|
|
|
|
Strike and dip
|
| |
|
| |
Strike and dip refer to the orientation or attitude of a geologic feature. The strike of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map this is represented with a short straight line segment oriented parallel to the compass direction of the strike. Strike can be given as either a quadrant compass bearing (N25°E for example) in terms of east or west of north or south, 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 335 is discarded), or the azimuth number followed by the degree sign (example of N25°E would be 25° or 335°).
Discussion
Ask a question about 'Strike and dip' Start a new discussion about 'Strike and dip' Answer questions from other users |
Encyclopedia
Strike and dip refer to the orientation or attitude of a geologic feature. The strike of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map this is represented with a short straight line segment oriented parallel to the compass direction of the strike. Strike can be given as either a quadrant compass bearing (N25°E for example) in terms of east or west of north or south, 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 335 is discarded), or the azimuth number followed by the degree sign (example of N25°E would be 25° or 335°). The dip gives the angle below the horizontal of a tilted bed or feature, and is given by the number (0°-90°) as well as a letter (N,S,E,W) with rough direction in which the bed is dipping. One technique is to always take the strike so the dip is 90° to the right of the strike, in which case the redundant letter following the dip angle is omitted. The map symbol is a short line attached and at right angles to the strike symbol pointing in the direction which the planer surface is dipping down. The angle of dip is generally included on a geologic map without the degree sign. Beds that are dipping vertically are shown with the dip symbol on both sides of the strike, and beds that are flat are shown like the vertical beds, but with a circle around them. Both vertical and flat beds do not have a number written with them.
Another way of representing strike and dip is by dip and dip direction. The dip direction is the azimuth of the direction the dip as projected to the horizontal (like the trend of a linear feature in trend and plunge measurements), which is 90º off of the strike angle. For example, a bed dipping 30º to the South, would have a East-West strike (and would be written 90º/30ºS using strike and dip), but would be written as 45/180 using the dip and dip direction method.
Strike and dip are determined in the field with a compass and clinometer or a combination of the two, such as a Brunton compass named after D.W. Brunton a Colorado miner. 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, 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.
Apparent dip is the name of a any dip which is not perpendicular to strike. True dip can be calculated from apparent dip using trigonometry if you know the strike. Geologic cross sections use apparent dip when they are drawn at some angle not perpendicular to strike.
|
| |
|
|
