Discovery Science: Earth – Mountains – Mountain Formation
Mountains are among the most impressive large landscapes on Earth. Young fold mountains, threatening volcanoes, and momentous fissures are all testaments to the tectonic processes that warped huge sections of the Earth’s crust. The process of mountain formation is called orogeny; the physical mountains themselves are orogens.
Mountain Formation
On a human scale, the time period over which mountains are created seems infinitely long. Even what appears at first glance to be fixed and unalterable is subject to constant change.
The most significant mountain ranges are not simply scattered across the Earth at random. Instead, most lie along active plate boundaries and belong to one of the Earth’s two large mountain systems. The Circum-Pacific system encircles the entire Pacific Ocean. It traces an arc from New Guinea, across Japan and the Aleutians, and into the American Cordilleras.
The latter extend along a 9,321 -mile (15,000- km) stretch from Alaska down to Tierra del Fuego in South America. The second, larger orogenic network, called the Alpine-Himalaya system, ranges east from northern Africa, over the Alps, and into the Himalaya before reaching Indonesia.
Phases of mountain formation
There have been three significant phases of mountain formation: the Caledonian, the Variscan, and the Alpine eras. The old fold mountains that were created 450 to 250 million years ago have been eroded and radically transformed by subsequent mountain formation. Of the once mighty Caledonians, only the Norwegian coastal plateau, the highlands of Scotland and Greenland, and the Appalachians remain today.
Almost all young fold mountains were formed during the worldwide Alpine orogeny, which began approximately 220 million years ago and still continues today. The world’s highest mountain ranges known today originated during this phase: the Alps in Europe, the Andes in South America, and the Himalaya in Asia.
Building up and breaking down
The most common high mountain formations are fold mountains, which emerge when two crustal plates come together. Under enormous pressure, sedimentary rock from the ocean floor is folded upward, pushing past younger layers of rock. This process occurs before the actual mountain range rises, and for most high mountain ranges the uplifting procedure is still occurring.
However, the effects of weather and erosion gnaw away at a mountain, wearing it down even as uplifting begins. Without these effects, the Alps would be 32,808 feet (10,000 m) high today. As erosion exposes the fold formations, the folds consisting of harder rock matter become mountain crests. In contrast, the softer rock is removed more quickly, which fosters valley and basin formation.