Earth's hard outer layer is called the crust. It is made up of large interlocking slabs called tectonic plates. The plates fit together like puzzle pieces far beneath our feet. Fold mountains are created where two or more of Earth's tectonic plates are pushed together. At these boundaries, rocks and debris are warped and folded into 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 language of fold mountains owes something to this simple tablecloth experiment. Some of the key structures in fold mountains are called 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 tablecloth experiment.

There is one huge difference, though, between the rock folds and the cloth folds. In the tablecloth experiment, the table itself does not fold. In the creation of fold mountains, Earth's crust itself is warped into folded forms.

Appalachians Were once Higher than Himalayas

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 in the process of being folded. Here, the Indian tectonic plate is pushing into the Eurasian plate.

The Andes are the world's longest mountain chain. They stretch along the entire west coast of South America, from Colombia to Argentina. Here, the dense crust of the Nazca plate is moving down below the less-dense crust of the South American plate. The Andes are mostly being folded and uplifted from the thicker rocks of the South American plate.

The Alps roughly mark the top of the "boot" of the Italian Peninsula. The mountains stretch across Italy, Slovenia, Austria, Germany, Switzerland, and France. Here, the tiny Adriatic microplate is crashing into the much larger Eurasian plate. The mountains include rocks that were once part of the ocean floor and were later lifted up 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 of the height of the Himalayas' Mount Everest.

You Can Question These Folds

Fold mountains are defined by complex geologic forms known as folds. There are many different types of folds. Geologists primarily categorize folds by their shape—do they have sharp turns or gentle curves? Do they fold inward or outward?

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 shaped like the letter "U," 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 the result of slope failure (a type of landslide). The slope failure happened when sediments were soft, before they became a single mass of rock. As the sediments transformed into stone, they became a slump.
• ptygmatic. Ptygmatic folds are a type of slump fold. They are created where the folding material is less solid than the material surrounding it. Many ptygmatic folds are created as rock melts and pushes into another rock layer.
• disharmonic. Disharmonic folds describe formations where different rock layers have different fold shapes.