Consider two intersecting, north-easterly striking and south-easterly dipping dikes Y1 and Y2, which are exposed on an east-west trending vertical wall of a granite (X) quarry as shown below.
The angle that the dikes make with the horizontal on the quarry wall is
Show Hint
When analyzing exposed rock faces or quarry walls, the angle observed from the vertical exposure is typically the apparent dip, not the true dip, unless the wall is perpendicular to the strike.
Step 1: Understanding the Terms
In geology, several terms describe the angles and orientations of rock layers, faults, and other geological features. The true dip refers to the steepest angle at which a geological feature inclines, measured perpendicular to the strike. The apparent dip, on the other hand, refers to the angle measured on a plane that is not perpendicular to the strike of the feature. This is typically the angle seen on the vertical exposure of a rock, like the quarry wall shown.
Step 2: Applying the Concept
In this case, the question asks about the angle that the dikes make with the horizontal as seen in the vertical wall of the quarry. Since the wall is vertical, the angle observed will be the apparent dip of the dikes, not the true dip.
Step 3: Comparing with Other Options True dip is the angle measured in the direction perpendicular to the strike, not as seen in the vertical quarry wall. Rake is the angle between a line (such as the line of intersection between two planes) and the horizontal plane, which does not apply in this case. Attitude of foliation refers to the orientation of planes of foliation in metamorphic rocks, not the angle of dikes.
