The Rock Cycle
The model describing how igneous, sedimentary, and metamorphic rocks form and transform over geological time.
The rock cycle describes how the three major rock classes — igneous, sedimentary, and metamorphic — form and transform over geological time. Igneous rocks crystallize from molten magma or lava, either deep underground (intrusive, coarse-grained) or at the surface (extrusive, fine-grained or glassy). Sedimentary rocks form from the accumulation, compaction, and cementation of sediments. Metamorphic rocks arise when heat and pressure alter existing rocks in the solid state, without melting them.
The cycle is not a single loop but a web of pathways. Weathering and erosion break down exposed rock into sediment; transport and deposition lay it down in layers; lithification — compaction and cementation — turns sediment into sedimentary rock. Deep burial or subduction exposes any rock to high pressure and temperature, producing metamorphic rock, and further heating melts rock into magma that rises, cools, and crystallizes into igneous rock, starting the cycle again.
The two energy sources that drive the cycle are Earth's internal heat, which powers plate motion and melting, and solar energy, which drives weathering and erosion. Plate tectonics is the engine that moves rocks between the surface and the deep interior along subduction zones and mid-ocean ridges, and it sets the rates at which the cycle operates.
The rock cycle is a conceptual model, not a literal itinerary: individual rocks can skip stages, move backward along the web, or remain in one class for billions of years. It remains the standard organizing framework for understanding mineral origins, ore-deposit formation, the sedimentary record of past climates, and the age relationships visible in the rock record.
Tags
earth science geology minerals plate tectonics rocks
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