Strike in Geology

Strike in geology is a reference to the attitude of a geologic feature such as a fault, bed or any other planar feature. The line of a bed (for example bedding of sedimentary rocks), fault (for example oblique-slip fault), or other planar feature that intersects with a horizontal plane is known as a strike line and the entire concept is known as a strike. Thus, strike definition geology or strike geology helps in understanding the attitude or the position of the linear structural characteristics of the above mentioned geologic features. Strike geology is also used in maps to depict the orientation of a particular feature.

Strike and Dip

Strike and dip are two references that are important for defining the attitude of any geologic feature. The strike and dip geology define the position of the structure with respect to a horizontal plane and both together provide a more accurate structural description rather than only strike geology. Strike and Dip are explained below along with their possible representations:

• Strike: The strike is the direction of the line representing the intersection between a fault, bed or any planar feature and a horizontal plane. On a geologic map, the strike line is given by the short straight line segment that is oriented parallel to the strike line. The strike is given as either a quadrant compass that bearing the strike line, for example, N25°E, and in terms of east or west of true or north (for example N25°E is given as 025), a single three-digit number representing the azimuth in which usually the lower number is given usually or the azimuth number is followed by the degree sign, for example, N25°E will be 025°.

• Dip: Dip is the steepest angle of descent of a tilted bed or a feature that is relative to the horizontal plane, and is given by the number of degrees (0° - 90°) and a letter (N, S, E, W) with rough direction in which the bed dips downward. Another technique is to take the strike which makes dip 90° to the right of the strike. When this method is used for representation the redundant letter that follows the dip angle is omitted according to the right-hand rule geology explains. In the map, the symbol is a short line that is attached and is at a right angle to the strike symbol pointing in the direction where the planar surface is dipping down.

The strike line and the dip of a plane that describes the attitude relative to a horizontal plane and a vertical plane perpendicular to the strike line are shown below in the given image:

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Of the two different approaches to represent strike and dip geology individually, is by dip and dip direction geology concept. It is known that the dip direction is the azimuth of the direction of the dip which is projected to the horizontal by 90° off the strike angle. An example of the dip and dip direction method is given by considering the case in which a bed that dips at an angle of 30° to the South. In this scenario, the feature will have an East-West strike and using the strike and dip direction it will be written as 090°/30° S. But according to the dip and dip direction method it will be written as 30/180.

Measurement of Strike and Dip Geology

The measurement of the strike and dip geology are determined by the field with a compass and clinometer or a combination of the two. An example of the combination is the Brunton Compass which is named after a Colorado miner, D. W. Brunton. To measure the dip and dip direction specialised compass-clinometers are used that perform the measurement in a single operation and are often these instruments are known as “stratum” or “Klar” compasses. 

Currently, strike and dip geology can also be measured using smartphones. There are several apps that are available and that make use of the internal accelerometer to provide the strike dip orientation measurements. Combined with the GPS function, such devices allow the strike dip readings that are recorded and later on can be downloaded onto a map. 

Uses of Dip and Strike Geology Concept

Any of the planar features mentioned above can be described with the use of strike and dip. Other such features that can also be described according to the strike and dip definition include sedimentary rock beddings, faults (including all types of faults such as an oblique-slip fault or normal dip slip fault or strike slip fault) and fractures, cuestas (a hill or ridge with a gentle slope), igneous dikes and sills, metamorphic foliations, etc. The strike definition geology describes and the dip direction method is applied usually to the horizontal features. The linear features are described by the “plunge” and “trend” concept, in which the “plunge” is the dip angle and the “trend” is analogous to the dip direction value. 

The apparent dip in the name of any of the dip measured in a vertical plane is not perpendicular to the strike line. For calculating the true dip from the apparent dip concepts of trigonometry can be used if the strike is known. An example where apparent dips are used is in the measurement or describe the orientation of geologic cross-sections when they are drawn at an angle that is not perpendicular to the strike.