures in the core that cause the lithospheric plates to move back and forth across the surface of the planet. If this heat energy were not carried upward to Earth's surface, where it can be released in some manner, the interior of the planet would melt. This does not happen because circular currents, called convection currents, carry the energy from the core upward through the mantle.
Convection takes place when material at a deeper level is heated to the point where it expands and becomes less dense (lighter) than the material above it. Once this occurs, the heated material rises. This process is similar to what happens in a pot of boiling water. As it begins to boil, water in a pot turns over and over. Heated water at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense. Once at the surface, the heated water cools and becomes dense (heavier), then sinks back down to the bottom to become reheated. This continuous motion of heated material rising, cooling, and sinking within the pot forms the circular convection currents.
Convection currents form in the planet's interior when rock surrounding the core heats up. Expanding and becoming less dense, the heated rock slowly rises through cooler, denser rock that surrounds it in the mantle. When it reaches the lithosphere, the heated rock moves along the lithosphere's base, losing heat. Cooling and becoming denser, the rock then sinks back toward the core, only to be heated once again.
The pressure exerted by the convection currents underneath the lithosphere causes the plates to move. The plates, which vary in size and shape, are in constant contact with each other. When one plate moves, other plates move in response. This movement and interaction of the plates either directly or indirectly creates the major geologic features on Earth's surface, including mountains.
Plate movement and mountain building
The boundaries where plates meet and interact are known as plate margins. It is here where mountain building primarily occurs. The type of mountain that develops is dependent on the nature of the plate interaction. Plates interact by moving toward each other (converge), moving away from each other (diverge), or sliding past each other (transform). Most mountains are formed when plates converge.
When continental plates (those under the continents or landmasses) converge, the rocks in the collision area are compressed, shattered, and folded. Although normally brittle, rocks in Earth's crust can bend and fold like warm toffee when placed under great pressure and heat for long periods of time (thousands to millions of years). As the plates continue to push into each other, the rock layers are folded into a wavelike series of high points and low points. Geologists call the upfold on a curve (the peak) the anticline and the troughlike downfold (the valley) the syncline. Since tectonic plates move only a few inches per year (about as fast as fingernails grow), the process forming folded mountains takes millions of years. Folded mountains stand high because the crust beneath them is thickened as the two plates pile on each other in the collision process.
The situation is different when a continental plate and an oceanic plate converge. The crust under the oceans is made primarily of basalt, a type of rock that is denser (heavier) than the granite rocks that make up the crust under the continents. Because of this difference in density, the oceanic plate subducts or slides under the continental plate where they are pushed together. As the oceanic plate sinks deeper and deeper into Earth, intense pressure and heat in the mantle melts the leading edge of rock on the plate. This molten rock (magma) is less dense than the rock that surrounds it underground. As a result, it begins to rise toward Earth's surface through weakened layers of rock, collecting in underground reservoirs called magma chambers. When pressure from the expanding magma exceeds that of the overlying rocks, the magma is forced through cracks or vents in the planet's surface, forming volcanic mountains. Lines of volcanic mountains are usually formed on the forward edge of the continental plate.
The Andes mountain system, the world's longest system on land, runs for more than 5,000 miles (8,000 kilometers) along the western coast of South America. It features many volcanic mountains. They have formed on the edge of the continental South American Plate where the leading edge of the oceanic Nazca Plate is subducting below it. As it sinks, some crust on the top layer of the Nazca Plate is also scraped off at the base of the Andes, adding height to the system.
Volcanic mountains may also form where tectonic pressure is stretching continental crust beyond its limits. As the crust splits apart, magma rises and squeezes through the widening cracks or faults. The rising magma, whether it erupts, puts more pressure on the crust, producing
|Highest Points on Land Around the World|
|Africa (Tanzania)||Kilimanjaro||19,340 feet (5,895 meters)|
|Antarctica||Vinson Massif||16,860 feet (5,139 meters)|
|Asia (Tibet/Nepal)||Everest||29,035 feet (8,850 meters)|
|Australia||Kosciusko||7,316 feet (2,230 meters)|
|Europe, Eastern (Georgia)||Elbrus||18,481 feet (5,633 meters)|
|Europe, Western (France)||Mont Blanc||15,771 feet (4,870 meters)|
|North America (Alaska)||McKinley||20,320 feet (6,194 meters)|
|Oceania (New Guinea)||Jaya Peak||16,503 feet (5,030 meters)|
|South America (Argentina)||Aconcagua||22,835 feet (6,960 meters)|
additional fractures. Ultimately, sections of the crust drop down between the faults, forming a rift valley. Volcanic mountains may then arise in or along the valley. Mount Kilimanjaro is an extinct volcano that stands along the East African Rift Valley in northeast Tanzania. It is the largest of many volcanoes in the area. Geologists believe that if the spreading of the rift valley continues, the edge of the present-day African continent will separate completely. The Indian Ocean will then flood the area, making the easternmost corner of Africa a large island.
