What is 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.
FAQs on Strike in Geology
1. What exactly is a 'strike' in geology?
In simple terms, the strike is the direction of a horizontal line on the surface of a tilted rock layer. Imagine a tilted book half-submerged in water. The line where the water surface meets the book's cover is the strike line. It describes the orientation or direction of the rock bed.
2. What is the difference between strike and dip?
Strike and dip are two measurements used together to describe a tilted rock layer. Here’s the main difference:
- Strike is the direction of a horizontal line on the rock's surface (e.g., North-South).
- Dip is the angle of the tilt, measured downwards from the horizontal plane. It tells you how steeply the rock layer is inclined.
3. How do geologists measure the strike of a rock formation?
Geologists use a special tool called a Brunton compass or a compass-clinometer. To find the strike, they place the edge of the compass horizontally against the tilted rock surface. The direction the compass needle points to, relative to North, gives the strike measurement. This measurement is usually expressed as an angle from North (e.g., N45°E).
4. Why is it important to know the strike and dip of rocks?
Understanding strike and dip is crucial for many real-world applications in geology. It helps professionals to:
- Find Resources: Locate oil, natural gas, and groundwater which are often trapped in tilted rock layers.
- Ensure Safety: Assess the risk of landslides by understanding how rock layers are angled on a slope.
- Plan Construction: Design stable foundations for buildings, dams, and tunnels by knowing the underlying rock structure.
- Understand Earth's History: Reconstruct how mountains were formed and how tectonic plates have moved over time.
5. Can you give a simple real-world example of strike?
Certainly. Imagine you are walking along a path on the side of a large, grassy hill. If you walk on a path that stays at the exact same elevation without going uphill or downhill, you are walking along the strike line of that hill. The steepest direction straight down the hill from your path would represent the direction of the dip.
6. How is strike shown on a geological map?
On a geological map, strike and dip are represented by a special symbol. It looks like a 'T' shape. The long line of the 'T' shows the direction of the strike. The shorter, perpendicular line points in the direction of the dip. A number next to the symbol indicates the dip angle in degrees. This allows geologists to visualize the 3D rock structure from a 2D map.
7. What is the strike of a rock layer that is perfectly flat?
This is a great conceptual question. A perfectly flat or horizontal rock layer does not have a strike. Because it has no tilt, its dip angle is zero. Without a tilt, you cannot define a unique horizontal line on its surface, so the concept of strike doesn't apply. It is horizontal in all directions.
