What is Plunge, Trend and Pitch of the Lineation ?

What is Lineation?

Before knowing about the lineation, let us know about the fabric. Fabric is built of minerals and mineral assemblage/aggregates with a preferred orientation that penetrates the rock at the microscopic to centimetre spacing scale. Fabric can be divided into three types, which are a) Planar Fabric, b) Linear fabric, and c) Random fabric.

Planar Fabric: 

A planar fabric is characterized as two-dimensional features like tabular minerals or platy minerals. e.g. foliation, cleavage, bedding, etc.

Linear Fabric: 

A linear fabric is characterized by elongate elements with preferred orientation. e.g. Lineation. Lineation is a type of a linear fabric that is best represented by a line. eg. Stretching, Slicken lines, Striations, Mineral lineation, Fold Axis, Crenulation lineation, etc.

Random Fabric: 

where its elements show no preferred orientation.

How to measure the linear fabric in the field?

The linear fabric or linear structures (e.g. lineation types, slicken lines, etc.) are measured by three geometrical elements, which are plunge, trend, and pitch/rake.

What is a Plunge?

The plunge is the angle of inclination of a linear structure with respect to the horizontal line. The plunge angle is measured along the vertical plane. During the measurement of the plunge, the compass must be placed perpendicularly over the linear structure (fig.).

What is the Trend?

The trend is the orientation of a line (this line is the impression of the vertical plane on the horizontal plane)  with respect to the true geographic north. The trend is always measured on the horizontal plane. So, the compass must be in a horizontal position while measuring the trend (fig.).

What is the Pitch/Rake?

The pitch/rake is an acute angle of the linear structure which is measured on the inclined plane with respect to the strike. This is carried out with the help of a protector (fig.).


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