Breaking Rocks and Faults
Diagrams showing faults that form at different tectonic settings. A – Original rock structure. B – Fault formed at a convergent boundary. C – Fault formed at a divergent boundary. D – Fault formed at a transform boundary. Original image A from Pearson Education, Inc (2010) modified by D. Lani Pascual (IUPUI / Earth Science).
When rocks can no longer hold onto the strain, the bonds break. But not just one bond breaks, billions of bonds break. You might even say billions of billions of bonds breaks. At the Earth’s surface, we see these broken bonds as broken rocks. They create features at the Earth’s surface called faults. Faults are simply planes along which rocks have broken and continue to move. Now, I do mean a plane. If you look at the figure to the right, you will see some very simple faults that were formed in hypothetical areas where the rocks underwent perfect elastic strain, breaking, and elastic rebound. (If they did deform and did not rebound, they would look a little differently.) Notice that the rocks broke along a surface of the rock structure. This surface is the plane along which the rocks broke. These planes can be kilometers long.
A – This is what our unstressed (happy?) rock looks like. You can see the nice sedimentary layers are still perfectly horizontal.
B – At a compressional or convergent boundary, the rocks are being squished together through compressional stress. This causes shortening strain. Here the release of this strain can cause the rocks to break and push up against each other along a diagonal plane. This forms what geologists call a reverse fault. Click here for a picture of a reverse fault.
C – At a divergent boundary where the rocks are being pulled apart, the rocks are undergoing tensile stress. The rocks hold this energy with lengthening strain. As the rocks are being pulled apart, like a rubber band, the rocks break along two planes. The area in between these two planes slips down, deeper into the asthenosphere. This process of dropping down of rocks is called subsidence and forms what geologists call a normal fault. Click here for a picture of a normal fault.
D – At transform boundaries, the rocks are undergoing shear stress. The rocks are resisting rotating or twisting, resulting in the rocks holding on to rotating strain (which is sometimes called shear strain). As these rocks break, they form what geologists call a strike-slip fault. Click here for a picture of a strike-slip fault.
These planes of rock breakage and movement are measured in terms of a slip rate or the distance the rocks move in millimeters per year. If you remember from our Plate Tectonics Module, scientists can measure plate movement by using global positioning satellite technology. Stations are set up at areas of known tectonic movement, like faults, and the movement of these stations is tracked over time. Below is a picture from the USGS showing the tracking of slip rates along the San Andreas Fault in California.
The larger map (left) is a blow-up of the area shown in the box to the right. The blue arrows show the movement of the Pacific Plate against the North American plate. Slippage along the San Andreas fault occurs at about 2-5 cm (20 -50 mm) per year. If you want to see a picture of what the fault looks like at the Earth’s surface, click here.