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Rocks change types. A piece of granite (igneous) on a mountainside can weather into sand, get carried downstream, pile up, and harden into sandstone (sedimentary). That sandstone can be buried deep, squeezed and heated, and transform into quartzite (metamorphic). If it's buried deep enough to melt, it becomes magma, cools, and forms a brand-new igneous rock. The rock cycle is real. Any rock can become any other type over a long enough time.

They are the same material, but the name depends on location. Molten rock below Earth's surface is called magma. Once it erupts and reaches the surface, it's called lava. This matters because magma cools slowly underground (bigger crystals) and lava cools quickly on the surface (smaller crystals or volcanic glass). Both produce igneous rock, but the speed of cooling changes what the rock looks like.

These are two separate steps that often get jumbled together. Weathering is the breaking of rock into smaller pieces (wind, water, freeze-thaw, plant roots, chemical reactions). The pieces stay in place. Erosion is the movement of those pieces to a new location, usually by water, wind, ice, or gravity. Break it first, move it second. Students who get this distinction right have a much easier time explaining how sedimentary rocks form.

Metamorphic rocks form when existing rocks are changed by heat and pressure WITHOUT fully melting. If the rock melts all the way, it becomes magma, and when it cools, it forms an igneous rock instead. The key feature of metamorphism is that the rock stays solid while its minerals rearrange and recrystallize. Marble, slate, gneiss, and schist are all rocks that were transformed, not melted.