Mountain is a landform that stands much higher than the surrounding terrain. Mountains generally are larger than hills, but features that people call hills in one place may be higher than features called mountains elsewhere. For example, the Black Hills of South Dakota and Wyoming stand higher above their surroundings than do the Ouachita Mountains of Arkansas and Oklahoma.
Mountains typically have steep slopes and sharp or slightly rounded peaks or ridges. Many geologists consider an elevated area to be a mountain if it includes two or more zones of climate and plant life at different altitudes. In general, the climate becomes cooler and wetter with increasing elevation. In most parts of the world, a mountain must rise about 2,000 feet (600 meters) above its surroundings to include two climate zones.
A mountain may stand as an isolated peak, such as a lone volcano, or it may form part of a mountain range. A group of mountain ranges forms a mountain system. The Pacific Mountain System, for example, includes the Cascade Range, the Olympic Mountains, the Sierra Nevada, and several other mountain ranges along the west coast of North America.
Mountains occur in the ocean as well as on land. Many islands are the exposed peaks of mountains that rise from the ocean floor. The world’s longest mountain system—the Mid-Atlantic Ridge—lies almost totally underwater. It stretches more than 10,000 miles (16,000 kilometers) from the North Atlantic Ocean nearly to Antarctica. Some of the ridge’s highest peaks form such islands as Iceland and the Azores.
The height of a mountain is usually expressed in terms of the elevation of its peak above sea level. The world’s highest mountain, Mount Everest, on the border of Tibet and Nepal, rises 29,032 feet (8,849 meters) above sea level. A mountain’s height can also be described in terms of its relief, the overall rise from its base to its peak. Mauna Kea, a volcano on the island of Hawaii, has the world’s largest relief. The volcano rises a total of 33,480 feet (10,205 meters) from its base on the floor of the Pacific Ocean to its peak 13,796 feet (4,205 meters) above the ocean surface—a relief of more than 6 miles (10 kilometers).
Other rocky worlds in our solar system also have mountains, some of which stand much taller than the highest mountains on Earth. Olympus Mons, an ancient volcano on the planet Mars, ranks as the tallest known mountain in the solar system. It rises about 16 miles (25 kilometers) above the surrounding plain, nearly three times the height of Mount Everest.
The importance of mountains
Mountain ranges are important because they affect the climate and water flow of surrounding regions. Mountains also provide a home for plants and animals and a source of such natural resources as lumber and minerals. Mountain ranges influence a variety of human activities, shaping patterns of transportation, communication, and settlement.
In affecting climate.
Mountain ranges strongly affect air movements and precipitation patterns. At higher altitudes, air pressure decreases, causing the temperature of the air to drop. Cold air cannot hold as much moisture as warm air can. As a result, when warm, moist air moves up one side of a mountain—known as the windward side—the air cools, and the water vapor it holds condenses into water droplets. Much of the water then falls on the windward slope as rain or snow.
By the time air passes the crest of a mountain, it has lost most of its moisture. For this reason, the side of the mountain away from the wind, called the leeward side, is drier than the windward side. The dry area beyond the leeward side of a mountain range is called a rain shadow. Many of the world’s deserts lie in rain shadows.
In maintaining water flow.
Because so much precipitation falls on mountain slopes, many rivers have their headwaters in mountain regions. The Rio Grande and the Colorado River, for example, receive nearly all their water from mountains. Much of the snow in high mountains melts only during the summer. Thus, mountains act as reservoirs, feeding streams and rivers even during periods of summer drought.
As a home for plants and animals.
Because mountains include diverse conditions at different elevations, they provide environments suitable to many kinds of plant and animal life. Few living things survive in the bitter cold of snow-capped mountain peaks. However, a variety of insects and other small animals, such as chinchillas and pikas, make their home just below the snowfields. A few sure-footed large animals, such as mountain goats and sheep, also live in these areas. These animals feed on shrubs, mosses, and other plants that grow above the timber line, the line above which the cold and wind prevents the growth of trees. Below the timber line, many mountains have large forests filled with a wide range of plant and animal life.
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As a wealth of natural resources.
Much of the world’s mineral resources come from mountainous areas. Mountains are formed by such geological processes as volcanic eruptions and earthquakes. These processes may bring valuable minerals near the surface, where they can be mined.
Through their effects on climate and water flow, mountains influence the availability of water over vast areas. Because of their steep slopes and abundant flowing water, mountainous areas also make suitable locations for building hydroelectric power plants, which convert the energy of moving water into electric power. Mountainous Norway, for example, produces nearly all its electric power in this way.
