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The rock cycle is the set of geologic processes by which rocks are continuously transformed among the three major rock types over geological time. Igneous rocks are produced by melting and solidification; they can be weathered and eroded to form sedimentary rocks, or buried and metamorphosed to form metamorphic rocks; metamorphic rocks can melt to restart the cycle. Energy driving the cycle comes from two sources: Earth's internal heat (drives volcanism, tectonics, and metamorphism) and solar energy (drives the hydrological cycle that causes weathering and erosion). The cycle has no fixed starting or ending point, and any rock type can transform into any other given sufficient time and the right conditions.
Drawing the cycle as a flow diagram with labeled pathways (uplift and erosion, burial and lithification, subduction and melting) and annotating each arrow with the responsible process builds a systems-level view of geology. Discussing real-world examples—granite exposed by erosion of the Sierra Nevada, now shedding sand toward the California coast—makes the timescales and processes concrete.
You now know the three major rock families — igneous, sedimentary, and metamorphic — and how each one forms. The rock cycle is the framework that connects them, showing that no rock type is permanent. Every rock on Earth is in transit between states, driven by processes that operate on timescales far longer than human experience but that are happening continuously right now.
Start with an igneous rock like granite, crystallized deep in the crust from cooling magma. Tectonic forces and millions of years of erosion expose it at the surface. Rain, ice, wind, and chemical reactions break it down — feldspar weathers to clay minerals, quartz grains are liberated as sand. Rivers carry these sediments to a basin — a lake, a delta, the continental shelf — where they accumulate layer upon layer. Over time, burial compresses the sediment, water circulating through the pore spaces deposits mineral cements, and the loose grains become lithified into sedimentary rock: sandstone from the quartz grains, shale from the clay. This pathway — weathering, transport, deposition, and lithification — is powered by solar energy driving the water cycle and gravity pulling sediment downhill.
Now suppose that sedimentary rock is caught in a tectonic collision. As continental plates converge, the rock is buried deeper, subjected to increasing temperature and pressure. Minerals recrystallize without melting, textures realign, and the rock transforms into a metamorphic rock — shale becomes slate, then schist, then gneiss as conditions intensify. If burial continues and temperatures exceed roughly 700–900°C, the rock begins to partially melt, producing magma that will eventually cool into new igneous rock and close the loop. This pathway — burial, heating, and metamorphism or melting — is powered by Earth's internal heat from radioactive decay and residual heat from planetary formation.
The crucial feature of the rock cycle is that it has no fixed sequence. Granite does not have to become sandstone before it can become gneiss; it can be metamorphosed directly if buried by tectonic forces without ever being weathered. A metamorphic rock exposed at the surface can weather into sediment without ever melting. A sedimentary rock can be melted by a volcanic intrusion and become igneous rock in a single step, bypassing metamorphism entirely. Every arrow in the cycle diagram represents a real geological process, and which pathway a rock follows depends entirely on which forces act on it. The rock cycle is not a conveyor belt — it is a network of possibilities, all operating simultaneously across the planet, recycling Earth's crustal material over billions of years while conserving mass throughout.