Fold (geology)

Rocks can bend into wavy shapes.

These bends are called folds. They happen when rock layers are squeezed. This squeezing can make the layers move.

Some folds are tiny. Other folds are as big as mountains. Some folds look like sharp peaks.

These are called chevron folds. Folds can also look like a single step. This is called a monocline. Folds help us see how the Earth changes over time.

Rocks can bend into wavy shapes. These shapes are called folds.

Folds happen when rock layers are squeezed. This squeezing can be very strong. It can make folds as small as tiny crinkles. It can also make folds as big as mountains.

Every fold has special parts. The sides of a fold are called limbs.

The middle part where the sides meet is the hinge. The highest point is the crest. The lowest point is the trough.

Folds come in many shapes. An anticline is a fold that curves upward. A syncline is a fold that curves downward.

Some folds look like a single step. We call these monoclines.

Many things cause folding. Squeezing layers can make them bend. Faults can also cause folds. This happens when rocks move along a crack. Even soft mud can fold before it turns into hard rock. This is called synsedimentary folding.

Large movements in the Earth's crust make these shapes.

Rocks are often seen as hard and unmoving. However, they can actually bend into wavy shapes called folds.

A fold is a stack of flat surfaces that become curved during permanent deformation. These shapes can be tiny, microscopic crinkles in the rock. They can also be huge, mountain-sized structures.

Folds happen when layers of rock are shortened or squeezed. This can occur in many different types of rocks. Folds are important because they help us understand how the Earth's crust moves.

Every fold has specific parts that help scientists describe them. The sides of a fold are called limbs.

The middle area where these limbs meet is the hinge. The very top of a fold is called the crest. The lowest point is called the trough.

Scientists also look at the interlimb angle to see how tight a fold is. A gentle fold has a wide angle between its limbs. A tight fold has a very small angle. Some folds are symmetrical, meaning both sides look the same. Other folds are asymmetrical and look different on each side.

There are many different types of folds based on their shape. An anticline is a fold where the layers curve upward.

In an anticline, the oldest rock layers are usually in the center. A syncline is a fold that curves downward like a bowl.

In a syncline, the youngest rock layers are in the center. A monocline looks like a single step in the rock layers.

Some folds are very extreme, like a recumbent fold that is almost lying on its side.

Other shapes include chevron folds, which have straight limbs and sharp angles.

Many different forces can cause rocks to fold. One common way is through layer-parallel shortening. This happens when rock layers are squeezed side-to-side. Folds can also form because of faults, which are cracks in the Earth. A fault-propagation fold happens when movement on a fault causes the layers above it to bend. Sometimes, folds form even before the rock is hard. This is called synsedimentary folding.

It happens when soft sediments, like mud, move and fold while they are still wet. Other forces, like rising hot magma, can also push against rock to create folds.

Understanding folds helps us see the history of our planet. When we see a fold belt, we know a large area of the Earth was squeezed. These folds are often found in orogenic zones, which are places where mountains form. Folds can even show us how old rocks moved in the past. For example, the direction of a fold can tell us which way the ground was sliding. By studying these shapes, we can piece together the story of how mountains and basins were made. It is like reading a map written in the stone itself.

In structural geology, a fold is a stack of originally planar surfaces that become bent or curved. These surfaces are often sedimentary strata, which are layers of rock. Folds represent a permanent deformation of the Earth's crust. They can range in size from microscopic crinkles to massive mountain-sized structures. Folds may appear as single, isolated shapes or in periodic sets called fold trains. When these folds are distributed across a large regional scale, they form a fold belt. These belts are common features in orogenic zones, which are the regions where mountains are built.

To understand a fold, geologists look at its specific geometric parts. The fold hinge is the line that joins the points of maximum curvature on a surface. This hinge line can be straight or curved. If you look at a fold perpendicular to its shortening direction, you can divide it into the hinge and the limbs. The limbs are the flanks or sides of the fold. These limbs converge at the hinge zone. Within this zone lies the hinge point, which is the point of minimum radius of curvature. The crest is the highest point of the fold surface, while the trough is the lowest point. An inflection point is a spot on a limb where the concavity reverses.

Geologists also use an axial surface to describe stacked folds. This is a plane that connects all the hinge lines of the folded surfaces. If this surface is a flat plane, it is called an axial plane. Some folds are cylindrical, meaning they can be generated by a fold axis. A fold axis is the closest approximation to a straight line that generates the fold's form when moved parallel to itself. Folds are also classified by their symmetry. Symmetrical folds have limbs of relatively equal length. Asymmetrical folds have highly unequal limbs and an axis at an angle to the original surface. The direction of overturning is often described by vergence.

Folds are categorized by their shape and tightness. Tightness is measured by the interlimb angle, which is the angle between the limbs. Gentle folds have an angle between 180° and 120°. Open folds range from 120° to 70°. Close folds range from 70° to 30°, and tight folds are between 30° and 0°. Isoclinal folds, or isoclines, have limbs that are essentially parallel, with angles between 10° and zero. Shapes can vary from chevron folds, which have angular axes, to cuspate folds with curved limbs. Some folds are circular or elliptical. Even the way layers change thickness matters. Concentric folds maintain uniform layer thickness, while similar folds show thinning limbs and a thickened hinge zone.

There are many specific types of folds based on how the layers dip. An anticline is a linear fold where layers dip away from the axial center. In an anticline, the oldest strata are in the center. A syncline is a linear fold where layers dip toward the axial center, placing the youngest strata in the middle. An antiform and a synform are similar, but their age orientation is unknown or inverted. A monocline is a linear fold where layers dip in only one direction between horizontal layers. Some folds are non-linear, such as domes, where strata dip away from the center in all directions. Basins are the opposite, with strata dipping toward the center.

Many different processes cause these deformations. One cause is layer-parallel shortening, where rocks are squeezed side-to-side. This can produce kink-bands, box-folds, or chevron folds depending on the rock properties. Folds also relate to faults. A fault-bend fold is caused by displacement along a non-planar fault. A fault-propagation fold occurs when displacement happens on an existing fault without the fault growing further. In some cases, folds form in soft, wet sediments before they harden into rock. These are called synsedimentary folds. This includes slump folding, where sediments move during deposition, and convolute bedding caused by rapid dewatering. Finally, igneous intrusions can deform surrounding rock, such as the folding seen above a laccolith.