![]() Hat tip to Lockwood DeWitt for giving me the idea for this post. But by adding this simple symbol, we can understand something about the 3-D orientation of the rocks. Without the strike and dip symbol, we would have no idea if those layers were dipping to the west, to the east, or were vertical. The different colors on the map represent the different rock layers exposed at the surface, just like looking at one of the block diagrams from above. It would look something like the example below. Just below that we would write the measured dip: “45”. ![]() Think of the tick mark as an arrow pointing in the direction a ball would roll if you could set it on that rock layer. If the rock was dipping 45 degrees to the east, we would add a tick mark at the midpoint of our strike line, pointing east. If we measured the rock striking north-south, we would draw a short, vertical line on the map in the location of our measurement (assuming up is north). Strike is easy enough, since it’s a compass direction. Once you’ve made the measurements, you can display them on a geologic map- which was our goal at the start of this post. Geologists carefully measure the strike and dip on an outcrop like that using a tool called a Brunton compass, pictured below. You can click on this image to see more photos from this spot. If you click this link, you can check out the area with Google Earth. Here’s an annotated image of that outcrop, just to make sure you can see the strike and dip. Be sure to zoom in and get a closer look. DIP: definition- The angle of inclination of a planar feature measured from a horizontal datum. The fracture itself is called a fault plane. Faulting occurs when shear stress on a rock overcomes the forces which hold it together. Here, sections of rock move past each other. Click the image below to explore a real outcrop of sandstone and siltstone from the Oregon coast that looks a lot like this. The strike is measured in that horizontal plane. A fault is a fracture, or break, in the Earths crust ( lithosphere ). When you look at the top of the block (the surface), you can see the strike. When you look at the side of the block, you can see the dip. The top of the block is still the surface of the Earth, but I’ve sliced out a piece of the block to help us visualize this better. Here’s a block made of tilted layers instead of horizontal ones. That means this layer is dipping to the east. (Don’t worry about why we use that word.) In this example, the strike is north-south. Draw a horizontal line across that layer- that’s what we call the “strike”. Again, it’s dipping downward to the right. Now, let’s try to imagine looking at that same tilted layer from directly above it. Strike is describes with end to end directions like North. Thus, a horizontal plan is devoid of strike as it cannot intersect horizontal plane. Strike describes the direction of surfaces along their line of intersection with a horizontal plane. We measure the dip as the angle between the layer and horizontal. Answer (1 of 5): Strike in geology means direction. The tilt of that layer is what we call “dip”. We describe this layer as dipping to the right, because that’s the direction it’s tilted downwards toward. Let’s picture a single layer of that rock and tilt it downward to the right. Picture a kangaroo hopping across it, if that helps or entertains you. Pretend the top of this block is the surface of the Earth. Let’s start with some nice, horizontal sedimentary rocks. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination (or depression angle ) measured downward from horizontal. The words may be confusing at first, but it’s really quite simple. In geology, strike and dip is a measurement convention used to describe the plane orientation or attitude of a planar geologic feature. ![]() The geological lingo for this is “strike and dip”. Here’s a basic primer on how that’s done. So when we’ve measured the orientation of a rock layer, we need a way to represent that on a map. While maps are useful in all kinds of ways, they’re a little lacking in the 3-D department. Those spatial relationships- what’s on top of what, how rocks are faulted or folded- give us all kinds of interesting information. As we will see below, the failure criterion for frictional sliding is defined by a similar equation, with similar magnitudes for the parameters.In order to understand geology, we need to think about the rocks below our feet in three dimensions. ![]() Cohesion for rock is typically about 0.1 MPa for silts or clays, but 10 MPa for other rocks. The cohesion term sets the minimum values of the shear fracture strength, which occurs when the normal stress is zero. The angle of internal friction for rock has a typical values \(30^\circ\), but ranges from \(20-45^\circ\). The term \(tan(\phi)\) can be thought of as an internal friction. Where \(\phi\) is the angle of internal friction, and \(c\) is the cohesion. ![]()
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