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Strike and dip is a measurement convention used to describe the
orientation Orientation may refer to: Positioning in physical space * Map orientation, the relationship between directions on a map and compass directions * Orientation (housing), the position of a building with respect to the sun, a concept in building de ...
, or attitude, of a
planar Planar is an adjective meaning "relating to a plane (geometry)". Planar may also refer to: Science and technology * Planar (computer graphics), computer graphics pixel information from several bitplanes * Planar (transmission line technologies), ...
geologic feature. A feature's strike is the
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematicall ...
of an imagined horizontal line across the plane, and its dip is the angle of inclination measured downward from horizontal. They are used together to measure and document a structure's characteristics for study or for use on a
geologic map A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...
. A feature's orientation can also be represented by dip and dip direction, using the azimuth of the dip rather than the strike value. Linear features are similarly measured with trend and plunge, where "trend" is analogous to dip direction and "plunge" is the dip angle. Strike and dip are measured using a compass and a clinometer. A compass is used to measure the feature's strike by holding the compass horizontally against the feature. A clinometer measures the features dip by recording the inclination perpendicular to the strike. These can be done separately, or together using a tool such as a Brunton transit or a
Silva compass Silva Sweden AB is an outdoors products company, most known for their high-grade compasses and other navigational equipment including GPS, GPS tools, Map, mapping software, and altimeters for aircraft. They also offer a Marine (ocean), marine range ...
. Any planar feature can be described by strike and dip, including sedimentary bedding,
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
s, faults,
joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...
,
cuesta A cuesta (from Spanish ''cuesta'' "slope") is a hill or ridge with a gentle slope on one side, and a steep slope on the other. In geology the term is more specifically applied to a ridge where a harder sedimentary rock overlies a softer layer ...
s, igneous
dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...
and sills, metamorphic
foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...
and
fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
, etc. Observations about a structure's orientation can lead to inferences about certain parts of an area's history, such as movement, deformation, or tectonic activity.


Elements

When measuring or describing the attitude of an inclined feature, two quantities are needed. The angle the slope descends, or dip, and the direction of descent, which can be represented by strike or dip direction.


Dip

Dip is the inclination of a given feature, and is measured from the steepest angle of descent of a tilted bed or feature relative to a horizontal plane. True dip is always perpendicular to the strike. It is written as a number (between 0° and 90°) indicating the angle in degrees below horizontal. It can be accompanied with the rough direction of dip (N, SE, etc) to avoid ambiguity. The direction can sometimes be omitted, as long as the convention used (such as right-hand rule) is known. A feature that is completely flat will have the same dip value over the entire surface. The dip of a curved feature, such as an
anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
or
syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...
, will change at different points along the feature and be flat on any fold axis.


Strike

Strike is a representation of the orientation of a tilted feature. The strike line of a
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. The strike of the feature is the
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematicall ...
(compass direction) of the strike line. This can be represented by either a quadrant compass bearing (such as N25°E), or as a single three-digit number in terms of the angle from true north (for example, N25°E would simply become 025 or 025°). A feature's orientation can also be represented by its dip direction. Rather than the azimuth of a horizontal line on the plane, the azimuth of the steepest line on the plane is used. The direction of dip can be visualized as the direction water would flow if poured onto a plane.


Apparent dip

While true dip is measured perpendicular to the strike, apparent dip refers to an observed dip which is not perpendicular to the strike line. This can be seen in outcroppings or cross-sections which do not run parallel to the dip direction. Apparent dip is always shallower than the true dip. If the strike is known, the apparent dip or true dip can be calculated using trigonometry: \alpha = \arctan(\sin\beta \times \tan\delta)
\delta = \arctan(\tan\alpha \div \sin\beta) where δ is the true dip, α is the apparent dip, and β is the angle between the strike direction and the apparent dip direction, all in degrees.


