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.

The Alps are fold mountains, the most common mountain type on Earth.
Photograph by James P. Blair, National Geographic
ancient
Adjective
very old.
anticline
Noun
layers of rock that have folded into a hill or crest.
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.
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.
dense
Adjective
having parts or molecules that are packed closely together.
disharmonic fold
Noun
rock formation in which different rock layers have different fold shapes.
ductile
Adjective
capable of withstanding a certain amount of force by changing form before fracturing or breaking.
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).
incline
Noun
slant, slope, or dip.
intrude
Verb
to thrust or bring into.
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.
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
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.