Lecture 2: Orientation of lines and planes

Orientation of Lines and Planes

Geological structures can be described using two basic geometric elements: lines and planes. Curved lines and surfaces can be approximated as collections of straight lines or planes. For example, a fold can be represented by a set of planes.

Orientation of planes

A plane’s orientation is described by its strike and dip:

• Strike is the compass azimuth of a horizontal line on the plane, measured from 0° to 360°:
  • 0°/360° = north
  • 90° = east
  • 180° = south
  • 270° = west
• Dip is the angle between the plane and the horizontal, measured in a vertical section perpendicular to the strike and in the direction of maximum inclination. It ranges from 0° for a horizontal plane to 90° for a vertical plane.

Measuring a plane in a vertical section that is not perpendicular to its strike gives an apparent dip, which is less than or equal to the true dip:

• A section perpendicular to the strike shows the true dip.
• A section parallel to the strike shows no dip; the plane appears horizontal.
• The apparent dip depends on the angle between the section and the strike.

To measure a plane with a compass, identify a horizontal line on the plane for the strike and the line of maximum inclination for the dip.

Reporting strike and dip

Strike alone is ambiguous because either end of the horizontal line can be used. The dip direction must therefore also be specified. The lecture focuses on the right-hand rule convention:

• Imagine placing the palm of your right hand on the plane.
• The thumb indicates the strike direction.
• The fingers point down dip.

Under this convention, the dip direction is 90° to the right of the strike. Two planes can have similar strike orientations but dip in opposite directions, so recording dip direction is important.

Measurement errors

Compass measurements can be imprecise. On a gently dipping plane, even a small error in identifying a horizontal line can produce a large error in strike. For example, a 2° error in identifying the horizontal on a plane dipping 5° can result in a 24° strike error.

Orientation of lines

A line’s orientation can be described by its trend and plunge:

• Trend is the azimuth of the line’s horizontal projection, measured from north.
• Plunge is the angle the line makes with its horizontal projection, measured in a vertical plane containing the line. It ranges from −90° to 90°:
  • −90° points upward.
  • 0° is horizontal.
  • 90° points downward.

With a compass, trend is found by determining the orientation of a vertical plane parallel to the line. Plunge is measured by placing the compass edge along the line and using the clinometer. Errors can occur, especially if the reference plane is not truly vertical.

Rake or pitch

For a line lying on a plane, rake (or pitch) can be used instead of trend and plunge. Rake is the angle between the line and the plane’s strike line, measured within the plane. It ranges from 0° to 180° and can be measured with a protractor placed on the plane.

When reporting rake, specify which end of the strike line the angle is measured from. For example, the striations on a fault plane may be described as having a rake of 130° east. Rake or pitch is distinct from plunge.

Pole to a plane

The pole to a plane is the downward-pointing normal to that plane. When strike and dip are given using the right-hand rule:

• Pole trend = strike − 90°.
• Pole plunge = 90° − dip.

The pole is a line that fully describes the plane’s orientation.

Speakers and sources

• Speaker: One lecturer or narrator; no individual name is identified in the subtitles.
• Visual sources: Diagrams and compass illustrations used to explain measurement and orientation.
• Referenced source: A structural geology textbook, identified in the auto-generated subtitles as “Reagan structural geology”; the name may have been transcribed incorrectly.

The lecture concludes by recommending relevant sections in a structural geology textbook and practice questions to reinforce the concepts.