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Primary and secondary structures; strike and dip

Short answer

Primary structures form during rock formation (e.g., bedding); secondary result from later deformation (e.g., folds). Strike is the horizontal line on an inclined plane, always perpendicular to dip direction. To find strike from dip direction, add or subtract 90° to get the bearing, then convert to quadrant bearing.

How do you differentiate primary geological structures and secondary geological structures? Describe the relationship of strike and dip of geological planes with illustrations. How do you find out the strike line when the dip direction is measured?

Primary and secondary structures

PointPrimary structuresSecondary structures
FormationDuring the formation of the rockAfter the formation, due to later stresses
ExamplesBedding, ripple marks, mud cracks, cross bedding, vesicles, columnar jointsFold, fault, joint, cleavage, foliation, unconformity
RocksIgneous and sedimentaryAll rocks
Study givesEnvironment of deposition and way upDeformation history

Relationship of strike and dip

  • Strike is the direction (bearing) of a horizontal line on an inclined plane.
  • Dip is the maximum inclination of the plane, measured at right angles to the strike, in the vertical plane.
  • Hence, strike and dip direction are always 90° apart, and the dip direction is perpendicular to the strike line.
  • Strike is the contour line of the plane (all points on it are at the same level), and dip is the line of steepest descent.
  • Any other direction on the plane has a smaller inclination, called the apparent dip.
             N
             |      strike line (horizontal)
             |    /
             |  /
   W --------+--------- E
           / | \
         /   |   \
       /     |    v  dip direction
             S       (90 deg to strike)

   Plan view; the dip amount is the angle seen in a vertical section

The attitude is written as strike direction/dip amount and dip direction, for example N30°E/40°SE.

Strike from the measured dip direction

  1. Measure the dip direction (bearing of the line of steepest descent) with a compass.
  2. Strike = dip direction + 90° or dip direction - 90° (both give the same line, in opposite senses).
  3. Convert to a quadrant bearing.

Example: dip direction N60°W (azimuth 300°). Strike = 300° - 90° = 210°, which is S30°W; the same line read the other way is 300° + 90° - 360° = 30°, which is N30°E. The strike is N30°E (S30°W).

✓ Answer checked by Claude Opus 5.5 (high effort)

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