Earth's hard outer layer is called the crust. It is made up of flat pieces called tectonic plates. These plates fit together like puzzle pieces. The plates move around very slowly. In some places, they push together. This causes rocks to fold into hills and mountains. These mountains are called fold mountains.

It takes millions of years to make a fold mountain. You can easily see how it works though. Cover a table with a cloth, or place a rug flat on the floor. Now push the edge of the cloth or rug. Wrinkles will form and fold on top of each other.

Young and Old, High and Low

There are different types of mountains. Fold mountains are the most common. Some of the most famous ranges are the Himalayas, Andes, and Alps.

The Himalayas, in Asia, stretch across China, India, and Pakistan. The crust beneath them is still being folded. The mountains were formed by two plates. The Indian plate is pushing into the Eurasian plate.

The Andes are the world's longest mountain chain. They stretch along the coast of South America. Here, the Nazca plate is moving below the South American plate. This is causing the Andes to get taller.

The Alps stretch across Europe. Here, the tiny Adriatic plate is crashing into the large Eurasian plate. The mountains include rocks that were once underwater. They were lifted up through plate folding.

All Kinds of 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. The most common folds are anticlines and synclines. An anticline is shaped like a question mark. A syncline is shaped like the letter "U."

Domes and basins are often considered types of folds. A dome is a series of anticlines. It looks like half of a large ball. A basin is a dip in Earth's surface.

Other types of fold include:

• monoclines. A monocline is like a bend in the earth. All rock layers dip in the same direction.
• chevron. A chevron is a pointed fold. Here, the rock layers look like zig-zags.
• slump. Slump folds are caused by landslides. The earth and rocks from the landslides press together over time. They become solid stone and form the 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 are different rock layers with different fold shapes.