Some regions, including many areas of the western United States, are too dry to support trees except in the cooler, wetter climate of the mountains. The lumber industry in such regions depends on timber grown in mountainous areas. Many animals valued by the fur industry or by hunters also live in the mountains.
As an influence on human activities.
In many parts of the world, mountains have long served as barriers, hindering transportation, settlement, communication, and military invasion. The isolation of mountain communities can contribute to the development of a great diversity of culture. In Switzerland’s Alps, for example, relatively isolated groups living in the same region speak hundreds of dialects (variations) of four different languages. 
Mountains also serve as important recreation areas. Each year, millions of people vacation in mountainous regions to camp, hike, ski, climb, or kayak, or just to enjoy fresh air and spectacular views.
How different types of mountains form
Mountains are created by tremendous forces in Earth operating over a period of about 1 million to 100 million years. Earth scientists have developed a theory called plate tectonics that explains the formation of mountains and other geological features. According to this theory, Earth’s outer shell consists of about 30 rigid pieces of various sizes called tectonic plates. The surfaces of these plates make up the continents—called continental crust—and the ocean basins—called oceanic crust. The plates are in slow, continuous motion. Most mountain building occurs along the boundaries between plates.
Geologists classify mountains into two major families: volcanic mountains, formed by the eruption of molten rock and its build-up as it cools, and tectonic mountains, built by geological forces that change the shape of, or deform, Earth’s crust. Both families consist of a number of different types of mountains. In addition, a single mountain system may develop from both volcanic and tectonic processes. For example, the Andes Mountains in South America contain some peaks created by volcanic eruptions and others formed by tectonic activity.
Volcanic mountains,
such as Washington’s Mount Rainier and Japan’s Mount Fuji, form when molten rock from deep within Earth erupts and piles up on the surface. As a result, volcanic mountains consist chiefly of igneous rocks, such as basalt and rhyolite, which are formed when molten material cools and solidifies. Igneous rocks are one of the three major types of rocks. The other types, sedimentary rocks and metamorphic rocks, can be found in mountains formed by other processes. 
Volcanic mountain building takes place in regions where molten rock rises up from Earth’s mantle, the hot, rocky layer beneath Earth’s crust. Most such activity occurs along the boundaries of tectonic plates, at mid-ocean ridges and subduction zones. Other volcanic mountain building occurs above hot spots.
At mid-ocean ridges.
Mid-ocean ridges form beneath the ocean in areas where two plates separate. Molten rock from Earth’s interior wells up between the plates, creating a submerged mountain range. The Mid-Atlantic Ridge formed in this way. As the molten rock cools and the plates continue to move apart, the solidified material becomes new ocean floor. Geologists estimate the total length of all of Earth’s mid-ocean ridges to be from 30,000 to 50,000 miles (50,000 to 80,000 kilometers).
At subduction zones.
Subduction zones are regions where two plates collide. At these boundaries, the oceanic edge of one plate is thrust beneath the edge of the other plate in a process called subduction. The sinking plate contains water that soaked into the rock and became chemically bound into minerals as the plate formed and moved beneath the ocean. As the plate subducts, it carries this water down into the mantle. There, the heated minerals release the water. The water rises into the overlying mantle, where it lowers the melting temperature of the rock. Melted rock then rises through the overlying plate, creating volcanic eruptions.
If continental crust lies atop this region, subduction zone mountain ranges can form along the continent’s edge. Such mountain ranges include the Cascade Range in North America and the volcanoes of the Andes Mountains. If the edges of both plates are oceanic, mountain-building activity at the subduction zone may form a chain of islands called an island arc. Such island arcs include the Aleutian Islands of Alaska and the Mariana Islands in the Pacific Ocean.
Above a hot spot.
Geologists believe that some mountain-building activity occurs above deep hot spots in the mantle. These hot spots contain partially molten rock that rises from deep inside Earth. From these locations, hot rock continues to rise through the mantle and the overlying plate, finally erupting as a volcano.
Over millions of years, as a plate moves over a hot spot, and as the hot spot itself moves within the mantle, eruptions create a trail of volcanic mountains. The Hawaiian Islands formed in this manner as the movements of the Pacific Plate and an underlying hot spot traced out a chain of volcanoes, a process that continues today.
