What are shells made of? what happens when they fall to the bottom of he ocean?

Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin. Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods.

An igneous rock can change under pressure to metamorphic.
a igneous rock can turn into another igneous rock by melting then resolidifying, it can turn into a sedimentary rock by getting weathered then recemented, or it can turn into a metamorphic rock by being subjected to great heat and pressure