Question

Which of the following change(s) when a dipping bed with a plunging lineation is rotated about a vertical axis?

• A. Dip amount of bed
• B. Plunge amount of lineation
• C. Plunge direction of lineation
• D. Strike of bed

Hint:

When rotating geological structures, remember that rotations about a vertical axis affect the plunge direction, but not the amount of plunge or strike.

Answer 1

The Correct Option is C, D

To understand the impact of rotating a dipping bed with a plunging lineation about a vertical axis, it is important to comprehend the geological concepts involved.

A dipping bed is a geological layer that is inclined at an angle relative to the horizontal plane. The lineation is a linear feature within the rock, in this case, a plunging lineation means that this line is inclined downwards at an angle relative to the horizontal. The various parameters associated with these features are:

• Dip amount of bed: The angle at which the bed inclines relative to the horizontal surface.
• Plunge amount of lineation: The angle that the lineation makes with the horizontal plane.
• Plunge direction of lineation: The compass direction in which the lineation plunges.
• Strike of bed: The compass direction of a horizontal line on the plane of the bed.

When a rotation about a vertical axis is applied:

1. The strike of the bed changes because strike is defined as the direction of the line where the plane of the bed intersects a horizontal plane. Rotating the bed around a vertical axis will change the compass direction of this horizontal line, thereby altering the strike.

2. The plunge direction of the lineation also changes. Since rotation about a vertical axis modifies the direction in which the lineation inclines, the compass direction of the plunge is altered as a consequence.

However:

• The dip amount of the bed does not change because rotation about a vertical axis affects only the orientation (the strike) on the horizontal plane, not the angle of inclination relative to horizontal.

• The plunge amount of the lineation remains constant, as this parameter describes the angle of inclination, which is unaffected by a rotation of the horizontal reference plane.

Therefore, upon rotating a dipping bed with a plunging lineation about a vertical axis, the plunge direction of lineation and the strike of bed will change, while the dip amount of bed and plunge amount of lineation will not change.