Tectonic mountains,
such as the Alps in Europe and the Sierra Nevada in California, result from geological forces that raise or fold Earth’s crust or otherwise change its shape. Some tectonic mountains form when plates collide or pull apart. Others develop from the lifting and wearing down of regions of the crust. Most tectonic mountains fit into one of four types: (1) fold-thrust mountains, (2) fault-block mountains, (3) dome mountains, and (4) erosion mountains. 
Fold-thrust mountains,
such as the Alps, the Appalachian Mountains of the eastern United States, and the Himalaya, form when two plates collide head-on. During the collision, the plates’ edges fold or crumple, and some layers of rock may be thrust over other layers. Fold-thrust mountains consist mainly of sedimentary rocks, such as limestone and shale. Sedimentary rocks form when sediments (loose pieces of minerals and rock) settle to the bottom of a body of water and harden. 
The thickest deposits of sedimentary rock generally accumulate along the edges of continents. As one plate subducts beneath another, any continents riding on the two plates collide. The accumulated layers of rock—and any previously formed volcanic mountains—crumple together, causing the rock layers to wrinkle up like a tablecloth that is pushed across a table. The resulting folds may range in character from gentle, wavelike patterns to sharp, complex folds. The compression may also produce extensive thrust faults, fractures in the rock layers in which one layer is pushed up and over another.
Much of the Appalachian Mountains consist of gently folded rock. Europe’s Alps, however, are so sharply folded that a person climbing in some areas may cross the same layer of rock several times during a single ascent. The first time the layer appears right side up, the next time upside down, then right side up again, and so on. In such intensely folded and faulted mountains, some rocks are pushed down so far that they are subjected to great pressure and temperature. The heat and pressure transform the sediments into such metamorphic rocks as schist and gneiss. Some of the rock may melt and then rise up into the overlying rocks, creating veins of granite and other igneous rocks.
Fault-block mountains
form where a plate is pulling apart or rifting. The stretching of Earth’s crust produces fractures called normal faults. Huge blocks of crust become tilted or pushed up along these faults, while neighboring regions drop down to form basins. Fault-block mountains include the Sierra Nevada, the Teton Range in Wyoming, the Wasatch Range in Utah, and the Harz Mountains in Germany. Most such mountains occur where a block is tilted up along one side of a single fault. But some blocks are pushed up between two separate faults. 
The uplifted blocks rapidly wear down, and the debris from this erosion accumulates at the base of the mountain, filling nearby basins. Most of the isolated mountains of the Basin and Range Province, in the southwestern United States and northern Mexico, are fault-block mountains separated by broad plains filled with such debris.
Both fault-block and fold-thrust mountains can also form along strike-slip faults, areas near plate boundaries where blocks of rock slide past each other horizontally, often producing earthquakes. The Peninsular Ranges in southern California and Baja California—which are fault-block mountains—and New Zealand’s Southern Alps/Kā Tiritiri o te Moana—a fold-thrust range—both developed on strike-slip faults.
Dome mountains,
such as the Black Hills of South Dakota and the Adirondack Mountains of New York, form where geological forces lift Earth’s crust into the shape of a broad bulge or dome. Because the dome rises above its surroundings, it becomes vulnerable to increased erosion. The layers of sedimentary rock covering the dome erode, exposing underlying igneous and metamorphic rock. This harder underlying rock erodes irregularly, forming peaks and valleys.
Erosion mountains.
A few mountains, such as the Catskill Mountains in New York, result from the erosion of a thick, flat pile of sedimentary rock. Such mountains are all that remains of a plateau that has been eroded away by rivers or glaciers.
Forces that shape mountains
Mountains may appear solid and enduring, but geological and climatic processes gradually and continuously alter a mountain’s height and shape. These forces can work together over millions of years to sculpt beautiful mountain landscapes such as gently sloping ranges or soaring, rugged peaks. The processes that shape mountains include: (1) isostasy << eye SAHS tuh see >> and uplift, (2) plate flexing, and (3) erosion.
Isostasy and uplift.
The elevation of a mountain is closely tied to the rise and fall of the underlying tectonic plate through a mechanism called isostasy. The rigid plates float on the asthenosphere—a hot, soft layer of Earth’s mantle—in much the same way that a wooden block will float on water. In the case of a wooden block, the water exerts an upward force of buoyancy that balances the downward force of gravity, causing the block to float. Floating freely, the block tends to rise or fall in the water until the buoyancy force exactly equals its weight. Likewise, the plates float on the molten rock of the asthenosphere, tending to rise or fall until they reach a condition of balance between weight and buoyancy called isostatic equilibrium.
