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mountainBritannica Elementary Article

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A mountain is a landform that rises high above its surroundings. Taller than a hill, it usually has steep slopes and a rounded or sharp peak. Mountains are rarely found alone. Groups of mountains are called ranges, and lines of ranges form mountain belts.


Mountain formation and types

Although most mountains are similar in some ways, ranges that have been created by different processes often have different features. Mountain ranges are formed either by the buildup of volcanic rock or by movements within the Earth's surface, or crust.

Many mountain ranges consist of chains of volcanoes. Most volcanic mountains are made up of rock that is thought to have melted deep within the Earth. This rock rose through the crust and flowed onto the surface in the form of lava. The buildup of lava and volcanic dust forms mountains that are typically steep and cone shaped. Mount Fuji in Japan, Mount Kilimanjaro in Africa, and Mount Rainier in the northwestern United States are examples of volcanic mountains.

Many other mountain belts have been built by the slow movement of the Earth's crust. According to a theory called continental drift, the Earth's surface is divided into several separate plates that are constantly moving at a very slow rate. The continents sit on top of these plates and move with them. At times these plates, and therefore the continents, collide and create new landforms. The Alps of Europe and the Himalayas of Asia, for example, are thought to have been created when parts of the crust collided. As the heavier landmass slipped beneath the lighter one, its front edge was scraped away. The scraped-off material piled up into mountains. (See also continental drift.)

The folding of layers of rock at the Earth's surface is a related mountain-building process. This folding creates long, parallel ridges and valleys called fold belts. Stronger rocks form the ridges, while weaker rocks are carved into valleys. The Appalachian Mountains of North America are an example of fold mountains. Fold belts commonly include segments where layers of older rocks have been pushed up and over younger rocks. Such segments are known as fold and thrust belts.

Faults, or fractures in the Earth's crust, cause the formation of fault-block mountains. Block faulting may occur when blocks of crust between two faults are thrust over neighboring valleys. The Rocky Mountains of the western United States were formed in this way. Block faulting also occurs where blocks are pulled apart. In these types of mountain ranges, one side is steep but the other has a gentle slope. This is because the blocks on one side drop down, creating steep slopes. The Sierra Nevada of California is one such range.


Mountains and human life

Mountain ranges are natural barriers. Roads are difficult to build across them, and railroads need expensive tunnels to cross even low ridges. Therefore mountain ranges tend to divide the people on either side of them. They often form frontiers between nations.

Life is hard in mountain lands. The high places of the world are cold and have little soil, making farming difficult. However, many mountain districts serve as vacation resorts where skiing and mountaineering can be enjoyed.