Fold mountains are created where two or more of Earth’s tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges.

Fold mountains are created through a process called orogeny. An orogenic event 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—wrinkles will develop and fold on top of each other. 

The vocabulary of fold mountains owes something to this simple tablecloth experiment. Some of the key structures in fold mountains are nappes. Nappes are common, dramatic folded rocks or rock formations. “Nappe” is French for “tablecloth” and it is believed the formations were named after the tabletop experiment. 

The huge difference between the rock folds and cloth folds is that in the tabletop experiment, the table itself does not fold. In the creation of fold mountains, Earth’s crust itself is warped into folded forms.

Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation.

At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation. For this reason, most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This can make them more susceptible to folding and warping. Most fold mountains are composed primarily of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present.

Young and Old, High and Low

Fold mountains are the most common type of mountain in the world. The rugged, soaring heights of the Himalayas, Andes, and Alps are all active fold mountains. 

The Himalayas stretch through the borders of China, Bhutan, Nepal, India, and Pakistan. The crust beneath the Himalaya, the most towering mountain range on Earth, is still the process of being compressed. Here, the Indian plate is colliding northward with the Eurasian plate. The sedimentary rocks of the Himalayas include shale and limestone. Metamorphic rocks of the region include schist and gneiss. Dikes of igneous rock also intrude throughout the rock formations of the Himalayas.

The Andes are the world’s longest mountain chain. They stretch along the entire west coast of South America, from Colombia in the north and through Ecuador, Peru, Bolivia, Chile, and Argentina to the south. Here, the dense oceanic crust of the Nazca plate is subducting beneath the less-dense continental crust of the South American plate. The Andes are mostly being folded and uplifted from the thicker, less-dense rocks of the South American plate. The sedimentary and metamorphic rocks of the Andes are dotted by active and dormant volcanoes.

The Alps roughly mark the top of the “boot” of the Italian Peninsula. The Alps stretch across Italy, Slovenia, Austria, Germany, Switzerland, Lichtenstein, Monaco, and France. Here, the tiny Adriatic microplate is colliding with the much larger Eurasian plate to the north. The J-shaped Adriatic microplate is a remnant of the African plate to the south, and today it carries the eastern Italian Peninsula as well as the entire Adriatic Sea. Alpine geology includes sedimentary and metamorphic rock, as well as igneous rocks that once were part of the ocean floor and were later uplifted in the process of folding. 

Not all fold mountains are soaring peaks. The Appalachians, stretching along North America’s east coast, are generally low-lying, gentle slopes. Millions of years ago, the Appalachians were taller than the Himalayas! Millions of years of erosion, however, have taken their toll. Today, some of the highest peaks of the Appalachians are less than a third of the height of Everest. 

The crust that is now the Appalachians began folding over 300 million years ago, when the North American and African continental plates collided. Plate tectonics created this ancient mountain range, then called the Central Pangean Mountains . . . and plate tectonics tore it apart. As tectonic activity ripped apart the ancient supercontinent Pangea, the African, Eurasian, and North American plates drifted apart.

The Appalachians are just one remnant of the Central Pangean Mountains. The Appalachians stretch from the province of Newfoundland, in southeastern Canada, through the southern state of Alabama in the United States. They are related to the gentle fold mountains of the Scottish Highlands (Eurasia) and the Little Atlas Mountains in Morocco (Africa)—their orogenic sisters from the Central Pangean Mountains.

Types of Folds

Fold mountains are defined by complexvital geologic forms known as folds. There are many, many different types of folds. Geologists primarily categorize folds by their shape—do they have sharp turns or gentle curves? Are the folds convex or concave?

A fold mountain usually displays more than one type of fold. Anticlines and synclines are the most common up-and-down folds that result from compression. An anticline has a ∩-shape, with the oldest rocks in the center of the fold. A syncline is a U-shape, 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 depression, or dip, in Earth’s surface. Like a syncline, a basin has its youngest rocks in its center.

Other types of folds include:

  • monoclines. A monocline is a type of fold in which all rock layers incline, or dip, in the same direction.
  • chevron. A chevron is a sharp, straight fold where rock layers look like zig-zags.
  • slump. A slump fold is a result of slope failure (a type of mass wasting or landslide). The slope failure happened when sediments were soft, before they became a single mass of rock. As the sediments lithified, they became a slump.
  • ptygmatic. Ptygmatic folds are a type of slump fold created where the folding material is much more viscous than the material surrounding it. Many ptygmatic folds are created as metamorphic rock melts and intrudes into another rock layer, forming a dike.
  • disharmonic. Disharmonic folds describe rock formations in which different rock layers have different fold shapes.

 

Fold Mountain
The Alps are fold mountains, the most common mountain type on Earth.
Foreland Basins
Fold mountains sometimes are characterized by foreland basins, depressions that run parallel to the mountain range. The Erbo Basin, in northern Spain, is a foreland basin that formed with the Pyrenees, a fold mountain chain created by the continental collision of the microcontinent of Iberia with the massive Eurasian plate. The Persian Gulf is a foreland basin that formed with the Zagros Mountains, a fold mountain chain created by the continental collision of the Arabian and Iranian tectonic plates.
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.