Model Answer
0 min readIntroduction
In structural geology, understanding the orientation of planar features like rock beds is crucial for interpreting geological history and predicting subsurface conditions. The 'dip' is a fundamental parameter describing this orientation. It represents the maximum angle of inclination of a plane from the horizontal. Accurate determination of dip, and distinguishing between true and apparent dip, is essential for geological mapping and resource exploration. This answer will define dip, explain the difference between true and apparent dip, and calculate the strike direction for a given dip.
Definition of Dip
The dip of a rock bed (or any planar geological surface) is the angle between the horizontal plane and the inclined plane, measured downwards from the horizontal. It is always expressed as a number between 0° and 90°. A 0° dip indicates a horizontal plane, while a 90° dip indicates a vertical plane.
True Dip vs. Apparent Dip
The true dip is the angle of inclination measured perpendicular to the strike line. It represents the maximum inclination of the bed. However, when a dipping plane is viewed along a direction other than perpendicular to the strike, the observed angle of inclination is called the apparent dip. The apparent dip is always less than or equal to the true dip. Multiple apparent dips can be measured from different directions, but there is only one true dip for a given plane.
The relationship between true dip (θ), apparent dip (θ’) and the angle (α) between the strike line and the direction of observation is given by: sin θ’ = sin θ * cos α
Calculating Strike Direction
The strike of a bed is the direction of a horizontal line on the dipping plane. It is expressed as a compass bearing (e.g., N30°E). If a bed dips 30° towards North 30° East, the strike direction is perpendicular to the dip direction.
To find the strike direction, we need to add 90° to the dip direction. Therefore:
- Dip direction: N30°E
- Adding 90°: N30°E + 90° = N120°E
Thus, the strike direction of the bed is N120°E. Alternatively, subtracting 90° from the dip direction also yields a valid strike direction (N30°E - 90° = S60°E). Both N120°E and S60°E represent the same strike line, just expressed in opposite directions.
It's important to note that strike and dip are always reported together to fully define the orientation of a planar geological feature.
Conclusion
In conclusion, understanding dip, differentiating between true and apparent dip, and accurately determining strike direction are fundamental skills in structural geology. These parameters are essential for interpreting geological structures, creating geological maps, and assessing subsurface geological conditions. The ability to calculate strike direction from dip information demonstrates a grasp of the geometric principles governing planar geological features.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.