How does movement of plates lead to the formation of folds and faults?



How does movement of plates lead to the formation of folds and faults?

In: Earth Science

Take two thin sheets, slowly crash then together. One goes up and the other goes down (this is a very basic way that my teacher used, correct me if it’s incorrect)

The earth isn’t sitting still. Inside it’s wiggly so the parts we live on can’t sit still. We live on plates. There’s land plates and ocean plates.

When land plates touch ocean plates, the heavy one goes down. Ocean plates are heavy, that can make it sink and make volcanoes like in Washington state or Japan.

When land plates are crashing together, they make big mountains, like mount Everest and the rest of Himalaya Mountains.

When land plates slide past each other, they get stuck and build up more and more energy. Then when they slip past each other they let out an earthquake, like 1906 or 1989 in San Francisco.

When land plates go apart, it stretches out. If you look at east Africa or the sea of Cortéz in between Baja California and the rest of Mexico.

When ocean plates go apart, that makes a cool long skinny mountain where lava comes up. One of those made Brazil and Africa move away but by bit until they got to where they are now.

The whole world keeps inching, sticking and earthquaking around all the time. You can’t feel it, see it or tell when there will be an earthquake. It keeps happening anyway.

Don’t worry too much. There are people watching the earthquake zones and the volcanoes. I’m places like Japan they build the houses bendy to stay up in Earthquakes. Many places where there’s a volcano, there are a lot of people paying attention to it.

Tectonic plates are floating above the surface of magma, and faults are placed where they meet, and folds are places where they overlap.

If they move the meet or overlap different plates

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](, 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.