Deciphering True Dip from Apparent Dip: Understanding Dip Determination in Structural Geology

In structural geology, the concept of dip is used to describe the angle between a geological feature and a horizontal plane. Dip is an important parameter used to understand the structure, orientation, and deformation history of geological structures. However, dip can be estimated in two different ways – true dip and apparent dip, which can cause confusion for many geologists. In this article we will discuss the differences between true dip and apparent dip and how they are determined in relation to each other.

The difference between true dip and apparent dip

True dip is the angle between a geological structure and a horizontal plane, measured in the direction perpendicular to the strike of the structure. True dip is the actual angle of inclination of a geological feature with respect to the horizontal plane. Apparent dip is the angle of inclination of a geological feature as it appears on a tilted surface, such as a geological map or section. Apparent dip is always less than true dip, except when the geological feature is vertical.
The difference between true dip and apparent dip can be explained with a simple analogy. Imagine a book on a table. The pages of the book represent the geological feature, and the table represents the horizontal plane. If you look at the book from the side, the angle between the pages and the table is the true dip. However, if you look at the book from an angle, the angle between the pages and your line of sight is the apparent dip.

Determining True Dip and Apparent Dip

Determining true dip and apparent dip depends on the orientation of the geological feature and the line of section. True dip can be accurately determined if the strike and dip of the feature are known and the section line is perpendicular to the strike. In this case, the angle between the feature and the horizontal plane can be calculated using trigonometry.

In many cases, however, the strike and dip of the geologic feature are not known and the section line is not perpendicular to the strike. In such cases, the apparent dip is estimated using the following formula

Apparent Dip = True Dip x cos (angle between section line and strike)

This formula takes into account the orientation of the section line with respect to the strike of the geological feature and allows the apparent dip to be estimated.

Conclusion

In summary, dip is an important parameter in structural geology that is used to understand the orientation, structure, and deformation history of geological features. Dip can be estimated in two ways – true dip and apparent dip. True dip is the actual angle of inclination of a geological feature with respect to the horizontal plane, while apparent dip is the angle of inclination of a geological feature as it appears on a sloping surface. The determination of true dip and apparent dip depends on the orientation of the geological feature and the line of section, and can be estimated using trigonometry. Understanding the differences between true dip and apparent dip is critical to the accurate interpretation of geologic structures.

FAQs

1. What is true dip in geology?

True dip is the angle between a geological structure and a horizontal plane, measured in the direction perpendicular to the strike of the structure. It is the actual angle of inclination of a geological feature with respect to the horizontal plane.

2. What is apparent dip in geology?

Apparent dip is the angle of inclination of a geological feature as it appears on a sloping surface, such as a geological map or a cross-sectional view. It is always less than the true dip, except when the geological feature is vertical.

3. How is true dip determined in relation to apparent dip?

True dip can be determined accurately if the strike and dip of the feature are known, and the line of section is perpendicular to the strike. If this is not the case, apparent dip can be estimated using the formula: Apparent Dip = True Dip x cos (angle between line of section and strike).

4. Why is understanding the difference between true dip and apparent dip important in geology?

Understanding the difference between true dip and apparent dip is important in geology because it allows for accurate interpretation of geological structures. Misinterpretation of the dip angle can lead to incorrect conclusions about the structure and deformation history of geological features.

5. What is the relationship between true dip and apparent dip?

Apparent dip is always less than true dip, except when the geological feature is vertical. The difference between true dip and apparent dip depends on the orientation of the geological feature and the line of section.

6. How can true dip and apparent dip be used to understand geological structures?

True dip and apparent dip can be used to understand the orientation, structure, and deformation history of geological features. By analyzing the dip angle and the relationship between true dip and apparent dip, geologists can infer the direction and magnitude of forces that have acted on the rocks, as well as the history of deformation and structural evolution of the rock mass.

7. Can apparent dip be greater than true dip?

No, apparent dip is always less than true dip, except when the geological feature is vertical. This is because apparent dip is the angle of inclination of the feature as it appears on a sloping surface, while true dip is the actual angle of inclination of the feature with respect to the horizontal plane.