Here's the whole idea in one breath: the ground isn't finished. Rock is constantly being made, destroyed and remade, travelling a great loop called the rock cycle — over deep time, stretches of millions of years too long to picture. The rock in your hand isn't a permanent object. It's one frame of a film that never stops.
The Earth Keeps Re-rocking Itself
Pick up a stone. It feels about as permanent as anything can — hard, cold, finished. But that stone is somewhere in a long journey. It might once have been molten, deep underground. Or it might be the squashed-together remains of a long-gone beach. And given enough time it'll be ground to grit, buried, cooked or melted and become something else again. Nothing here is the end of the line — the whole crust is being recycled, endlessly, just far too slowly for us to notice in a lifetime.
Three Families, One Loop
Every rock belongs to one of three families, and the family name just records how that rock was last made. An igneous rock cooled and hardened from molten rock — magma underground or lava at the surface (granite, or dark basalt). A sedimentary rock is built from sediment — sand, mud, shell, grit — that piled up in layers, got buried, squashed and cemented into stone (sandstone, with its visible stripes). A metamorphic rock is an existing rock changed in place by fierce heat and pressure deep in the crust, without ever fully melting — marble (once limestone) or slate (once mud).
And here's the loop — they aren't three separate things, they turn into one another. Heat enough and any rock melts to magma, which cools into igneous rock. Expose any rock at the surface and weathering breaks it down to sediment, which gets deposited and buried and cemented into sedimentary rock. Bury any rock deep and heat and pressure cook it into metamorphic rock. Push it deeper still and it melts again, and the loop closes.
Click the processes in the toy and watch a single stone ride the whole circle. Same material, same loop — you're only choosing which process acts next, and the family changes itself in front of you. That's the whole engine of this topic, and everything else is just learning to read it.