How does movement of plates lead to the formation of folds and faults?
In: Earth Science
The same forces which move the plates are transmitted throughout the tectonic plates. That is, the results are most noticeable at plate boundaries where there are lots of faults in particular, but the stresses get carried through rock for hundreds (sometimes even thousands) of kilometres. You gotta understand that the plates aren’t really rigid bodies either, the surface of them can fracture and they can get squished and squeezed and stretched throughout. This is why you can have faults (and folds) away from plate boundaries.
This is more true for the continental portions of plates, which can be surprisingly squishy. Oceanic crust is more likely to fracture and have faults, whereas continental crust will get all deformed in ways that still confuse geologists sometimes.
A compressive stress (squishing) is, at its simplest, caused by convergence of two plates. This can make continental bits of either plate fold up, or fault so that sections can slide over each other and the crust effectively thickens up. Both of these processes occur in mountain belts, where two plates are moving towards each other. So what makes one happen over the other? Strengths of the rocks involved is one factor, but mostly it comes down to strain rate, temperature and pressure, as I’ll try to explain.
Imagine a chocolate bar. Preferably one of those ones with a gooey, chewy centre of some kind. You put it in the fridge all day and come back to it in the evening. What happens when you pull the ends towards each other? It snaps. Well done, you’ve just ruptured your chocolate bar, because it was **brittle**. Say you left it on the radiator for a few minutes and then tried the same, not forcing it too quickly. You would bend the chocolate bar because it has become **ductile**. So we have the same material with the same force being applied to it in the same way (the same stress) and it behaved differently due to temperature differences.
Our brittle chocolate bar was the upper crust, our ductile chocolate bar the lower crust, where temperatures are a bit hotter (and the confining pressure helps things not to just rupture/fracture with gradual stresses at least). Bend that warm chocolate bar suddenly and it will still rupture, just like earthquakes can occur in rock layers that are being folded if a sudden release of built up stress is permitted (so, if some point gives way suddenly).
The difference in the way the upper and lower crust behave in response to stresses have led to us naming the relevant depths as the brittle zone and the ductile (or plastic) zone. The brittle-ductile transition zone is illustrated nicely [here](https://i.imgur.com/VJXTtpM.png), where a fault in which rock layers are slipping past each other turns into a shear zone, where the rock is smeared out and the size of the grains in the rock is reduced by various physical and chemical processes. These sorts of processes can operate on the atomic scale and work best with higher temperatures, they are what allow a rock to fold and deform plastically as well as the shear zone thing.
Latest Answers