This schematic diagram is a much more up to date version of what we think is going on in the interior of the Earth, at least in the mantle. The take home message here is motion. See all these little black arrows everywhere. They are showing you that the mantle is not actually just statically sitting there. It is moving around all the time. The thing that drives that motion is internal heat. The core has a lot of excess heat from the formation of the Earth and from the decay of radioactive elements. It needs to get rid of that heat somehow. The way it does it is by convection. That means moving hot material from one place to another where it can give that heat away. From the core mantle boundary up first of all you have got this weird D double prime layer where strange things happen to seismic waves that get in there. Here is a plume of material that is buoyantly rising because it is hot. This has been posited to be the source for hot spot volcanoes like this one in the picture here. We also have arrows that show things that are sinking. Right here is a cross section of a subduction zone and you can see the slab is sinking. A lot of slabs get sort of hung up around 670 km depth. This is where the mantle has an increase in density and so it is harder for a sinking slab to get through there but they do get through most of the time. When they do the material that composes them piles up down here so it can later be recycled into whatever the rest of the mantle is doing. The take home message here again is motion. But I want you to also remember that we are talking about solid rock here. It is by no means a liquid, so that motion is happening on very long timescales.