Trend and plunge

The measurement of a linear feature's orientation is similar to strike and dip, though the terminology differs because "strike" and "dip" are reserved for planes. Linear features use ''trend'' and ''plunge'' instead. Plunge, or angle of plunge, is the inclination of the feature measured downward relative to horizontal. Trend is the feature's azimuth, measured in the direction of plunge. A horizontal line would have a plunge of 0°, and a vertical line would have a plunge of 90°. A linear feature which lies within a plane can also be measured by its
rake Rake may refer to: * Rake (stock character), a man habituated to immoral conduct * Rake (theatre), the artificial slope of a theatre stage Science and technology * Rake receiver, a radio receiver * Rake (geology), the angle between a feature on a ...
(or pitch). Unlike plunge, which is the feature's azimuth, the rake is the angle measured within the plane from the strike line.


Maps and cross-sections

On
geologic maps A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with str ...
, strike and dip can be represented by a T symbol with a number next to it. The longer line represents strike, and is in the same orientation as the strike angle. Dip is represented by the shorter line, which is perpendicular to the strike line in the downhill direction. The number gives the dip angle, in degrees, below horizontal, and often does not have the degree symbol. Vertical and horizontal features do are not marked with numbers, and instead use their own symbols. Beds dipping vertically have the dip line on both sides of the strike, and horizontal bedding is denoted by a cross within a circle. Interpretation of strike and dip is a part of creating a
cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Ab ...
of an area. Strike and dip information recorded on a map can be used to reconstruct various structures, determine the orientation of subsurface features, or detect the presence of
anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
or
syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...
folds.


Measurement


Conventions

There are a few conventions geologists use when measuring a feature's azimuth. When using the strike, two directions can be measured at 180° apart, at either clockwise or counterclockwise of north. One common convention is to use the "right-hand rule" (RHR) where the plane dips down towards the right when facing the strike direction, or that the dip direction should be 90° clockwise of the strike direction. However, in the UK, the right-hand rule has sometimes been specified so that the dip direction is instead counterclockwise from the strike. Some geologists prefer to use whichever strike direction is less than 180°. Others prefer to use the "dip-direction, dip" (DDD) convention instead of using the strike direction. Strike and dip are generally written as 'strike/dip' or 'dip direction,dip', with the degree symbol typically omitted. The general alphabetical dip direction (N, SE, etc) can be added to reduce ambiguity. For a feature with a dip of 45° and a dip direction of 75°, the strike and dip can be written as 345/45 NE, 165/45 NE, or 075,45. The compass quadrant direction for the strike can also be used in place of the azimuth, written as S15E or N15W.


Tools

Strike and dip are measured in the field using a
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
and with a
clinometer An inclinometer or clinometer is an instrument used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction. It is also known as a ''tilt indicator'', ''tilt sensor'', ''tilt meter'', ''slope ...
. A compass is used to measure the azimuth of the strike, and the clinometer measures inclination of the dip. Dr. E. Clar first described the modern compass-clinometer in 1954, and some continue to be referred to as Clar compasses. Compasses in use today include the Brunton compass and the
Silva compass Silva Sweden AB is an outdoors products company, most known for their high-grade compasses and other navigational equipment including GPS, GPS tools, Map, mapping software, and altimeters for aircraft. They also offer a Marine (ocean), marine range ...
.
Smartphone apps A mobile application or app is a computer program or software application designed to run on a mobile device such as a phone, tablet, or watch. Mobile applications often stand in contrast to desktop applications which are designed to run on des ...
which can make strike and dip measurements are also available, including apps such as
GeoTools GeoTools is a free software (LGPL) GIS toolkit for developing standards compliant solutions. It provides an implementation of Open Geospatial Consortium (OGC) specifications as they are developed. GeoTools is a contributor to the GeoAPI project ...
. These apps can make use of the phone's internal
accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is accele ...
to provide orientation measurements. Combined with the
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
functionality of such devices, this allows readings to be recorded and later downloaded onto a map. When studying subsurface features, a dipmeter can be used. A dipmeter is a tool that is lowered into a
borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petro ...
, and has arms radially attached which can detect the microresistivity of the rock. By recording the times at which the rock's properties change across each of the sensors, the strike and dip of subsurface features can be worked out.


References


Further reading

* {{Structural geology, state=collapsed Structural geology