What is a fold in geology?

In geology, a fold is a bend or warp in Earth's crust's stratified rocks. Originally, sediments formed these rocks in flat, horizontal layers, but in many places, these layers are now warped. Folds are typically classified by the orientation of their axes and their appearance in cross-sections. An anticline is a fold that is convex upward, while a syncline is concave upward. Linear folds are common in mountainous regions and are thought to be caused by compressional forces acting parallel to Earth's surface and perpendicular to the fold. Some geologists believe that many folds result from strata sliding from a vertically uplifted area due to gravity. The push of an advancing glacier can also fold weakly consolidated rocks, and the compaction of sedimentary rocks over buried hills can create gentle folds.

How do folds form in layers of rock?

Folds in rock layers form through the warping of originally flat, horizontal sedimentary strata. This occurs due to compressional forces parallel to Earth's surface, often in mountainous regions. The intensity of warping varies, ranging from barely perceptible inclinations to pronounced, nearly parallel strata, as seen in recumbent folds. Several factors can cause folds, including gravity-induced sliding from uplifted areas, pressure from advancing glaciers, and compaction of sediments over buried hills. In reality, folds typically arise from a combination of these processes rather than a single cause.

What are the main types of folds?

Geologic folds are undulations or waves in the Earth's crust within stratified rocks. Folds are generally classified according to the attitude of their axes and their appearance in cross sections perpendicular to the trend of the fold. Main types of folds: * Anticline An anticline is a fold that is convex upward. * Syncline A syncline is a fold that is concave upward. * Monocline A monocline is an oblique geologic fold. * Symmetrical fold A symmetrical fold is one in which the axial plane is vertical. * Asymmetrical fold An asymmetrical fold is one in which the axial plane is inclined. * Overturned fold An overturned fold, or overfold, has the axial plane inclined to such an extent that the strata on one limb are overturned. * Recumbent fold A recumbent fold has an essentially horizontal axial plane. * Isoclinal When the two limbs of a fold are essentially parallel to each other and thus approximately parallel to the axial plane, the fold is called isoclinal. * Dome A dome is such a fold that is convex upward; this means that its strata dip outward from a central area. * Basin A basin is a circular fold that is concave upward—i.e., the strata dip inward toward a central area.

What is the difference between an anticline and a syncline?

In geology, folds are undulations or waves in the Earth's crust within stratified rocks. An anticline is a convex upward fold, resembling an arch, where the rock layers bend downward away from the crest. Conversely, a syncline is a concave upward fold, forming a trough, with the rock layers dipping toward each other from either side.

How can folds tell scientists about Earth's past movements?

Folds in rock layers reveal Earth's past movements by indicating how stratified rocks have been warped from their original horizontal state due to compressional forces. Geologists study folds to understand the forces that have deformed the Earth's crust over time. The orientation and shape of folds provide information about the direction and intensity of these forces. For example, overturned folds suggest strong directional pressure, while the presence of anticlines and synclines can indicate regional compression or uplift. Analyzing folds helps scientists reconstruct the sequence of geological events and understand the tectonic history of a region.

Where can folds be found around the world, and why are they important?

Folds, undulations in Earth's crust's stratified rocks, appear worldwide in mountain regions formed by compressional forces. Examples include the Alps and Himalayas , products of continental collisions, and the Rocky Mountains . Folds are essential for understanding geological history, revealing past tectonic activity and crustal deformation. They also trap valuable resources like petroleum and natural gas, accumulating in the crests of anticlines or along faults associated with folds, making their study economically significant.

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Images

Layered metamorphic rock formation with dramatic folds exposed on a roadside cliff next to a highway.

Diagram showing three types of geological folds: a syncline with a downward trough, an anticline with an upward crest, and a monocline with a single bend, all labeled with axes, limbs, and axial plane.

Diagram showing four types of geological folds in layered rock, from left to right: symmetrical fold, asymmetrical fold, overfold, and recumbent fold, with the inclination of the axial plane decreasing across the sequence. Red lines indicate the axial planes and axes, and the layers are shaded in brown tones.

Figure 20: The forms of three types of folds.

At a Glance

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Quick Facts

Related Topics:: anticline; fold axis; anticlinorium; synclinorium; syncline

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geology

Also known as: folding, rock fold, tectonic fold

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Top Questions

What is a fold in geology?
In geology, a fold is a bend or warp in Earth's crust's stratified rocks. Originally, sediments formed these rocks in flat, horizontal layers, but in many places, these layers are now warped.

Folds are typically classified by the orientation of their axes and their appearance in cross-sections. An anticline is a fold that is convex upward, while a syncline is concave upward.

Linear folds are common in mountainous regions and are thought to be caused by compressional forces acting parallel to Earth's surface and perpendicular to the fold. Some geologists believe that many folds result from strata sliding from a vertically uplifted area due to gravity. The push of an advancing glacier can also fold weakly consolidated rocks, and the compaction of sedimentary rocks over buried hills can create gentle folds.
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How do folds form in layers of rock?
Folds in rock layers form through the warping of originally flat, horizontal sedimentary strata. This occurs due to compressional forces parallel to Earth's surface, often in mountainous regions. The intensity of warping varies, ranging from barely perceptible inclinations to pronounced, nearly parallel strata, as seen in recumbent folds.