Stress from the movement of tectonic plates can fracture continental crust. This stress or unequal pressure may be in different forms: tensional stress, which stretches or pulls rock; compressional stress, which squeezes and squashes rock; and shear stress, which changes the shape of rock by causing adjacent parts to slide past one another. When sudden stress near Earth's surface fractures brittle rock, it creates a fault in the crust.
Fault-block mountains form when tensional stress fractures the crust, separating it into blocks between faults. Pressure from magma moving underneath the surface can move the large blocks of rock (called fault blocks) either up or down. A block that is uplifted between faults is known as a horst; one that sinks is a graben (pronounced GRAH-bin). A large horst that is uplifted high between two parallel normal faults can form a fault-block mountain. More often, a fault-block mountain is created when one edge of a fault block is tilted upward at the fault a great distance in relation to the block on the other side. Sometimes these resulting landforms are referred to as tilted fault-block mountains.
Magma welling up beneath continental crust may not be able to reach Earth's surface through vents or move blocks of the crust located alongside faults. Instead, its high temperature and pressure may simply cause the overlying crust to fold and bubble gently outward into a dome shape. Over
millions of years, the magma beneath the dome cools and hardens into a solid core. During the same period, erosion wears away the softer materials on top, leaving the rugged, harder material beneath exposed as an upwarped mountain.
Spotlight on famous forms
Black Hills, South Dakota and Wyoming
Some 65 million years ago, an upwelling of magma under Earth's crust in the present-day Great Plains region formed the Black Hills. These rugged mountains, which rise some 2,500 feet (760 meters), cover an area of approximately 6,000 square miles (15,540 square kilometers). The highest point in the Black Hills is Harney Peak, which stands 7,242 feet (2,207 meters) in elevation.
The Black Hills region contains large amounts of many minerals, including a few rare ones. Uranium, feldspar, mica, and silver are among the important minerals found in the area. Gold was discovered in the Black Hills in 1874. The Homestake Mine, the largest gold mine in the United States, produced more than $200 million worth of gold between 1876 and 2002.
The discovery of gold led to an invasion of white settlers into the Black Hills, forcing out local Native Americans from the area. The Lakota, Northern Cheyenne, and Omaha tribes believe the mountainous region is a sacred landscape. Two landforms the Native Americans consider particularly sacred in the Black Hills are Bear Butte and Devils Tower. Bear Butte, which rises 1,253 feet (382 meters) above the surrounding plain, is made of magma that rose, deformed the crust, then cooled and solidified. Devils Tower, standing 1,267 feet (386 meters), is a volcanic neck, the inner remains of an ancient volcano.
Cascade Range, Pacific Northwestern United States
The largest collection of volcanic mountains in the contiguous United States (connected forty-eight states) is the Cascade Range. This mountain chain extends about 700 miles (1,125 kilometers) in length. It runs south from British Columbia, Canada, through the U.S. states of Washington and Oregon before it becomes the Sierra Nevada mountain range in northeastern California. It parallels the edge of the Pacific Ocean, lying 100 to 150 miles (161 to 241 kilometers) inland from the West Coast of the United States.
The Cascade Range formed more than 30 million years ago when the oceanic Juan de Fuca Plate sunk beneath the continental North American Plate as the two plates converged. Rising magma from the leading edge of the oceanic plate created an arc of volcanic mountains. Many of these form the range's highest peaks. Mount Rainier, at 14,410 feet (4,392 meters) is the highest point in the Cascades. Other notable volcanic mountains in the range include Lassen Peak, Mount Hood, Mount Shasta, and Mount St. Helens, which last erupted in 1980. Since the Juan de Fuca Plate continues to sink beneath the North American Plate at a rate of about 1.6 inches (4 centimeters) per year, many of the volcanoes in the range are still active.
Mount Everest, on the border of Tibet and Nepal
The collision between the Indian Plate and the Eurasian Plate some 40 to 50 million years ago led to the formation of the Himalayan Mountains. The highest mountain system in the world, it features thirty mountains that rise above 25,000 feet (7,620 meters). The highest mountain in the system, and the highest on land anywhere in the world, is Mount Everest. Its frozen summit stands 29,035 feet (8,850 meters) above sea level.
Geologists estimate that when the Indian Plate moved into the Eurasian Plate, it did so at rates of up to 6 inches (15 centimeters) per year. Most plates move at rates one-fourth as fast. At present, the Indian Plate is still inching northward, and the Himalayan Mountains are rising by as much as 0.4 inch (1 centimeter) per year.
