system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can someone explain tectonic plates and what they physically are and what they do?
gpt
Ever seen ice on a frozen lake? Similar idea.

The crust of the Earth is, more or less, just where it is so cold that the molten rock has gotten a chance to freeze over.  This crust of frozen rock (as in it is now solid again instead of flowing around like it does further down) helps insulate the interior of the Earth.

Thin layer of solid rock up on top where it is cold. A lot of hot molten stuff below.  The further down you go, the hotter it gets.

Hot things tend to expand and become less dense than when they are cold. This means that molten rock near the core of the Earth gets really hot and floats up towards the solid rock crust. There it cools off again and, eventually, drops.

This means there is a current.  Much like the ocean. Except these currents move really slowly.  Rock is a lot denser than water and it takes a lot longer for the flow to happen.

All this movement just below the crust causes it to fracture. In some places the hot stuff even pushing its way out. This is where we get volcanoes.  But, let's get back to the crust being broken apart.

The crust, that is the solid part near the surface, is being pulled and pushed in different directions as this hot current moves underneath it and tries to drag the solid part along with it. This means that the crust of the Earth is separated into several huge plates and they are bumping into each other.

But it is taking place incredibly slowly.

Now let's move away from ice and talk about cars.  Two cars hit each other with a head on collision. Their bumpers aren't the same height. What happens?  When they hit the car with the higher bumper tends to go up. The one with the lower bumper tends to go down.

Okay. Now two cars with the bumper in the same place hit. What happens? Depends on how they hit, really. Sometimes it creates a push upwards and the front of both cars lift. Sometimes it causes a push downwards and both cars bend towards the road.

Similar things happen when plates collide. But it is much, much, much slower. How slow? Slow enough that the impact doesn't cause the rocks to shatter. Instead the rock faces of the plates keep pushing at each other until the rock actually bends.

Yes, solid rock will bend. It just takes a lot of constant force taking place over a long, long time.

So, when two plates collide into each other they don't always hit evenly. Sometimes one gets shoved under the other. Sometimes both sides go up. Sometimes both sides go down. It depends on how they collide, at what angle, and how the plates are matched.

When the force is directed upwards the rock is bent up to become mountains. When it is bent downwards it can form valleys or trenches.

Because the places between the plates offer a natural connection to the hot magma below, most volcanoes occur along the plate boundaries.  Earthquakes happen along the plate boundaries (fault lines) and that is due to the fact that these rocks that are moving past one another aren't that well lubricated. Even though the movement is slow, there is still a lot of energy needed to move something the size of a continent. If you press the palms of your hands together tightly and slide them across one another you'll notice a lot of slipping. It will move, the bumps on your hands will catch one another causing it to stop, and then the steady pressure causes you to push past this and it moves faster all at once as the energy is released. Same idea. The movement gets stuck or slowed down by something. Once it gets past this obstacle a lot of energy is released all at once and there is a lot of movement right there along the fault line. Something that was moving at an inch a year suddenly moved a foot or so. That releases a lot of energy at once and earthquakes are the shock waves from this.

So, that's the best answer I've got. Rock goes from liquid to solid because its too cold where we are for rock to stay a liquid. Still, there is a lot of movement down below at it causes the crust to break up a bit and twist, turn, and crash together in places.