Strike and Dip
Geologists use a special symbol calledand to represent inclined . and map symbols look like the capital letter T, with a short trunk and extra-wide top line. The short trunk represents the and the top line represents the . is the angle that a or layer plunges into the Earth from the horizontal. A number next to the symbol represents angle.
One way to visualize the Azimuth. In Figure 2, the strike is measured as either 37 degrees, or 37 + 180 = 217 degrees. Convention is to use the smallest angle (in this case, 37 degrees), but either is correct.is to think about a line made by standing water on the inclined layer. That line is horizontal and lies on a compass direction that has some angle with respect to true north (see Figure 3). The is indicated by azimuth (orientation) on the map – see
Theof the inclined layer represents the direction the layer or bed is tilting into the Earth . The direction of would be the direction a ball would roll if set on the layer and released. In Figure 2, the layers are dipping to the SE. In this case, the dip is labelled “27 degrees,” so we know it is tilted exactly 27 degrees into the Earth. Sometimes dip angles are not labeled, but we can still determine the dip direction (but not the magnitude) from the dip symbol.
A horizontal rockhas a of 0° and a vertical has a of 90°.
Figure 3: Strike and Dip 3D model (click on link):
Rules of Strike and Dip
- Strike is always parallel to the bedding direction at that location.
- The dip is always drawn perpendicular to strike in map view. It may be drawn at an angle to show perspective in a 3D block diagram.
- The dip CAN be labelled with the dip angle, or only the direction can be indicated. The dip always shows which way layers are tilting into the Earth.
- Special symbols are used for horizontal beds and vertical beds.
Tilted beds or layers occur when plate tectonic forces cause horizontal layers to be pushed up or dropped down unevenly. This results in a tilting or incline of the original horizontal beds.
Figure 5. A 3D virtual outcrop of tilted beds (click on link): https://sketchfab.com/3d-models/vom-2-65d74feafc1f4e99a87d0c9d17070fa5
Here are some examples of tilted beds:
3D interactive model of Figure 6: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYqcLLnAEM
3D interactive model of Figure 7: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYyeKJpAEM
3D interactive model of Figure 8: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYqezDmgEM
3D interactive model of Figure 9: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYyaOkogEM
3D interactive model of Figure 10: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwY-cORowEM
3D interactive model of Figure 11: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYibyKoQEM
Some questions, like Questions 5-6, are intentionally left as non-interactive.
Question 5: Which strike and dip symbol is correct in Figure 11?
3D interactive model of Figure 12: http://app.visiblegeology.com/model.html#ahFzfnZpc2libGUtZ2VvbG9neXIPCxIFTW9kZWwYmp7DmgEM
Question 6: Which strike and dip symbol is correct in Figure 12?
Text modified from http://opengeology.org/textbook/9-crustal-deformation-and-earthquakes/ CC-BY-SA.