Rocks are not forever. Melted rock cools into igneous rock. Broken bits of rock settle in layers and are pressed and cemented into sedimentary rock. Heat and squeezing deep underground change either kind into metamorphic rock. The rock cycle is driven by two energy sources: heat inside Earth and the Sun-powered movement of water, ice and air. This week the crew sorts a handful of loose rocks by grain, layers and crystals, and builds a rock cycle model from cards.