Discovery Science: Earth – Mountain Ranges
No mountain range is the same as any other; each has its own specific evolutionary history and unique characteristics. Nevertheless, in their genesis many follow a more or less standard formation process.
The largest mountain ranges were created through the collision of continental plates, which fold sedimentary layers upward. If the compressional movement creates enough pressure, “roofs” tear away from the lower layers. These roofs can then be pushed over one another across a distance of several miles. The clash between the Indian and the Eurasian plates around 50 million years ago led to the lifting of the Himalaya.
However, this process is not yet finished, and the “roof of the world” grows annually by about half a millimeter. The Alps are an outcome of the colliding Eurasian and African continental plates; the current form of these mountains was created by lifting that occurred about 65 million years ago. Further surface transformation happened during the Neogene ice ages. The South American Andes demonstrate the results of a collision between an oceanic plate, called the Nazca plate, and the continental crust.
Due to subduction, parts of the heavy oceanic crust were scraped away and deposited on the continental plate in a wedge formation. Simultaneously, the continental crust was shaved back. Then a belt made of volcanic and magmatic rock that had solidified in the depths was shifted inland. Many such belts, parallel to each other, were formed in this manner and are thus called cordillera, or “strings of mountains.”
Volcanic mountain ranges
When two sections of oceanic crust collide, volcanic arcs such as the Aleutians, the Kuril Islands, and the Philippines are formed in the subduction zones. When crustal plates drift away from each other, volcanic mountain ranges result; the largest of this type are found along mid-oceanic ridges. Ongoing volcanic activity also produces such mountain ranges on the continents, not only at the bottom of the sea.
Movements caused by plate tectonics along the sides of the East African Rift have created huge cracks where magma wells up. The mightiest mountain in Africa ascends above this area: the Kilimanjaro massif, which is 19,340 feet (5,895 m) high.
The smallest of the giants
The low mountain ranges vary between 4,900 and 6,500 feet (1,500 and 2,000 m) high. They are often residual mountain ranges, meaning that over millions of years, the older mountains have been worn down by weather and erosion.
These old, rigid remains do not yield and break under the pressure caused by expansion in the Earth’s crust. Examples of these kinds of mountain ranges include the low mountains of Germany, the African Ruwenzori range (now called the Rwenzori Mountains), and the Appalachians.
ISOSTASY
Mountains float on the elastic lithosphere like icebergs on the ocean. In both cases, only a part of the whole can be seen above the surface. As an iceberg melts, it floats upward, so that its height relative to the ocean’s surface stays the equal.
The same process occurs in mountainous regions that are worn down or where glaciers are melting and are no longer so heavy.