Earth's hard outer layer is called the crust. It is made up of large slabs called tectonic plates. The plates fit together like puzzle pieces. Fold mountains are created where two or more plates are pushed together. At these boundaries, rocks are folded into hills and mountains.

Fold mountains are created through a process called orogeny. It takes millions of years to create a fold mountain. However, you can easily see how it works. 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 fold on top of each other.

Most Famous Mountain Ranges Are Fold Mountains

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

The Himalayas, in Asia, stretch through the borders of China, India, and Pakistan. The crust beneath the Himalayas is still 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 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 rocks of the South American plate.

The Alps, in Europe, stretch across Italy, Austria, 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 in the process of folding.

Not all fold mountains are tall peaks. For example, the Appalachians in North America are low, gentle slopes. Long ago, the Appalachians were taller than the Himalayas. The mountains were worn down over millions of years and now they are much smaller.

Folds Are Classified by Their Shape

Fold mountains are defined by complex rock formations known as folds. There are many different types of folds. Scientists usually classify 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 folds. An anticline is shaped like a question mark. The oldest rocks are located in the center of the fold. A syncline is shaped like the letter "U." The youngest rocks are located in the center of the fold.

Domes and basins are often considered types of folds. A dome is a series of symmetrical anticlines. It is 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. In a monocline, all rock layers dip in the same direction.
• chevron. A chevron is a fold in which 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. Sediments are smaller pieces of matter, like dirt, that settle in an area. As the sediments turned 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. Melted rock sometimes pushes into other rock layers. This creates a ptygmatic fold.
• disharmonic. Disharmonic folds describe mixed rock formations. Here, different rock layers have different fold shapes.