The mantle is mostly solid rock. It is much thicker and hotter than the crust. There are many minerals in the mantle of the earth. The earth's crust is made up of twenty different plates that fit together like a jigsaw puzzle. These plates are always moving, but only a few centimeters each year.
Plates rub together and they rub apart from each other.
When plates collide they melt and magma is formed. Magma is hot melted rock from when two of the earth's plates collide and rub together. As magma builds, it forms an underground lake of hot liquid. Use water filled balloon. Do not tie the top, only pinch it off.