What is Strike and Dip ? | It’s important features and measurements

Contents: Strike and Dip

Strike and Dip

Strike and dip definition | difference between dip and strike

In geology strike refers to the direction of the line formed by the intersection of geological (mostly rock) surface with an imaginary horizontal plane passing through the surface.

Dip direction | Convert strike and dip to dip and dip direction

While DIP is the acute angle that a geological surface (mostly rock) makes with the horizontal plane. In order to find dip direction, one can find strike line and drop a perpendicular line on the strike line in the direction of descent, so that the angle between strike and dip direction is always 90.

Example of Strike and dip | Geology dip and strike problems :

Suppose a bed is dipping 30 degree to south would have east-west strike and written as 090/30 S using strike and dip. Other way of finding dip is to pour the water on the planar surface and see the direction of descent, that direction is of maximum descent, hence DIP. For strike

Strike and Dip
Strike and Dip
Image credit :Brews ohareStrikeLine&DipCC BY-SA 3.0

Why Strike and dip is important?

Importance of Strike and dip:

As geology is the study of present geological feature and linking it with the past to understand how earth has evolved as a planet. As earth undergoes tectonic activity over millions of years, this leads to the development of various topographic features. Out of which there exist some planar features in the form of sedimentary beds, faults and fractures, cuestas, igneous dikes or metamorphic facies etc. These planar features have lot to tell once we know their attitude and orientation. Strike and dip are the physical quantity that helps describing them. 

How to measure strike and dip with a compass? | How to measure strike and dip with a protractor

Measurement of dip and strike:

In the field strike and dip are measured with compass and clinometer or by the combinations of these two. One such example is Brunton-compass. 

Measurement of dip and strike using Clinometer | Dip direction

First of all the compass clinometer has to be in clinometer mode. It can be done by setting the compass clinometer in east-west direction by turning the bezel. This leads to east ( 90 ) and west ( 270 ) point along the mark.

Hold the clinometer vertically and move the clinometer until it shows zero reading. Zero reading is given by the free hand of the clinometer. This direction is the strike direction. Perpendicular to the strike direction is the dip direction.

Measuring the dip amount | Dip angle

Dip is the acute angle between the horizontal plane and dip direction. In order to measure the dip amount, again the bezel is turned to keep it in clinometer mode. Then the free compass ( moves under the effect of gravity ) shows the amount of dip.

Strike and Dip Symbols:

How do you find dip and strike from a geological map ? |How do you find the true dip ?

Calculating strike and dip from contour map:

Contour map consists of contour lines that joins points of same elevation (height) from a given common level such as mean sea level. This is called topogrpahy map. However, it can be interpreted for other physical quantity also.  In geology, contour map helps measure strike and dip.

Generally, strike line is a line connecting two or more point on a gelogical surface having same elevation. They mimic the contour lines on geological map. The physical essence of strike line is of vital importance. Parallel strike lines represents a planar surface. As for a planar surface line of same elevation will never cross each other. 

However, dip is found by V rule. V-shaped trace is formed on the map when a dipping surface intersects a valley. A trace is the intersection of geological surface with topography. For a vertically dipping surface trace is always straight on the map regardless of topography. 

In the contour map, the direction of V points depends on both the direction of the dip of the beds and also on the steepness of dip. However, exception occur when slop of valley is in same direction of dip and the dip amount is lesser that that of valley slope. When dip direction of the beds is parallel to slope direction of the valley floor then symmetric V is formed on the map. However, this is an unusual phenomenon geologically.

Image credit : en:User:MapXpertCntr-map-1, marked as public domain, more details on Wikimedia Commons

In the above figure, contour interval is 20 ft. Concentric lines are making strike lines. One with the highest value lies in the center.

Classification of strata based on V rule :

  • vertical srtata: no V
  • horizontal strata : V parallel to contours
  • strata dip upstrream : V points upstream
  • strata dip downstream : V points downstream
  • strata dip downstream shallower than slope of valley floor : V points upstream and is very narrow.

How to measure dip and strike using Brunton compass ?

  • Place the sides of the compass against the plane of the outcrop.
  • Adjust the compass orientation so that its bottom edge remains flat with respect to the plane, in addition air bubble in the Bull’s eye level is centered.
  • Read both ends of the compass needle to get the strike direction value.
  • After determining the strike rotate the compass by 90 degree in the downhill direction.
  • Place the Brunton compass flat on the plane and centre the air bubble. The compass scale directly gives the dip direction. Make sure dip direction is 90 degree with respect to strike.

How to measure strike and dip with a silva compass?

The principle remains the same as Brunton compass.

Image credit : “Compass confusion” by henrikjon is licensed under CC BY 2.0

Strike dip fault :

Geological fault involves the movement of rocks with respect to each other ( in the form of rock blocks) that undergo fracture. The moving blocks are called foot walls and hanging walls.

Strike Slip

A fault is said to be strike-slip when blocks move along strike direction with respect to each other.

