15.4: How Do Geologists Measure Geologic Structures?
How Do Geologists Measure Geologic Structures?
Each of the boundaries between rock units represents a geologic contact Links to an external site., which is simply the surface between two different rock units. Rock layers are often at an angle, not horizontal, indicating that changes have occurred since deposition. In order to measure and describe layers, geologists apply the concepts of strike and dip.
Strike & Dip
Strike Links to an external site. refers to the line formed by the intersection of a horizontal plane and an inclined surface. To determine strike, find where the dipping layer intersects the horizontal surface and draw a line parallel to this line of intersection on the top of the block (i.e. our horizontal surface). In Figure 15.4, look at the tilted sedimentary layers. Strike is the line on the horizontal plane created when the dipping green layer intersects the Earth’s surface. To better display the horizontal surface, it has been represented by water.
Figure 15.4: A depiction of the strike and dip of some tilted sedimentary beds. The dipping beds are shown partially covered with water so that you can visualize a horizontal line on the rock surface. The notation for expressing strike and dip on a map is also shown. (CC-BY 4.0, Steven Earle Links to an external site.)
Dip Links to an external site. is the angle between that horizontal plane and the tilted surface (Figure 15.4). To determine dip, pretend that there is a drop of water between one bed and the next (for example, along the intersection of the green bed and the orange bed). In which direction would the water roll if it followed that contact? That is the direction of dip, towards the left in this case.
Figure 15.5: Right, a strike and dip symbol on tilted sedimentary beds. Left, the strike and dip symbol from the diagram on at compass rose to illustrate directional and use of the symbol. (CC-BY 4.0, Chloe Branciforte via Google Earth)
The symbol for strike and dip is illustrated in greater detail on Figure 15.5. Note that the dip symbol (shorter line) is perpendicular to the strike symbol; dip degree (number) will vary depending on measurements. You will work more with measuring strike and dip in the next chapter.
When using strike and dip, it is important to remember that with these symbols a geologist can interpret and draw a cross-sectional view from a map-view. Examine Figure 15.6, do you notice any patterns between the map and cross-sectional views? Locate the two peach-colored layers in map view and their associated strike and dip symbols. Notice the dips point away from each other. Now, trace a peach-colored layer down to the cross-sectional view; the layers are no longer straight. Instead the layers actually dip away from each other and there is a distinct rainbow pattern. With a protractor measure the angle of dip for the peach-colored layer. You are measuring how many degrees off the horizontal that layer is. In this case the peach-colored layer is 28° off the horizontal, and is what the number associated with the strike and dip symbol signifies. Consider a swiss or jelly roll cake Links to an external site.. The cake begins as flat, horizontal layers of cake and filling. Those layers are then deformed by the baker rolling and filling the cake. The final cake has a distinct rolled shape, which when cut to be served will reveal the curved layers inside.
Figure 15.6: A depiction of an anticline and a dike in cross-section and in map view with appropriate symbols. (CC-BY 4.0, Steve Earle Links to an external site.)
Useful in map interpretation is the application of the Rule of V’s to determine dip direction. Recall that due to erosion, sharp-pointed V’s are usually in stream valleys, with the stream channel passing through the point of the V, and the V pointing upstream. As a result, when a stream crosses tilted strata, it will cut a V-shape into the rock layers. The point of the V is in the direction of dip. In vertical strata, no V-shapes are made (Figure 15.7).
Figure 15.7: In image A, the stream is encountering tilted strata, which results in a V shape that points towards the dip direction (in this case, to the right). In image B, the strata are vertical, so the stream cannot create a V shape. (CC-BY-SA 3.0, Randa Harris Links to an external site.)