Fold mountains are formed by the effects of folding on layers within the upper part of the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

. Before the development of the theory of

plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

and before the internal architecture of thrust belts became well understood, the term was used to describe most mountain belts but has otherwise fallen out of use.

Formation

Fold mountains form in areas of

thrust tectonics Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the Tectonics, tectonic processes associated with, the shortening and thickening of the Crust (geology), crust or lithosphere. It is one of the three main ...

, such as where two

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

s move towards each other at convergent plate boundary. When plates and the continents riding on them collide or undergo

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

(that is – ride one over another), the accumulated layers of rock may crumple and fold like a tablecloth that is pushed across a table, particularly if there is a mechanically weak layer such as

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

. Since the less dense

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

"floats" on the denser mantle rocks beneath, the weight of any crustal material forced upward to form hills,

plateau In geology and physical geography, a plateau (; ; : plateaus or plateaux), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. ...

s or mountains must be balanced by the buoyancy force of a much greater volume forced downward into the mantle. Thus the continental crust is normally much thicker under mountains, compared to lower-lying areas. Rock can fold either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines. Severely folded and faulted rocks are called nappes. In asymmetric folding there may also be recumbent and overturned folds. The mountains such formed are usually greater in length instead of breadth.

Examples

* The

Jura mountains The Jura Mountains ( ) are a sub-alpine mountain range a short distance north of the Western Alps and mainly demarcate a long part of the French–Swiss border. While the Jura range proper (" folded Jura", ) is located in France and Switzerla ...

– A series of sub-parallel mountainous ridges that formed by folding over a

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...

decollement due to thrust movements in the foreland of the

Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...

*The 'Simply Folded Belt' of the Zagros Mountains – A series of elongated anticlinal domes, mostly formed as detachment folds over underlying thrusts in the foreland of the Zagros collisional belt, generally above a basal decollement that formed in

of the late

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

to Early Cambrian Hormuz Formation *The Akwapim-Togo ranges in

Ghana Ghana, officially the Republic of Ghana, is a country in West Africa. It is situated along the Gulf of Guinea and the Atlantic Ocean to the south, and shares borders with Côte d’Ivoire to the west, Burkina Faso to the north, and Togo to t ...

*The Ridge-and-Valley Appalachians in the eastern part of United States. *The

Ouachita Mountains The Ouachita Mountains (), simply referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of highly deformed Paleozoic strata constituting the Ouachita Fold and Thru ...

of Arkansas and Oklahoma.

References

{{Authority control Mountain geomorphology Structural geology Orogeny

Formation

Examples

See also

References