An object’s elevation at isostatic equilibrium depends in part on its density. For example, a block of light wood, such as balsa wood, will float higher on the water than will an identical block of heavy wood, such as oak. Similarly, a continental region of a plate, which is less dense than an oceanic region, will float higher above the asthenosphere, helping the continents to rise above the ocean basins.
The height at which an object floats also depends on its thickness. A thick wooden block will float higher above the water than will a thin wooden block made of the same material. Mountain-building processes generally involve the build-up of mass and thickness—the mountains—over a certain area of plate. Without the effects of isostasy, the additional mass might cause that area to sink entirely, canceling any change in elevation. With isostasy, the thickening can actually produce a buoyant, upward influence called uplift. Most mountain ranges have buoyant crustal roots beneath them that cause the underlying plate to bulge into the asthenosphere. These roots provide the uplift that enables the mountains to tower above the surrounding terrain. 
Plate flexing.
Although isostasy prevents mountains from sinking entirely, the load exerted by large mountains can cause the plates to flex downward significantly, lowering the height of the mountains in turn. The Hawaiian Islands, for example, have flexed the Pacific Plate downward by about 1 to 3 miles (2 to 5 kilometers). The weight of the Himalaya, along with the forces of compression as the Indian subcontinent collides with Asia, has bent the northern edge of the Indian Plate down by about 12 miles (20 kilometers).
For a given load, stronger plates flex less than weaker plates. As a result, the strength of the plate that a mountain rides on limits the height the mountain can reach.
Erosion
by water and ice constantly wears away uplifted terrain. In warm, wet climates, or at low elevations, rivers can carve hills and gorges into the land. At higher elevations, glaciers can form, cutting jagged peaks and deep valleys as they move down the rock. As elevation increases, erosion by glaciers intensifies, imposing another limit on the height that a mountain can reach.
Erosion gradually lowers the average elevation of an uplifted region. But as erosion removes material, the underlying plate will rise over time, maintaining isostatic equilibrium.
Major mountain systems
Many of Earth’s mountain systems are still geologically active, but others have ceased building and are slowly eroding down to plains. Some systems consist of both volcanic and tectonic mountains and may include multiple types from either family. Individual ranges may even include more than one type of mountain. The Front Range of the Rocky Mountains in Colorado, for example, initially consisted of fold-thrust mountains. Later, geological forces pushed up large blocks of Earth’s crust in the range, creating fault-block mountains.
The Appalachian Mountains
extend from Alabama to the Gaspé Peninsula in Canada—a distance of about 1,500 miles (2,400 kilometers). They consist mainly of folded and faulted sedimentary and metamorphic rock layers. Many scientists think that the Appalachian Mountains were formed by three collisions of the North American Plate with the Eurasian and African plates and much smaller “microplates,” beginning about 435 million years ago. The last collision, between the North American and African plates, occurred about 250 million years ago. Since then, the continents have been rifted apart, and the Atlantic Ocean has formed due to spreading along the Mid-Atlantic Ridge, leaving Europe and Africa far from North America. As a result, geological activity stopped in the Appalachians, and they are now simply eroding.
The Rocky Mountains
stretch for about 3,300 miles (5,300 kilometers) from New Mexico into Alaska. The Rockies include fold-thrust, fault-block, and volcanic mountains. They began to form about 100 million years ago and some areas were lifted again in the past 25 million years, but mountain building seems to have ended since then. See Rocky Mountains.
The Pacific Mountain System
consists of two parallel chains of mountains that run for about 2,500 miles (4,000 kilometers) from southern California to Alaska. One chain, the Coast Ranges, includes the Olympic Mountains of Washington, many islands off the coast of British Columbia, and part of Alaska’s coast and coastal islands. The Coast Ranges consist chiefly of fold-thrust mountains composed in large part of marine sediments and volcanic rocks. The North American Plate scraped these materials from the oceanic crust of other plates that subducted beneath it. The other chain in the Pacific system includes the Sierra Nevada of California, the Cascade Range of Oregon and Washington, and the Coast Mountains of British Columbia. Both chains began forming about 240 million years ago and have been further uplifted within the past 63 million years. Many of these mountains continue to grow.
