The oldest mountain on the earth came into being about 400 million years ago. These are called Caledonian Mountains.
Fold mountains are created where two or more of Earth’s tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges.
Fold mountains are created through a process called orogeny. An orogenic event takes millions of years to create a fold mountain, but you can mimic it in seconds. Cover a table with a tablecloth, or place a rug flat on the floor. Now push the edge of the tablecloth or rug—wrinkles will develop and fold on top of each other.
The vocabulary of fold mountains owes something to this simple tablecloth experiment. Some of the key structures in fold mountains are nappes. Nappes are common, dramatic folded rocks or rock formations. “Nappe” is French for “tablecloth” and the formations were named after the tabletop experiment.
The huge difference between the rock folds and cloth folds is that in the tabletop experiment, the table itself does not fold. In the creation of fold mountains, Earth’s crust itself is warped into folded forms.
Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation.
At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation. For this reason, most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This makes them more susceptible to folding and warping. Most fold mountains are composed of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present.
Young and Old, High and Low
Fold mountains are the most common type of mountain in the world. The rugged, soaring heights of the Himalayas, Andes, and Alps are all active fold mountains.
The Himalayas stretch through the borders of China, Bhutan, Nepal, India, and Pakistan. The crust beneath the Himalaya, the most towering mountain range on Earth, is still the process of being compressed. Here, the Indian plate is colliding northward with the Eurasian plate. The sedimentary rocks of the Himalayas include shale and limestone. Metamorphic rocks of the region include schist and gneiss. Dikes of igneous rock also intrude throughout the rock formations of the Himalayas.
The Andes are the world’s longest mountain chain. They stretch along the entire west coast of South America, from Colombia in the north and through Ecuador, Peru, Bolivia, Chile, and Argentina to the south. Here, the dense oceanic crust of the Nazca and Antarctic plates is subducting beneath the less-dense continental crust of the South American plate. The Andes are mostly being folded and uplifted from the thicker, less-dense rocks of the South American plate. The sedimentary and metamorphic rocks of the Andes are dotted by active and dormant volcanoes.