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Lesson Explainer: Folds Geology

In this explainer, we will learn how to describe the different types of folds and highlight their geological and economic importance.

Folds are types of geological structures that arise due to the curving or bending of Earth’s crust. Folds are secondary tectonic structures that have formed after the deposition of rocks in response to compressional stress.

We call this structure a fold because visually the rock appears as a wavelike structure, as shown in Figure 1, which shows folded limestone in Crete, Greece. These limestone beds were once layered horizontally, and then tectonic activity affected this area, which caused these layers to be folded.

Folded limestone on Crete, Greece
Figure 1: Folded limestone in Crete, Greece.

Key Term: Fold

A fold is a wavelike structure formed by the bending of any type of rock due to compressional stress.

Folds are common in nature and can be found in any type of rock. They can be very simple, such as a single fold in a body of rock. In most cases, folds are complex structures that have many curves or connected bends. This is because rocks are usually subjected to repeated compressions, as shown in Figure 2, and even fractures, depending on the rate of deformation.

Figure 2: A diagram showing horizontal layers subjected to compressional stress, which is indicated by the arrows.

Example 1: Understanding the Definition of a Fold

Which of the following statements about folds is correct?

  1. Folds only occur in sedimentary rocks and are often found as a single bend.
  2. Folds can occur in any type of rock and are often found as a single bend.
  3. Folds can occur only in sedimentary and metamorphic rocks.
  4. Folds only occur in sedimentary rocks and are often found as several connected bends.
  5. Folds can occur in any type of rock and are often found as several connected bends.

Answer

Folds are secondary geologic structures that form due to bending or curving horizontal layers upward or downward as a result of compressional stress.

They are rarely found in nature as single folds, though this is possible. Most commonly, however, they are found as sets of multiple bends connected together.

So, the correct description of folds is that they can occur in any type of rock and are often found as several connected bends, so option E is the correct answer.

Folds vary in extent or size. Some expand hundreds of kilometres in width, and others measure a few centimetres or less, as shown in Figure 3. Folds appear clearly in sedimentary rocks. This is because they consist of layers of different nature and colors, making the folding more apparent.

Folded sedimentary rock layers exposed near the Bobotov Kuk mountain
Minor fold crenulation rock-lance cover
Figure 3: Folded sedimentary rock layers, showing how folds can extend for either many kilometres or a few centimetres.

Example 2: Identifying the Types of Rocks that Folds Exist in

Which type of rock are folds found in?

  1. Only in igneous rocks
  2. Only in sedimentary rocks
  3. Only in metamorphic rocks
  4. Only in sedimentary and igneous rocks
  5. In all types of rocks

Answer

If any kind of rock, sedimentary, metamorphic, or igneous, is subjected to compressional stress, it will be bent or folded. However, folding appears quite clearly in sedimentary rocks. This is because they consist of layers of different nature and colors, making the folding more apparent.

So, the correct answer is option E: folds are found in all types of rocks.

Folds are characterized by several structural elements. In this explainer, we will focus on hinge points, axial planes, fold axes, and fold limbs, as shown in Figure 4.

Figure 4: A diagram showing a fold’s structural elements.

A fold hinge point is the point located at the maximum curvature of the fold.

Key Term: Fold Hinge Point

The fold hinge point is the point of maximum curvature on a folded surface.

A fold axial plane is an imaginary plane or surface that bisects the fold into two halves. It may be vertical, horizontal, or inclined according to the fold shape.

Key Term: Fold Axial Plane

The fold axial plane is an imaginary plane that bisects a fold into two halves that are as similar as possible.

The fold axis is an imaginary straight line that joins all the hinge points on a folded surface. Each layer of any fold has its own axis, with the axial plane containing all these axes.

Key Term: Fold Axis

A fold axis is an imaginary straight line of intersection of the axial plane with the surface of all different layers.

The sides of a fold are called the limbs. They extend out from the fold’s axis and are found on either side of the axial plane.

Key Term: Fold Limbs

Fold limbs are the flanks of a fold and are found on both sides of the axial plane.

Folds can be classified according to many factors which include the following:

  1. the appearance of folds exposed in the field,
  2. the position of certain structural elements of the fold,
  3. the type and nature of the tectonic forces that affect the rocks during the folding process.

There are different types of folds created by compressional stress. This depends on the way the rocks bend as shown in Figure 5. The most common types of folds are anticlines and synclines.

Figure 5: A diagram showing how compressional stress leads to rock layers curving upward to form a syncline or curving downward to form an anticline.