Dip Slip

A fault is said to be dip-slip when blocks move along dip direction with respect to each other.

Geologic maps definition | Geological cross section |Topographic cross section |Cross section map

this refers to the graphical representation of geological bodies in the subsurface which are sequentially oriented. It is shown by intersection of geological bodies in the subsurface with a vertical plane. It is an indispensable component of geological map that shows the approximate model of real distribution of rocks with depth and is based on the information available on the surface. It also helps identify the eroded material above the topography.

From economic or social point of view, geological are very important. They form the basis for planning engineering works and gigantic construction.

How to draw strike and dip on a map?

Construction of geological cross-section map involves the interpretation of rock distribution in the subsurface or above the surface. The avaiable data is limited leading to its interpolation with the objective of building relaible and coherent model of gelogical rocks.

The Geological data is available in two forms:

  1. First one is obtained directly from the field, that involves knowing the orientation and attitude of the outcrops available. It is done by measuring strike, dip amount and dip direction of the outcrop. It also involves finding the thickness of stratigraphical units, lateral relationship between them and the type of contact between the beds. Features such as unconformity, folds, fractures etc. are identified.
  • Otherwise, data is inherited from the existing geological map that includes geological formation, type of rock, angle of intersection with topographic surface.

Although it is bit easier to get the data on the topography rather than finding the data in the subsurface. Advances has been done to find the subsurface data. Drilling cores provide an efficient information about the rock succession in depth.

However, this tedious and costly method of knowing the subsurface. Another method is geophysical imaging, wherein different geophysical methods measure the different physical properties of the rocks in the subsurface by indirect method.  Physical properties are the direct indicator of the type of rocks to some extent. Also imaging leads to the thickness and orientation of rock beds.

Cross section of the Earth :

Like geological cross-section for a local region, cross-section of entire earth has been evaluated. It srarts from the surface of the earth to the centre of the earth.  It gives the visualization of composition of the earth depth wise in terms of geology. The planet earth is severly complicated and a dynamic system. Going down in the earth leads to two major divisions:

  1. based on mechnaical properties: lithopshere, asthenosphere, mantle and core.
  2. based on chemical properties: the uppermost part is crust followed by upper mantle, lower mantle, outer core and inner core.
Cross section of earths

Original horizontality definition | law of original horizontality example

The principle of original horizontal states that the layers of sediment deposits must have begun in a horizontal fashion leading to the formation of strata. This must have happened under the effect of gravity. It turned to be an effective method of dating. The succession of beds lying horizontally must have chronological relation. The bottom being the oldest and top the newest. However, because of the tectonic forces and other geological movements beds are not horizontal. So its universality is not valid. Mostly, we find tilted beds

Anticline and Syncline :

These are up and down folds in the form of crest and trough. They are usually together and is caused by compressive stress. However, because of erosion it is not explicitly seen everywhere. So the geologists use pattern of rocks on the surface to identify anticline and syncline.

Example of Anticline and Syncline

Image credit : “Anticlines and Synclines” by U.S. Geological Survey is licensed under CC BY 2.0

Symbols of folds on a geological map

In anticline the oldest rock lies in the centre while in syncline the youngest rock lies in the centre.

Image credit : Anticline and Syncline By Original image by Pearson Scott Foresman, Converted to SVG by Offnfopt – Derivative work of File:Anticline (PSF).png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=40152960
Image credit : Pearson Scott ForesmanAntecline (PSF), marked as public domain, more details on Wikimedia Commons

Thus anticlines and synclines, are the kinds of geological folds. They carries a lot of information as core of the anticlines are suppossed to be the  host of hydrcarbons ( oil and gas ) which is economically important.

Anticline and Syncline symbol on a geological map | Map Symbol :

Structural map:

Geological structures such as such as folds, faults, fractures, synclines , anticlines, lineaments etc.  are important to identify to understand how past movements have shaped the present terrain. Structural mapping involves the the identification of these features. Also it helps in knowing the oil and gas reserves by locating the point that favors its formation. Structures are used for examining potential hazards such as Earthquakes, landslides and volcanic activity etc.

How does a cross bed form?

Cross beds are layering within the main bed that makes some angle with the main bedding plane. They are also known as cross-stratification. It is indicative of some activities that occurred after the formation of bedding plane. Usually it forms during the deposition on the inclined surfaces of the bedforms such as ripples and dunes. It tells that the depositional environent contained a flowing medium such as aeolian ( air ) or fluvial (water).

Cross – bedding
Image credit : By Dr. Igor Smolyar, NOAA/NESDIS/NODC. – NOAA Photo Library: amer0038, Public Domain, https://commons.wikimedia.org/w/index.php?curid=17954251

Strike and dip strata | Vertical strata:

in general stratification is horizontal. More often, it becomes tilted because of the tectonic forces acting on the bed. However, vertical strata is the extreme case of tectonic activity where horizontal strata get transfomed till it becomes vertical. However, this is rare and is often not seen. When the surrounding rocks are eroded then it becomes visible.

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