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 it is believed 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 can make them more susceptible to folding and warping. Most fold mountains are composed primarily 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 Himalayas, the most towering mountain range on Earth, is still 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 plate 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 with active and dormant volcanoes.
The Alps roughly mark the top of the "boot" of the Italian Peninsula. The Alps stretch across Italy, Slovenia, Austria, Germany, Switzerland, Lichtenstein, Monaco, and France. Here, the tiny Adriatic microplate is colliding with the much larger Eurasian plate to the north. The J-shaped Adriatic microplate is a remnant of the African plate to the south, and today it carries the eastern Italian Peninsula as well as the entire Adriatic Sea. Alpine geology includes sedimentary and metamorphic rock, as well as igneous rocks that once were part of the ocean floor and were later uplifted in the process of folding.
Not all fold mountains are soaring peaks. The Appalachians, stretching along North America's East Coast, are generally low-lying, gentle slopes. Millions of years ago, the Appalachians were taller than the Himalayas. Millions of years of erosion, however, have taken their toll. Today, some of the highest peaks of the Appalachians are less than a third the height of Everest.
The crust that is now the Appalachians began folding over 300 million years ago, when the North American and African continental plates collided. Plate tectonics created this ancient mountain range, then called the Central Pangean Mountains, and plate tectonics tore it apart. As tectonic activity ripped apart the ancient supercontinent Pangea, the African, Eurasian, and North American plates drifted apart.
The Appalachians are just one remnant of the Central Pangean Mountains. The Appalachians stretch from the province of Newfoundland, in southeastern Canada, through the southern state of Alabama in the United States. They are related to the gentle fold mountains of the Scottish Highlands (Europe) and Morocco's Little Atlas Mountains (Africa) — their orogenic sisters from the Central Pangean Mountains.
A Questionable Shape
Fold mountains are defined by complex, vital geologic forms known as folds. There are many, many different types of folds. Geologists primarily categorize folds by their shape—do they have sharp turns or gentle curves? Are the folds convex or concave?
A fold mountain usually displays more than one type of fold. Anticlines and synclines are the most common up-and-down folds that result from compression. An anticline is shaped like a question mark, with the oldest rocks in the center of the fold. A syncline is a U-shape, with the youngest rocks in the center of the fold.
Domes and basins are often considered types of folds. A dome is a series of symmetrical anticlines, roughly shaped like half a sphere. Like an anticline, the oldest rocks in a dome are found in the center. A basin is a depression, or dip, in Earth's surface. Like a syncline, a basin has its youngest rocks in its center.
Other types of fold include:
- monoclines. A monocline is a type of fold in which all rock layers incline, or dip, in the same direction.
- chevron. A chevron is a sharp, straight fold where rock layers look like zig-zags.
- slump. A slump fold is a result of slope failure (a type of mass wasting or landslide). The slope failure happened when sediments were soft, before they became a single mass of rock. As the sediments lithified, they became a slump.
- ptygmatic. Ptygmatic folds are a type of slump fold created where the folding material is much more viscous than the material surrounding it. Many ptygmatic folds are created as metamorphic rock melts and intrudes into another rock layer, forming a dike.
- disharmonic. Disharmonic folds describe rock formations in which different rock layers have different fold shapes.