Geological Folds: Definition, Causes & Types

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  • 0:07 What are Folds?
  • 0:43 Types of Folds
  • 2:21 Classification of Folds
  • 3:40 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Under the right conditions and over millions of years, rock can be deformed so that it is folded into a different shape. In this video lesson, you will identify different types of folds as well as what causes them to occur in rock.

What Are Folds?

We often think of rock as hard, brittle material. Throw a rock hard enough on the ground, and it will likely break into pieces. But under the right conditions, rock can actually 'flow' in a way that keeps it from breaking. When rock deforms in such a way that it bends instead of breaking, we call this a fold. Folds come from pressure on rocks that occur over very long periods of time (think millions of years), so instead of a quick action like throwing the rock on the ground, it's more like standing on it for millions of years until the pressure is finally enough that it changes shape.

Types of Folds

Folds are created in rock when they experience compressional stress. This is when the rock is being pushed inward from both sides. This is like if you put a spring between your hands and push them together. As you push, you're compressing the spring, and rock can be compressed in the same way over long periods of time. There are different types of folds created by compressional stress depending on which way the rock bends.

An anticline is a fold that arches up as both sides of the rock are pushed inward. You can remember that the anticline creates this type of fold because the arch looks like an 'A' (for anticline).

A syncline is a fold that sinks down as both sides of the rock are pushed inward. You can remember that a syncline creates this type of fold because the fold 'sinks' downward, which sounds like 'syncline.'

A monocline is a fold where the rock layers form an S-shape as the sides of the rock are compressed. You can remember this type of fold because all the layers of rock are still horizontal, going in one direction instead of bending vertically upward or downward like anticlines and synclines. And since 'mono' means 'one,' monoclines are layers in only 'one direction.'

We also have domes, which are like anticlines but instead of an arch, the fold is in a dome shape, like an inverted bowl. Similarly, there are also basins, which are like synclines but again, instead of a sinking arch, the fold is in the shape of a bowl sinking down into the ground.

Classification of Folds

Not only do folds come in different shapes, but they also form at different angles. Symmetrical folds are folds with the same angle. This makes sense since symmetry means the same on both sides. We can also have the reverse of symmetrical, which occurs in asymmetrical folds, or folds with different angles.

Isoclinal folds are similar to symmetrical folds, but these folds both have the same angle and are parallel to each other. 'Iso' means 'the same' (symmetrical), and 'cline' means 'angle,' so this name literally means 'same angle.' So, isoclinal folds are both symmetrical and aligned in a parallel fashion.

Overturned folds occur when the folding is so intense that the fold appears to have turned over on itself. Similarly, we can have recumbent folds, which are even more extreme than overturned folds. These are folds that are nearly horizontal. 'Recumbent' means 'lying down,' so you could think of this fold as lying down sideways.

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