Several factors can cause folds, including gravity-induced sliding from uplifted areas, pressure from advancing glaciers, and compaction of sediments over buried hills. In reality, folds typically arise from a combination of these processes rather than a single cause.
1 Britannica Source
1. fold
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What are the main types of folds?
Geologic folds are undulations or waves in the Earth's crust within stratified rocks. Folds are generally classified according to the attitude of their axes and their appearance in cross sections perpendicular to the trend of the fold.

Main types of folds:
* Anticline An anticline is a fold that is convex upward.
* Syncline A syncline is a fold that is concave upward.
* Monocline A monocline is an oblique geologic fold.
* Symmetrical fold A symmetrical fold is one in which the axial plane is vertical.
* Asymmetrical fold An asymmetrical fold is one in which the axial plane is inclined.
* Overturned fold An overturned fold, or overfold, has the axial plane inclined to such an extent that the strata on one limb are overturned.
* Recumbent fold A recumbent fold has an essentially horizontal axial plane.
* Isoclinal When the two limbs of a fold are essentially parallel to each other and thus approximately parallel to the axial plane, the fold is called isoclinal.
* Dome A dome is such a fold that is convex upward; this means that its strata dip outward from a central area.
* Basin A basin is a circular fold that is concave upward—i.e., the strata dip inward toward a central area.
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1. Definition of monocline
2. fold
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What is the difference between an anticline and a syncline?
In geology, folds are undulations or waves in the Earth's crust within stratified rocks. An anticline is a convex upward fold, resembling an arch, where the rock layers bend downward away from the crest. Conversely, a syncline is a concave upward fold, forming a trough, with the rock layers dipping toward each other from either side.
2 Britannica Sources
1. Definition of syncline
2. fold
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How can folds tell scientists about Earth's past movements?
Folds in rock layers reveal Earth's past movements by indicating how stratified rocks have been warped from their original horizontal state due to compressional forces. Geologists study folds to understand the forces that have deformed the Earth's crust over time. The orientation and shape of folds provide information about the direction and intensity of these forces. For example, overturned folds suggest strong directional pressure, while the presence of anticlines and synclines can indicate regional compression or uplift. Analyzing folds helps scientists reconstruct the sequence of geological events and understand the tectonic history of a region.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where can folds be found around the world, and why are they important?
Folds, undulations in Earth's crust's stratified rocks, appear worldwide in mountain regions formed by compressional forces. Examples include the Alps and Himalayas, products of continental collisions, and the Rocky Mountains.

Folds are essential for understanding geological history, revealing past tectonic activity and crustal deformation. They also trap valuable resources like petroleum and natural gas, accumulating in the crests of anticlines or along faults associated with folds, making their study economically significant.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

fold, in geology, undulation or waves in the stratified rocks of Earth’s crust. Stratified rocks were originally formed from sediments that were deposited in flat horizontal sheets, but in a number of places the strata are no longer horizontal but have been warped. Sometimes the warping is so gentle that the inclination of the strata is barely perceptible, or the warping may be so pronounced that the strata of the two flanks may be essentially parallel or lie nearly flat (as in the case of a recumbent fold). Folds vary widely in size; some are several kilometres or even hundreds of kilometres across, and others measure just a few centimetres or less. The tops of large folds are commonly eroded away on Earth’s surface, exposing the cross sections of the inclined strata (see also erosion).

Folds are generally classified according to the attitude of their axes and their appearance in cross sections perpendicular to the trend of the fold. The axial plane of a fold is the plane or surface that divides the fold as symmetrically as possible. The axial plane may be vertical, horizontal, or inclined at any intermediate angle. An axis of a fold is the intersection of the axial plane with one of the strata of which the fold is composed. Although in the simpler types of folds the axis is horizontal or gently inclined, it may be steeply inclined or even vertical. The angle of inclination of the axis, as measured from the horizontal, is called the plunge. The portions of the fold between adjacent axes form the flanks, limbs, or slopes of a fold.

An anticline is a fold that is convex upward, and a syncline is a fold that is concave upward. An anticlinorium is a large anticline on which minor folds are superimposed, and a synclinorium is a large syncline on which minor folds are superimposed. A symmetrical fold is one in which the axial plane is vertical. An asymmetrical fold is one in which the axial plane is inclined. An overturned fold, or overfold, has the axial plane inclined to such an extent that the strata on one limb are overturned. A recumbent fold has an essentially horizontal axial plane. When the two limbs of a fold are essentially parallel to each other and thus approximately parallel to the axial plane, the fold is called isoclinal.

Many folds are distinctly linear; that is, their extent parallel to the axis is many times their width. Some folds, however, are not linear but are more or less circular in plan. A dome is such a fold that is convex upward; this means that its strata dip outward from a central area. A basin is a circular fold that is concave upward—i.e., the strata dip inward toward a central area.

The long linear folds that are characteristic of mountainous regions are believed to have resulted from compressional forces acting parallel to the surface of Earth and at right angles to the fold (see also mountain). Some geologists believe that many folds are the result of strata sliding from a vertically uplifted area under the influence of gravity. The push exerted by an advancing glacier also may throw weakly consolidated rocks into folds, and the compaction of sedimentary rocks over buried hills gives rise to gentle folds. In nature, folds are rarely produced by a single process but by a combination of processes.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by John P. Rafferty.

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