In 1865, Mount Everest was given its English name in honor of George Everest (1790–1866), who had served as the English surveyor general of India. Tibetans call the mountain Chomolongma, meaning Mother Goddess of the World. Nepalese call it Sagarmatha, meaning Goddess of the Sky. Both hold the mountain to be sacred.
Farthest from the Center of Earth
Although the summit of Mount Everest in the Himalayan Mountains is the highest elevation on land, it is not the farthest point from the center of Earth. That distinction goes to the volcanic mountain Chimborazo (pronounced cheem-bor-AH–so) in the Andes mountain system in central Ecuador. The highest mountain in Ecuador, Mount Chimborazo rises to a height of 20,703 feet (6,310 meters). While it is 8,832 feet (2,540 meters) lower than that of Mount Everest, its summit is actually farther away from the center of Earth because of the equatorial bulge caused by the rotation of the planet.
Earth is not a perfectly round sphere or ball; it bulges around the equator (a shape scientists call an oblate spheroid). As it revolves around the Sun, Earth also rotates or spins on its axis like a top. At the equator, the rate of this motion is slightly more than 1,000 miles (1,609 kilometers) per hour. This constant circular motion of Earth creates centrifugal force, which is the tendency of an object traveling around a central point to fly away from that point. Riders on a merry-go-round experience this same force. The centrifugal force created by Earth's rotation causes the middle of the planet to bulge slightly and the north and south poles to flatten slightly. The diameter of Earth from the North Pole to the South Pole is 7,900 miles (12,711 kilometers), but through the equator it is 7,926 miles (12,753 kilometers).
Mount Chimborazo lies very near the equator; Mount Everest lies farther north. Because of the equatorial bulge, the summit of Mount Chimborazo is 7,153 feet (2,180 meters) farther away from the center of Earth than the summit of Mount Everest.
The first mountain climbers to reach the summit of Mount Everest were Edmund Hillary (1919–) of New Zealand and Tenzing Norgay (1914–1986) of Nepal on May 28, 1953. Since that time, more than 1,600 climbers have reached the summit of the world's tallest mountain. A record-breaking 54 climbers reached the peak on May 16, 2002. Not everyone who attempts to reach the top of Mount Everest is successful or even returns from the incredibly dangerous feat. The bodies of more than 160 climbers remain on the mountain.
Sierra Nevadas, California
The Sierra Nevada mountain range is the largest fault-block mountain formation in the United States. It runs mainly along California's eastern border for about 400 miles (643 kilometers). Its width varies from 40 to 80 miles (64 to 129 kilometers). The highest and most rugged mountains in the range occur in its southern portion. Here, 11 peaks rise more than 14,000 feet in elevation. The summits of many of these high mountains are continuously covered with snow. The highest peak in the range, Mount Whitney, rises to 14,495 feet (4,418 meters). It is the tallest mountain in the contiguous United States (connected forty-eight states).
The blocks of crust that formed the range tilted upward along its eastern side. As a result, its eastern slope rises steeply while its western slope descends gradually to the hills in California's central valley. Forests filled with aspen, cedar, fir, pine, and sequoia trees dominate the western slope.
To followers of the religions of Buddhism and Hinduism, Mount Kailas is a sacred place. Part of the Himalayan mountain system, Mount Kailas rises some 22,280 feet (6,790 meters) above the Tibetan Plateau. What is unique about the mountain is that it sits on the highest part of the plateau, and it stands isolated: Over the course of several days, an individual may walk completely around the mountain at its base. Four of Asia's largest rivers also have their sources within 62 miles (100 kilometers) of the mountain. They flow away from the mountain in four different directions, like spokes from the hub of wheel: the Indus to the north, the Karnali to the south, the Yarlung Zangbo to the east, and the Sutlej to the west.
Hindus believe Mount Kailas is the dwelling place of Shiva, one of the greatest gods of Hinduism. Along with the gods Brahma and Vishnu, Shiva forms the Hindu Supreme Being. Although difficult to define simply, Hinduism is based on the concept that all living things in the universe are in a constant cycle of creation, preservation, and destruction. Shiva is the god of destruction.
Tibetan Buddhists believe that Mount Kailas is the earthly form of mythical Mount Sumeru, the cosmic axis or center of the universe. This is the place where all planes, or realms of existence—spiritual and physical—are united.
The beauty of the Sierra Nevadas is immeasurable. Two of the nation's most-scenic national parks, Sequoia National Park and Yosemite National Park, are located within the range.
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Rotter, Charles. Mountains: The Towering Sentinels . Mankato, MN: Creative Education, 2003.
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