An anticline is a type of fold created in a rock when it is subjected to compressional stress that causes the rocks to bend or curve downward as shown in Figure 6. The oldest layers are found in the fold’s core or center.

Key Term: Anticline

An anticline is a fold that is convex upward, and its oldest rocks are in its core.

An anticline in nature (Sot de Chera, Valencia, Spain)
Figure 6: An anticline in nature (Sot de Chera, Valencia, Spain).

A syncline fold is the opposite of an anticline fold. A syncline fold is created in a rock when it is subjected to compressional stress that causes the rock to bend or curve upward as shown in Figure 7. The youngest layer is found in the fold’s core.

Key Term: Syncline

A syncline is a fold that is concave upward, and its youngest rocks are in its core.

Syncline in nature (mudstone and sandstone cliff, Wales, United Kingdom)
Figure 7: Syncline in nature (mudstone and sandstone cliff, Wales, United Kingdom).

Example 3: Understanding How Folds Are Classified

Which of the following is used to classify a fold?

  1. The geological history of the rock before it was folded
  2. The position of the fold’s structural elements
  3. The volume of rock that has been folded
  4. The amount of the fold that is exposed in the field
  5. Where on Earth’s surface the fold has formed

Answer

Folds can be simple single folds or highly complex folds in morphology and geometry, but, in most cases, they are complex.

Folds can occur in any area and in any type of rock; they also can vary in size. Some folds are several kilometres in width and others measure just a few centimetres or less.

Geologists classify folds according to many aspects; one of them is the position of the structural elements of the fold (axis, axial plane, etc.).

For example, when a fold’s limbs dip away from the axial plane causing the fold to convex upward, it is classified as an anticline, and vice versa.

So, if we see our choices in the question, the correct answer is the position of the fold’s structural elements, that is, option B.

Example 4: Identifying Synclines and Anticlines in Nature

Which geological structure is shown in this picture?

A geological fold in sedimentary rock
  1. Anticlines
  2. Synclines
  3. Cross-bedding
  4. Ripple marks
  5. Both anticlines and synclines

Answer

Folds can be identified in nature as layers that are convex upward or concave upward.

An anticline is a fold where the rocks are bent or curved downward and the oldest rocks are found in the core of the bends. However, a syncline is the inverse of an anticline where the rocks have bent or curved upward and the youngest rocks are found in the core of the bends.

If we look at the picture, we will see both the synclines and anticlines connected. So, the correct option is E: both anticlines and synclines.

Folds are very important structures, as they can act as structural traps and reservoirs in petroleum exploration. Petroleum migrates and accumulates in the crest or the hinge of the fold. Because of the nature of folds, the majority of oil and gas accumulations occur in anticlinal traps as shown in Figure 8.

Synclines are also favorable locations for collecting and storing groundwater, which form aquifers or groundwater basins. Also, folds have economic importance in mineral exploration, as valuable minerals accumulate in the hinges of the fold.

Figure 8: Subsurface illustration for hydrocarbon traps in anticlines.

Folds can be used for reconstructing geological history and determining the chronology of rocks. We can use anticlines and synclines to determine the relative age of rock layers. This is because in anticlines the oldest rocks exist in the fold core and in synclines the youngest rocks exist in the fold core.

Example 5: Identifying the Chronological Importance of Folds

How is a syncline used to help determine geological chronology?

  1. The rocks in the core of the fold are the oldest.
  2. The rocks at the axial plane are older than those at the fold limbs.
  3. The rocks in the core of the fold are the youngest.
  4. The axial plane intersects the oldest rocks only.
  5. The axial plane intersects the youngest rocks only.

Answer

Synclines are a type of fold in which the youngest rocks are found in the fold’s core with its layers concave upward.

Being able to identify the relative age sequence of rocks by observing a syncline means it is very useful when determining geological chronology.

So, the correct answer is C: the rocks in the core of the fold are the youngest.

Let’s summarize what we have learned so far.

Key Points

  • Folds are wavelike structures formed by the bending of any type of rock due to compressional forces.
  • Folds can occur in any type of rock but appear quite clearly in sedimentary rocks.
  • The structural elements of the folds include the hinge points, axial plane, fold axis, and fold limbs.
  • Folds are classified according to their appearance in the field and the positions of their structural elements in nature.
  • The most common types of folds are anticlines and synclines.
  • Anticlines have layers that are convex upward with the oldest layers found in the fold’s core.
  • Synclines have layers that are concave upward with the youngest layers found in the fold core.
  • Folds are very important structures as they can act as traps for oil, natural gas, ore deposits, and groundwater.

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