Mountains

The Himalayas, for example, rose as a result of the compression and deformation that accompanied the collision of the Indo-Australian Plate with the Eurasian Plate. Compression generated by the collision of the African Plate and the Eurasian Plate formed Europe’s Alps and the Jura Mountains.

Some mountain belts, such as the Andes in South America, result from the convergence of a continental plate and an oceanic plate. In these cases, the heavier oceanic plate is subducted, or forced under, the continental plate and partially melts, generating new magma. This magma solidifies as light, relatively buoyant rock beneath the mountains and helps cause uplift. Similarly, most of North America’s Rocky Mountains were formed in response to the subduction of oceanic plates beneath the plate margin of western North America. This kind of mountain building is often called orogeny.

Local uplift can also result from continental extension or rifting, the process that eventually breaks continents up into two or more pieces. Usually, rifting within continents is confined to long, narrow zones bounded by normal faults with a central downdropped block and uplifted sides. The Great Rift Valley of eastern Africa is a famous example of a continental rift. Basin and mountain structures such as those of Nevada in the United States and the Mexican state of Sonora are also due to crustal extension and normal faulting, but over a broad area rather than confined to a narrow rift valley. Crustal extension also occurs in the oceanic realm. In fact, the mid-oceanic rift system, which is almost entirely under water, is the longest continuous mountain belt on the earth, extending into all the major oceans.

The products of erosion—sediments—are carried towards the sea by rivers and streams. Coarse sediment in the form of boulders and gravel often settles at the foot of mountains in alluvial fans. Finer sediments in the form of sand and mud are more easily carried longer distances. Eventually, mountains are eroded down to the level of the surrounding land, and most of the resulting sediment solidifies into sedimentary rock along river courses and near the seashore.

Glaciers are masses of ice that are so large they actually flow under their own weight. They constitute a slow-moving but very powerful erosive force. The bottom of the thick ice digs into the surface, picking up rocks and boulders, which add to the grinding and scouring process. The motion of glaciers has created huge U-shaped valleys and fjords in different parts of the world. Glaciers exist on many mountains, such as Grossglockner in the Austrian Alps, Mount Rainier in the Cascade Range of the northwest US, and Illimani Peak in the Real Mountains in Bolivia.

Humans significantly increase soil erosion on mountains through deforestation and overgrazing, particularly on steep slopes. Soils lose cohesion and easily succumb to water erosion. 

Once-protected surfaces become more vulnerable to constant winds, and landslides become more frequent. In addition, hiking and vehicular traffic in sensitive areas or off designated paths lead to damaging erosion

The effects of erosion can create topographically rugged areas. Rocks of different composition resist erosion differently, meaning that areas of relatively hard rock may stand high above areas of softer, more easily eroded rock. The Massif Central in France and the Lake District in the United Kingdom are good examples of mountains created by the forces of erosion. In the US, a part of the Ozark Plateau—also known as the Ozark Mountains—in Arkansas and Missouri is another example.

Exceptionally high mountains, such as the Himalayas in Asia, markedly affect patterns of climate and weather over vast areas of the earth because they stand as barriers to regularly circulating air masses. Moisture carried inland by the westerlies from the Pacific Ocean, for example, falls as rain and snow on the windward sides of the Sierra Nevada of North America and the Andes of southern Chile. The leeward, or inland, side is drier, and the land beyond is frequently arid. This weather pattern is called the orographic effect.

The influence of mountains on the development of the western US demonstrates the importance of mountains to the history and economy of various nations. 

The political significance of mountains has been evident throughout human history. Mountain barriers with their relatively narrow and easily defensible passes have made various ranges natural political boundaries, second in strategic importance only to oceans and seas.

For more information on Mountains, go to:

• Hydrological Cycle
• Rivers
• Rocks and Rock Cycle