The Andes Mountains
stretch along the west coast of South America for about 4,500 miles (7,200 kilometers). They consist largely of igneous rocks formed by volcanic eruptions within the past 65 million years. The volcanic activity resulted from the subduction of the Nazca Plate beneath the South American Plate. The Nazca Plate underlies the Pacific Ocean west of South America. Mountain building continues in the Andes. The Andes form part of the Ring of Fire, as does the Pacific Mountain System and the mountains of Antarctica. The ring is a belt of subduction zones that encircles the Pacific and includes most of the world’s volcanoes. The mountains of Antarctica include an extension of the Andes called the Antarctic Peninsula.
The Tethyan Mountain System
extends for well over 7,000 miles (11,000 kilometers) across Africa, Europe, and Asia. It includes the Atlas Mountains of northwestern Africa, the Alps and the Carpathian Mountains in Europe, and the Caucasus Mountains between Europe and Asia. In Asia, it continues through the Zagros and Alborz mountains, the Pamirs, the Karakoram Range, and the Himalaya.
The mountains of the Tethyan << TEH thee uhn >> system consist largely of highly deformed sedimentary and igneous rocks that have been folded and faulted within the past 80 million years. Earth scientists consider the system to be a result of the African, Arabian, and Indian plates colliding into the Eurasian Plate. Earthquakes occur frequently along the Tethyan system, indicating that mountain building is still taking place.
Mountains on other planets or moons
Many other bodies in the solar system have mountains on their surfaces. All of the other inner, rocky planets—Mercury, Venus, and Mars—feature mountains, as does Earth’s moon. In addition, mountains appear on several moons of the outer, gas-giant planets.
On Venus,
six mountainous areas cover more than one-third of the planet’s surface. The tallest mountain range, called Maxwell Montes, reaches a peak 7 miles (11 kilometers) in height. Scientists believe that the mountains of Venus formed by the folding and faulting of the planet’s surface. However, because Venus has no water, its mountains lack the erosion-carved features found on Earth’s mountains.
On Io,
a large moon of Jupiter, the surface is dotted with fault-block mountains and volcanic mountains. Scientists think the fault-block mountains formed when regions of the crust sank toward Io’s center and became compressed. In addition, built-up internal heat in these regions may have caused the overlying crust to expand. These processes fractured the crust, producing faults and mountains. Io’s mountains soar to 55,000 feet (17,000 meters) tall, nearly twice the height of Mount Everest.
On Titan,
the largest moon of Saturn, mountain ranges occur south of Titan’s equator. The biggest known range runs for about 90 miles (150 kilometers) and measures about 1 mile (1.5 kilometers) tall. Scientists think the mountains formed from material that welled up as Titan’s tectonic plates pulled apart, similar to the way mid-ocean ridges formed on Earth. White material, believed to be methane snow, tops the mountain peaks.
Studying mountains
Geologists and geographers study mountains to learn important and useful knowledge about Earth. Planetary scientists may also study mountains on other planets or moons to learn similar information about these bodies.
Why scientists study mountains.
Analyzing the structure, composition, and distribution of a planet’s mountains yields information about the geological forces that shape the planet. It also provides clues about the makeup of the planet’s interior. Because mountains are formed by major geological events, geologists study them to learn about the planet’s history.
The study of Earth’s mountains is necessary in the ongoing search for useful mineral resources. Knowledge of mountain geology helps engineers design and build mountain roads, railroad beds, tunnels, and dams.
Measuring mountains.
Accurately determining the location and height of mountains ranks as a particularly important challenge for geographers and mapmakers. To do this, they often use aerial photography, in which an airborne camera takes a series of overlapping photographs of an area. Mapmakers then use a process called photogrammetry to create maps from the photographs.
For Earth, high-flying aircraft and artificial satellites provide accurate data about location and altitude by means of instruments called altimeters. Some altimeters send radio signals or laser light down to the surface and receive the waves after they have been reflected from the ground. These altimeters compute the distance to the surface based on the speed of the waves and the time they took to travel to the ground and back.
Space probes carrying altimeters have measured mountains on other planets and on moons. In the late 1990’s and early 2000’s, for example, the Mars Global Surveyor spacecraft used a laser altimeter to help map that planet’s surface.
Mountains can also be measured using the Global Positioning System. The system’s artificial satellites, orbiting Earth, send radio signals to special receivers, which can be placed on mountains. The receivers then transmit information to stations below, where computers determine the mountain’s altitude. Mountain surveyors measure mountains using a technique called laser ranging, in which laser beams are bounced off objects to determine their distance.
