12.3: Strike and Dip
- Page ID
To learn many of the concepts associated with structural geology, it is useful to use block diagrams. As you examine these blocks, note the different ways that you can view them. If you look at a block from along the side, you are seeing the cross-section view (like what you see along roads that have been cut through the mountains). If you look at the block from directly above it, you are looking at a map view (Figure 12.5).
As you think about how rocks have changed through the process of deformation, it is useful to remember how they deposited in the first place. We will briefly review some of the geologic laws that you learned in the Introduction Lab. Sedimentary rocks, under the influence of gravity, will deposit in horizontal layers (Law of Original Horizontality). The oldest rocks will be on the bottom (because they had to be there first for the others to deposit on top of them), and are numbered with the oldest being #1 (Law of Superposition). The wooden block in Figure 12.6 displays how sedimentary rocks originally deposit. Each of the boundaries between the colored rock units represents a geologic contact, which is simply the surface between two different rock units. Earth’s rock layers are usually not this uncomplicated. Rock layers are often at an angle, not horizontal, indicating that changes have occurred since deposition. Examine Figure 12.7 to see tilted rocks.
In order to measure and describe layers like this, geologists apply the concepts of strike and dip. Strike refers to the line formed by the intersection of a horizontal plane and an inclined surface. Dip is the angle between that horizontal plane (such as the top of this block) and the tilted surface (the geologic contact between the tilted layers). In Figure 12.8, look at the tilted sedimentary layers. A 'strike' is a 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. The dip angle is measured from the horizontal surface to the dipping bed (outlined in orange on this image).
Now, let’s apply this concept to the block of dipping beds that you just looked at (Figure 12.7). 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). To determine dip, pretend that there is a drop of water between one bed and the next, for example, along the intersection of the pale blue bed and the red bed. In which direction would the water roll if it followed that contact? That is the direction of dip—towards the right in this case. The symbol for strike and dip is given along the top of the block. Note that the dip symbol (shorter line) is perpendicular to the strike symbol (Figure 12.9).
Useful in map interpretation is the application of the Rule of V’s to determine dip direction. As 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 created (Figure 12.10).