Strike and dip

Strike and dip

Strike and dip refer to the orientation or "attitude" of a geologic feature. The "strike" of a stratum or planar feature is a line representing the intersection of that feature with the horizontal. On a geologic map this is represented with a short straight line segment oriented parallel to the compass direction of the strike. Strike is usually given as a compass bearing (N25°E for example) in terms of east or west of north, or as a single three digit number representing the azimuth, where the lower number is usually given (where the example of N25°E would simply be 025, and the other value of 205 is discarded). The "dip" gives the angle below the horizontal of a tilted stratum or feature. The symbol is a short line attached and at right angles to the strike symbol pointing in the direction of inclination. Typically the angle of dip is included on a geologic map.

Strike and dip are determined in the field with a compass and clinometer or combination known as a Brunton compass.

Another way of representing strike and dip is by dip and dip direction, where the latter is simply the azimuth of the dip. It can be obtained from strike by simply counting 90° around in the relevant direction. For example, for strata dipping 45º to the south and striking east-west, the strike and dip value would be 45ºS/090, which converts to a dip and dip direction reading of 45/180. Compass-clinometers which measure dip and dip direction in a single operation (as pictured) are often called "stratum" or "Klar" compasses after a German professor.

Any planar feature can be described by strike and dip. This includes sedimentary bedding, geologic faults and fractures, cuestas, igneous dikes and sills, metamorphic foliation and any other planar feature in the Earth. Linear features are measured with very similar methods, where "plunge" is the dip angle and "trend" is analogous to the dip direction value.

ee also

*Escarpment
*Dip slope
*Cuesta

References

* Lahee, Frederic H., 1961, "Field Geology", 6th ed., McGraw-Hill
* Tarbuck, Edward J. and Frederick K. Lutgens, 2002, "Earth: an Introduction to Physical Geology", 7th ed. Prentice Hall, ISBN 0-13-092025-8


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