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.

 

Fold Mountain
The Alps are fold mountains, the most common mountain type on Earth.
ancient
Adjective

very old.

anticline
Noun

layers of rock that have folded into a hill or crest.

Noun

a dip or depression in the surface of the land or ocean floor.

Noun

natural or artificial line separating two pieces of land.

categorize
Verb

to arrange by specific type or characteristic.

chevron
Noun

zig-zag or V-shaped pattern.

collision
Noun

crash.

complex
Adjective

complicated.

compress
Verb

to press together in a smaller space.

concave
Adjective

curving inward.

Noun

one of the seven main land masses on Earth.

continental crust
Noun

thick layer of Earth that sits beneath continents.

convergent plate boundary
Noun

area where two or more tectonic plates bump into each other. Also called a collision zone.

convex
Adjective

curving outward.

Noun

rocky outermost layer of Earth or other planet.

dense
Adjective

having parts or molecules that are packed closely together.

Noun

a barrier, usually a natural or artificial wall used to regulate water levels.

disharmonic fold
Noun

rock formation in which different rock layers have different fold shapes.

Noun

shape that is half of a sphere.

ductile
Adjective
capable of withstanding a certain amount of force by changing form before fracturing or breaking.
Noun

act in which earth is worn away, often by water, wind, or ice.

Noun

areas of the Earth's crust that have been bent and forced up by movement of tectonic plates.

geologist
Noun

person who studies the physical formations of the Earth.

geology
Noun

study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

Noun

land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

Noun

rock formed by the cooling of magma or lava.

incline
Noun

slant, slope, or dip.

intrude
Verb

to thrust or bring into.

Noun

the fall of rocks, soil, and other materials from a mountain, hill, or slope.

lithify
Verb

to change into stone or rock.

mass wasting
Noun

downward movement of rock, soil, and other material.

metamorphic rock
Noun

rock that has transformed its chemical qualities from igneous or sedimentary.

mimic
Verb

to copy another organism's appearance or behavior.

mineral
Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

monocline
Noun

step-shaped fold in a rock formation in which all rock layers gently dip in the same direction.

mountain
Noun

landmass that forms as tectonic plates interact with each other.

mountain range
Noun

series or chain of mountains that are close together.

nappe
Noun

large mass or fold of rock that has been thrust from its position by tectonic activity. Also called a thrust sheet.

oceanic crust
Noun

thin layer of the Earth that sits beneath ocean basins.

orogenic event
Noun

process of a specific mountain range or ranges being formed.

orogeny
Noun

the way mountains are formed.

outcropping
Noun

layer of rock visible above the surface of the Earth.

Pangea
Noun

(300 million years ago) ancient supercontinent that contained all present-day continents and began to break up about 200 million years ago.

peak
Noun

the very top.

Noun

piece of land jutting into a body of water.

Noun

division of a country larger than a town or county.

ptygmatic fold
Noun

rock formation (fold) created where the folding material is much more viscous than the material surrounding it. 

remnant
Noun

something that is left over.

rock
Noun

natural substance composed of solid mineral matter.

rugged
Adjective

having an irregular or jagged surface.

salt
Noun

(sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.

Noun

solid material transported and deposited by water, ice, and wind.

Noun

rock formed from fragments of other rocks or the remains of plants or animals.

slope
Noun

slant, either upward or downward, from a straight or flat path.

slump fold
Noun

rock formation (fold) formed by the collapse of soft sediments on the edge of a continental boundary.

sphere
Noun

round object.

stress
Noun

physical or mental factor (or set of factors) that disturbs the body's normal state of functioning or ability.

subduct
Verb

to pull downward or beneath something.

supercontinent
Noun

ancient, giant landmass that split apart to form all the continents we know today.

susceptible
Adjective

able to be influenced to behave a certain way.

symmetrical
Adjective

having the same arrangement of parts on either side.

syncline
Noun

layers of rock that have folded to create a dip or area between hills.

tectonic plate
Noun

massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

toll
Noun

amount of loss or suffering from an event.

uplift
Noun

elevation of the Earth's surface due to tectonic or other natural activity.

viscous
Adjective

liquid that is thick and sticky.

vital
Adjective

necessary or very important.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

warp
Verb

to bend out of shape.