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 plates are pushed together. At these boundaries, rocks are warped and folded into hills and mountains.

Fold mountains are created through a process called orogeny. It 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. You will see wrinkles develop and fold on top of each other.

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

Himalayas, Andes and Alps Are Fold Mountains

Fold mountains are the most common type of mountain in the world. Some of the most famous active ranges are the Himalayas, Andes, and Alps.

The Himalayas stretch through the borders of China, Bhutan, Nepal, India, and Pakistan. The crust beneath the Himalayas 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. Here, the Nazca plate is moving down below the South American plate. The Andes are mostly being folded up from the thicker rocks of the South American plate.

The Alps stretch across Italy, 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. They were lifted up over time in the process of folding.

Not all fold mountains are soaring peaks. The Appalachians, stretching along North America's East Coast, are generally low, gentle slopes. Long ago, the Appalachians were taller than the Himalayas. However, millions of years of erosion have taken their toll. Today, some of the highest peaks of the Appalachians are less than a third the height of the Himalayas' Mount Everest.

Domes and Basins Are Folds, too

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

A fold mountain usually has more than one type of fold. Anticlines and synclines are the most common up-and-down folds. 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 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 dip in the same direction.
• chevron. A chevron is a 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. This occurred 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 around it. Many ptygmatic folds are created as rock melts and pushes into another rock layer.
• disharmonic. Disharmonic folds describe mixed rock formations. Here, different rock layers have different fold shapes.